United Kingdom
National Hydrogen Safety Assessment
Mar 2026
Publication
Hydrogen is being proposed as a suitable low carbon alternative to natural gas. It has the potential to reduce green-house gas emissions across Great Britain’s industrial landscape but also for heating buildings. The Department of Energy Security and Net Zero (DESNZ) statistics show domestic heating currently accounts for 37% of the country’s CO2 emissions.
Hydrogen and natural gas are very similar when compared with other fuel sources. They are both gaseous at distribution operating pressures they can both be burned to release energy and they are both non-corrosives. This means that a piped distribution system and combustion appliances can be used to exploit their captured energy. They also differ from one another in that hydrogen has a broader range of flammability and lower ignition energy but is also much more buoyant and has higher diffusivity. Hydrogen does not have the potential to produce carbon monoxide when burnt the Health & Safety Executive’s (HSEs) incident statistics suggest that by 2032 there will still be 1.5 fatalities per year attributed to carbon monoxide poisoning as a direct result of using methane based natural gas for heating and cooking.
A Clean Heat Policy Decision from the Government is due in 2025. Ahead of this DESNZ has commissioned the Health and Safety Executive to carry out a Comprehensive Formal Assessment (CFA) of all safety and technical evidence on hydrogen due in mid-2025. The National Hydrogen Safety Assessment (NHSA) is being submitted to HSE along with other evidence for assessment as part of the CFA.
The purpose of the NHSA is to describe the minimum general arrangements that will be required to demonstrate that a network and its connected properties can be converted to hydrogen and be operated safely with risks appropriately managed for those working on the system or whose safety may be impacted including the general public (section 3). The NHSA applies to the transmission distribution and end use of hydrogen but does not include production or storage. At this time no specific parts of a network are planned for conversion and the NHSA therefore sets out the general approach to be taken for conversion and operation rather than detailed plans.
Subject to a successful policy decision on hydrogen for heating supported by HSE’s Comprehensive Formal Assessment of safety evidence the approach will be to develop detailed plans and safety cases to allow progression to delivering hydrogen conversion. The differences between natural gas and hydrogen mean that to transition from one gas to the other with the intention of utilising the same piped distribution system and end-utilisation requires due diligence assessment and where necessary updates to the system both to the assets themselves or the way they are operated.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Hydrogen and natural gas are very similar when compared with other fuel sources. They are both gaseous at distribution operating pressures they can both be burned to release energy and they are both non-corrosives. This means that a piped distribution system and combustion appliances can be used to exploit their captured energy. They also differ from one another in that hydrogen has a broader range of flammability and lower ignition energy but is also much more buoyant and has higher diffusivity. Hydrogen does not have the potential to produce carbon monoxide when burnt the Health & Safety Executive’s (HSEs) incident statistics suggest that by 2032 there will still be 1.5 fatalities per year attributed to carbon monoxide poisoning as a direct result of using methane based natural gas for heating and cooking.
A Clean Heat Policy Decision from the Government is due in 2025. Ahead of this DESNZ has commissioned the Health and Safety Executive to carry out a Comprehensive Formal Assessment (CFA) of all safety and technical evidence on hydrogen due in mid-2025. The National Hydrogen Safety Assessment (NHSA) is being submitted to HSE along with other evidence for assessment as part of the CFA.
The purpose of the NHSA is to describe the minimum general arrangements that will be required to demonstrate that a network and its connected properties can be converted to hydrogen and be operated safely with risks appropriately managed for those working on the system or whose safety may be impacted including the general public (section 3). The NHSA applies to the transmission distribution and end use of hydrogen but does not include production or storage. At this time no specific parts of a network are planned for conversion and the NHSA therefore sets out the general approach to be taken for conversion and operation rather than detailed plans.
Subject to a successful policy decision on hydrogen for heating supported by HSE’s Comprehensive Formal Assessment of safety evidence the approach will be to develop detailed plans and safety cases to allow progression to delivering hydrogen conversion. The differences between natural gas and hydrogen mean that to transition from one gas to the other with the intention of utilising the same piped distribution system and end-utilisation requires due diligence assessment and where necessary updates to the system both to the assets themselves or the way they are operated.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Dispersion of Helium Releases in Domestic Properties
Mar 2026
Publication
The UK’s pathway to net zero is likely to include the use of hydrogen in place of natural gas within the gas network. From the perspective of domestic end users of gas if a conversion is to take place then the behaviour of hydrogen after an escape within a property needs to be fully understood. Several studies have been carried out in recent years to understand the dispersion characteristics of hydrogen in the event of an internal escape within a building. From a domestic perspective this has always taken place in laboratory settings or purpose-built test houses which are probably not entirely reflective of UK housing stock. This work pulls together previous work on domestic ventilation and gas dispersion analysis to provide further evidence to show how a significantly lighter than air gas will move around a range of real domestic properties in the event of an internal gas escape. Reference is then made to work carried out on the consequences of ignition of hydrogen to show how the use of risk mitigation measures such as ventilation and hydrogen detectors can be used to support early conversion projects.
The outputs of previous research into the use of hydrogen in domestic properties suggested several recommendations on a hydrogen conversion. These recommendations included the following measures:
• Integrity assurance of pipework and gas appliances i.e. ensuring that the gas system within the property is leak-tight and all appliances are fitted with appropriate safety devices.
• Odourisation this will continue to occur to the same concentration (ca. 6 mg/m3).
• Excess Flow Valves (EFVs) to stop the gas flow in the event of a large leak.
• Appropriate ventilation and additional ventilation in small spaces such as cupboards.
• Gas detection in the form of an alarm or alarms with a 1000 ppm set point located in appropriate and central locations to alert the householder if a gas leak occurs.
To solidify these recommendations this experimental programme was carried out to investigate the propagation of a buoyant gas in a variety of as lived in houses under a range of leak scenarios. As such the work focussed on reducing the risk/protecting individuals in situations where a leak has already occurred via the optimal application of ventilation and gas detection.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
The outputs of previous research into the use of hydrogen in domestic properties suggested several recommendations on a hydrogen conversion. These recommendations included the following measures:
• Integrity assurance of pipework and gas appliances i.e. ensuring that the gas system within the property is leak-tight and all appliances are fitted with appropriate safety devices.
• Odourisation this will continue to occur to the same concentration (ca. 6 mg/m3).
• Excess Flow Valves (EFVs) to stop the gas flow in the event of a large leak.
• Appropriate ventilation and additional ventilation in small spaces such as cupboards.
• Gas detection in the form of an alarm or alarms with a 1000 ppm set point located in appropriate and central locations to alert the householder if a gas leak occurs.
To solidify these recommendations this experimental programme was carried out to investigate the propagation of a buoyant gas in a variety of as lived in houses under a range of leak scenarios. As such the work focussed on reducing the risk/protecting individuals in situations where a leak has already occurred via the optimal application of ventilation and gas detection.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Hydrogen Conversion Strategy-Pipework – Literature Review and Interim Report
Mar 2026
Publication
The purpose of this study is to consider identify and mitigate risks associated with a conversion (repurposing) of a natural gas (NG) installation to a 100% hydrogen (H2) installation within the current UK gas network. The scope of this project focuses on domestic gas installations downstream of the ECV to the appliance inlet.
The report analyses accident and leak data to explore the types of leak that occur in current NG domestic pipework. The differences in the physical properties of hydrogen and methane are tabulated and the differing behaviour of hydrogen and methane leaks is analysed.
This report concentrates on spontaneous leaks in the gap between small and the large which will trip the EFV. The dispersion of gas from the different types of leak and available ventilation models are discussed in detail.
British Standard and IGEM guidance documents are discussed in detail along with previous studies of hydrogen and NG leak behaviour.
The review recommends modifications to the original project work programme. In particular WP3 will be modified to examine methods of testing internal pipework to ensure that it is suitable for repurposing to hydrogen. This is an essential output from this project. This will include consideration of a range of options:
• Visual inspection of visible pipework
• Low pressure tightness testing
• Higher pressure strength testing
• Novel techniques such as thermal imaging or visual inspection using borescopes
• The use of other gases (helium) to check pipework soundness.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
The report analyses accident and leak data to explore the types of leak that occur in current NG domestic pipework. The differences in the physical properties of hydrogen and methane are tabulated and the differing behaviour of hydrogen and methane leaks is analysed.
This report concentrates on spontaneous leaks in the gap between small and the large which will trip the EFV. The dispersion of gas from the different types of leak and available ventilation models are discussed in detail.
British Standard and IGEM guidance documents are discussed in detail along with previous studies of hydrogen and NG leak behaviour.
The review recommends modifications to the original project work programme. In particular WP3 will be modified to examine methods of testing internal pipework to ensure that it is suitable for repurposing to hydrogen. This is an essential output from this project. This will include consideration of a range of options:
• Visual inspection of visible pipework
• Low pressure tightness testing
• Higher pressure strength testing
• Novel techniques such as thermal imaging or visual inspection using borescopes
• The use of other gases (helium) to check pipework soundness.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Use of Automatic Isolation Valve (AIV) Systems with Hydrogen, Final Report
Mar 2026
Publication
Kiwa,
Steer Energy and
Cadent
Automatic Isolation Valve (AIV) systems are common gas safety systems based around a powered isolation valve a control system and some form of measurement input that is often a gas detector or proving system. AIVs are a standard safety system already found in some commercial and industrial Natural Gas installations.
This project investigates their suitability for small commercial installations using hydrogen with a focus on the UK village trial sites. The project scope included a review of commercial installations in the two hydrogen village trial sites in Whitby and Redcar and assessed the potential benefits of using AIVs as a risk mitigation method in those sites.
A small number of AIV systems were found during the site surveys and the project considers the benefits of adding additional systems to improve installation safety with the hydrogen trials. The project scope also included an investigation of AIV systems suited for hydrogen and three such units were to be specified and tested.
The initial focus of the project was to assess the performance of AIVs with hydrogen. However it was later realised that the key to this assessment was the correct placement of detectors which required an understanding of how hydrogen moves and disperses compared to Natural Gas. The project was therefore refocused to include experimental studies using methane as a substitute for hydrogen and computational fluid dynamics (CFD) studies to model the movement and dispersion of hydrogen.
This report summarises the output of the project; additional details on individual elements of work are provided in the appendices. Appendix A gives details of the risk assessment methodology of the project. Appendix B comprises the interim project report and baseline project review. Appendix C has the full experimental programme of work. Appendix D covers the scenario modelling and village trial site visits. Appendices E and F provide manufacturer guidance on detector placement. Appendix G gives the reports on the CFD study.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This project investigates their suitability for small commercial installations using hydrogen with a focus on the UK village trial sites. The project scope included a review of commercial installations in the two hydrogen village trial sites in Whitby and Redcar and assessed the potential benefits of using AIVs as a risk mitigation method in those sites.
A small number of AIV systems were found during the site surveys and the project considers the benefits of adding additional systems to improve installation safety with the hydrogen trials. The project scope also included an investigation of AIV systems suited for hydrogen and three such units were to be specified and tested.
The initial focus of the project was to assess the performance of AIVs with hydrogen. However it was later realised that the key to this assessment was the correct placement of detectors which required an understanding of how hydrogen moves and disperses compared to Natural Gas. The project was therefore refocused to include experimental studies using methane as a substitute for hydrogen and computational fluid dynamics (CFD) studies to model the movement and dispersion of hydrogen.
This report summarises the output of the project; additional details on individual elements of work are provided in the appendices. Appendix A gives details of the risk assessment methodology of the project. Appendix B comprises the interim project report and baseline project review. Appendix C has the full experimental programme of work. Appendix D covers the scenario modelling and village trial site visits. Appendices E and F provide manufacturer guidance on detector placement. Appendix G gives the reports on the CFD study.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Hydrogen Risk Mitigations – GDN risk mitigations selection proposal
Mar 2026
Publication
Hydrogen is being proposed as a suitable low carbon alternative to natural gas. It has the potential to reduce green-house gas emissions across Great Britain’s industrial landscape but also for heating buildings. The Department of Energy Security and Net Zero (DESNZ) statistics show domestic heating currently accounts for 37% of the country’s CO2 emissions. Hydrogen also helps reduce fatalities associated with carbon monoxide. Discussions with the Health & Safety Executive (HSE) on incident statistics suggest that by 2032 there will still be 1.5 fatalities per year attributed to carbon monoxide poisoning as a direct result of using methane based natural gas for heating and cooking.
Hydrogen and natural gas are very similar when compared with other fuel sources. They are both gaseous at distribution operating pressures they can both be burned to release energy and they are both non-corrosives. This means that a piped distribution system and combustion appliances can be used to exploit their captured energy. They also differ from one another in that hydrogen has a broader range of flammability and lower ignition energy but is also much more buoyant and has higher diffusivity than natural gas; hydrogen also releases no carbon when burnt. The differences mean that to transition from one gas to the other with the intention of utilising the same piped distribution system and end-utilisation requires due diligence assessment and where necessary updates to the system – be that to the assets themselves or the way they are operated.
A strategic Government decision on the role of hydrogen in decarbonising heat is due in 2025. Ahead of this DESNZ has commissioned the Health and Safety Executive to carry out a Comprehensive Formal Assessment (CFA) of all safety and technical evidence on hydrogen due in mid-2025. The deadline to have all evidence to be assessed with the HSE is September 2024 (albeit work will continue after this date but not be part of the CFA). One of the safety demonstration requirements is an assessment of the change in risk posed by a transition to hydrogen. Whilst other projects focus on the risks related to transmission assets the emphasis of this piece of work has been on below 7 bar distribution assets and its end-users excluding industrial users and storage who would require bespoke risk assessments for their processes.
Several projects have been undertaken or contributed to hydrogen distribution and end-use risk assessments. DESNZ’s Hy4Heat Programme included a safety assessment in 2021. NGN’s H21 project and concurrent development of the Hydrogen Village Trial submissions helped progress the risk assessments in 2022 and 2023. They included the development of quantitative risk assessments (QRA) being carried out using the DNV modelling tool CONIFER which was developed specifically to differentiate between natural gas and hydrogen distribution systems.
Recognising the influx of additional safety and technical evidence throughout 2023 and 2024 leading up to the CFA deadline of September 2024 a final risk assessment was commissioned to cover all of Great Britain. It is the most thorough risk assessment the gas industry has ever carried out. All the Gas Distribution Networks (GDNs) and some of the Independent Gas Transporters (IGTs) have contributed to the study.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Hydrogen and natural gas are very similar when compared with other fuel sources. They are both gaseous at distribution operating pressures they can both be burned to release energy and they are both non-corrosives. This means that a piped distribution system and combustion appliances can be used to exploit their captured energy. They also differ from one another in that hydrogen has a broader range of flammability and lower ignition energy but is also much more buoyant and has higher diffusivity than natural gas; hydrogen also releases no carbon when burnt. The differences mean that to transition from one gas to the other with the intention of utilising the same piped distribution system and end-utilisation requires due diligence assessment and where necessary updates to the system – be that to the assets themselves or the way they are operated.
A strategic Government decision on the role of hydrogen in decarbonising heat is due in 2025. Ahead of this DESNZ has commissioned the Health and Safety Executive to carry out a Comprehensive Formal Assessment (CFA) of all safety and technical evidence on hydrogen due in mid-2025. The deadline to have all evidence to be assessed with the HSE is September 2024 (albeit work will continue after this date but not be part of the CFA). One of the safety demonstration requirements is an assessment of the change in risk posed by a transition to hydrogen. Whilst other projects focus on the risks related to transmission assets the emphasis of this piece of work has been on below 7 bar distribution assets and its end-users excluding industrial users and storage who would require bespoke risk assessments for their processes.
Several projects have been undertaken or contributed to hydrogen distribution and end-use risk assessments. DESNZ’s Hy4Heat Programme included a safety assessment in 2021. NGN’s H21 project and concurrent development of the Hydrogen Village Trial submissions helped progress the risk assessments in 2022 and 2023. They included the development of quantitative risk assessments (QRA) being carried out using the DNV modelling tool CONIFER which was developed specifically to differentiate between natural gas and hydrogen distribution systems.
Recognising the influx of additional safety and technical evidence throughout 2023 and 2024 leading up to the CFA deadline of September 2024 a final risk assessment was commissioned to cover all of Great Britain. It is the most thorough risk assessment the gas industry has ever carried out. All the Gas Distribution Networks (GDNs) and some of the Independent Gas Transporters (IGTs) have contributed to the study.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
380 Air Ingress in Isolated Installations, Interim Report
Mar 2026
Publication
The precursor to this project was an event leading to a damaged meter at the in January 2024 after the installation had been isolated for 27 days over the holiday period. The incident required two key events: an air/fuel mixture within the flammable range for hydrogen in the installation pipework and an ignition of that air/fuel mixture. The consequence of this was an overpressure in the system leading to damage occurring in the gas meter.
The initial investigations into the event suggested that there may be a mechanism allowing air ingress to occur in an isolated gas installation. The ignition itself was suspected to have been caused by an attempt to light the hydrogen fire with subsequent flashback into the installation.
This report details work carried out to date by Steer Energy supported by Enertek to investigate air ingress mechanisms in gas installations. The project is split into two overall work programmes:
• a detailed experimentally led study of the air ingress phenomenon to identify and understand gas exchange mechanisms in an isolated system leading to the creation of a flammable mixture in an installation.
• an investigation into the potential for ignition of a flammable mixture in an installation by an appliance or component in that system.
Both programmes will then conclude with mitigating measures for each event providing assurance for safe operation of hydrogen in gas installations during roll out.
This interim report provides details of the work carried out to date on the project. The work is not completed however good progress has been made in understanding the causes of both events described above.
Over 100 individual tests have been carried out air ingress has been demonstrated and a good understanding of the mechanisms has been gained. The effects have been seen with hydrogen heliumand methane. Key observations are:
• no air ingress has been seen while pressure remains above ambient.
• no air ingress has been seen when systems are connected to an upstream gas supply.
We can therefore conclude that air ingress does not occur whilst positive pressure is maintained in a gas installation and this can be achieved by maintaining the upstream supply.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
The initial investigations into the event suggested that there may be a mechanism allowing air ingress to occur in an isolated gas installation. The ignition itself was suspected to have been caused by an attempt to light the hydrogen fire with subsequent flashback into the installation.
This report details work carried out to date by Steer Energy supported by Enertek to investigate air ingress mechanisms in gas installations. The project is split into two overall work programmes:
• a detailed experimentally led study of the air ingress phenomenon to identify and understand gas exchange mechanisms in an isolated system leading to the creation of a flammable mixture in an installation.
• an investigation into the potential for ignition of a flammable mixture in an installation by an appliance or component in that system.
Both programmes will then conclude with mitigating measures for each event providing assurance for safe operation of hydrogen in gas installations during roll out.
This interim report provides details of the work carried out to date on the project. The work is not completed however good progress has been made in understanding the causes of both events described above.
Over 100 individual tests have been carried out air ingress has been demonstrated and a good understanding of the mechanisms has been gained. The effects have been seen with hydrogen heliumand methane. Key observations are:
• no air ingress has been seen while pressure remains above ambient.
• no air ingress has been seen when systems are connected to an upstream gas supply.
We can therefore conclude that air ingress does not occur whilst positive pressure is maintained in a gas installation and this can be achieved by maintaining the upstream supply.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Air/Oxygen Ingress in Domestic Gas Installations, Phase 1 Summary Paper
Mar 2026
Publication
Air ingress is not a new phenomenon that is exclusive to hydrogen - it already occurs in natural gas installations today (but has not previously been recorded as a challenge). Subsequent ignition events are highly unlikely requiring a very specific combination of actions and conditions to occur. However the likelihood and potential consequences of an ignition event occurring are greater with hydrogen systems (if unmitigated) compared to natural gas. Therefore additional safeguards are recommended for hydrogen systems to appropriately manage these risks. This work has concluded that theoretically any ignition events in hydrogen installations in the future should be prevented by requiring additional assurances within the formal testing and certification process of hydrogen appliances. However as good-practice extra layers of protection are also being recommended as precautionary measures - to inhibit the mechanisms of air ingress and to minimise the consequences of any (now unforeseeable) flash-back events respectively. Air ingress can be successfully mitigated in the short-term using existing technology and methods. Opportunities exist for significant improvements on mitigation measures in the future by deploying evolving technologies and ‘smart’ systems.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Kiwa Energy Domestic Ventilation Report
Mar 2026
Publication
The report provides an evaluation of the effect of the levels of ventilation on hydrogen concentrations that could result from leaks.
A mathematical model has been developed using data collected from air tightness testing of real properties to simulate maximum gas in air concentrations achieved within a domestic property following a leak of hydrogen. The model is based on well-established theories in fluid dynamics and has been validated (by the model developers) by comparison with real experimental data gained from the Hy4Heat H100 and HyHouse programmes of work. A comprehensive range of scenarios have been modelled to assess leaks of varying severity under a range of ventilation regimes. Modelled hydrogen leak rates range from 0.02 m3/h to 51.7 m3/h and levels of property air permeability range from 4.2 to 24.7 m3/h/m2 @ 50 Pa by convention 0.21 to 1.24 Air Changes per hour (ACH).
As expected spaces with greater ventilation are predicted by the model to result in lower maximum gas in air concentrations and adding vents (internal and / or external) is effective at reducing concentrations within a room.
A general finding from air permeability tests of dwellings was kitchens and hallways within properties were less airtight than living rooms. This implies greater ventilation to/from these spaces. Consideration has been given in this work to the co-use of audible alarms where they are most likely to alert the occupiers. If pipework and appliances are in rooms connected to a central space typically a hallway an alarm (with integral detector) located at the highest point of that space should detect hydrogen from a leak in the connected rooms. In general this is the most likely location to form the audible alarm to be heard in the various rooms in a dwelling. However there will be cases where appliances and pipework are not in a room connected to a central location or where a large build-up of gas may occur (e.g. due to high ceilings relative to door height) before tracking to a central location. Adding internal high-level vents to facilitate tracking to the alarm location should aid timely detection of a leak in these cases. An alternative could be to install more alarm units which may be more practical and preferred.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
A mathematical model has been developed using data collected from air tightness testing of real properties to simulate maximum gas in air concentrations achieved within a domestic property following a leak of hydrogen. The model is based on well-established theories in fluid dynamics and has been validated (by the model developers) by comparison with real experimental data gained from the Hy4Heat H100 and HyHouse programmes of work. A comprehensive range of scenarios have been modelled to assess leaks of varying severity under a range of ventilation regimes. Modelled hydrogen leak rates range from 0.02 m3/h to 51.7 m3/h and levels of property air permeability range from 4.2 to 24.7 m3/h/m2 @ 50 Pa by convention 0.21 to 1.24 Air Changes per hour (ACH).
As expected spaces with greater ventilation are predicted by the model to result in lower maximum gas in air concentrations and adding vents (internal and / or external) is effective at reducing concentrations within a room.
A general finding from air permeability tests of dwellings was kitchens and hallways within properties were less airtight than living rooms. This implies greater ventilation to/from these spaces. Consideration has been given in this work to the co-use of audible alarms where they are most likely to alert the occupiers. If pipework and appliances are in rooms connected to a central space typically a hallway an alarm (with integral detector) located at the highest point of that space should detect hydrogen from a leak in the connected rooms. In general this is the most likely location to form the audible alarm to be heard in the various rooms in a dwelling. However there will be cases where appliances and pipework are not in a room connected to a central location or where a large build-up of gas may occur (e.g. due to high ceilings relative to door height) before tracking to a central location. Adding internal high-level vents to facilitate tracking to the alarm location should aid timely detection of a leak in these cases. An alternative could be to install more alarm units which may be more practical and preferred.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Hydrogen in Multi-occupancy Buildings Feasibility Study - Final Report
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project ‘MOBs Work Pack 2 Asset Information Review’ considered the requirements for converting MOBs to Hydrogen and identified gaps in technical evidence. SGN is leading a feasibility project with some applied testing to address the evidence gaps identified.
This report summarises the work which has been undertaken as part of Work Pack 3 – QRA and Testing. This includes the development of Quantitative Risk Assessments (QRAs) for Hydrogen MOBs conversion and an overall impact review of the conversion of MOBs.
Work Pack 3 included the development of a Quantitative Risk Assessment (QRA) for Hydrogen MOBs conversion which should:
• Easily integrate with wider network QRAs (e.g. GB QRA) to help complete understanding of safety across the entire system
• Take account of the network and end user parts of the system in the building
• Provide a record of underlying evidential basis or where this is lacking the justified assumptions or simplifications made in reviewing existing evidential basis (e.g. can it be assumed that occupants will react to a Hydrogen leak in the same way as Natural Gas?)
• Provide a quantified assessment of risk either in the form of:
o Absolute risk
o Comparative risk
o ALARP (“as low as reasonably practicable”)
• Obtain agreement from HSE on conclusions of QRA.
Work Pack 3 also included a review of the overall impact of conversion of MOBs considering:
• The cost and practicality of converting the MOB stock
• The safety of Hydrogen in MOBs vs alternatives
• This study could draw on feasibility type studies – i.e. feasibility review of the conversion of a limited number of real MOBs
• Overall recommendation for the suitability of Hydrogen versus alternatives with potential split between different categories of building
• Further recommendations for transition NIA to SIF project:
o Trials
o Further confirmatory evidential work
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This report summarises the work which has been undertaken as part of Work Pack 3 – QRA and Testing. This includes the development of Quantitative Risk Assessments (QRAs) for Hydrogen MOBs conversion and an overall impact review of the conversion of MOBs.
Work Pack 3 included the development of a Quantitative Risk Assessment (QRA) for Hydrogen MOBs conversion which should:
• Easily integrate with wider network QRAs (e.g. GB QRA) to help complete understanding of safety across the entire system
• Take account of the network and end user parts of the system in the building
• Provide a record of underlying evidential basis or where this is lacking the justified assumptions or simplifications made in reviewing existing evidential basis (e.g. can it be assumed that occupants will react to a Hydrogen leak in the same way as Natural Gas?)
• Provide a quantified assessment of risk either in the form of:
o Absolute risk
o Comparative risk
o ALARP (“as low as reasonably practicable”)
• Obtain agreement from HSE on conclusions of QRA.
Work Pack 3 also included a review of the overall impact of conversion of MOBs considering:
• The cost and practicality of converting the MOB stock
• The safety of Hydrogen in MOBs vs alternatives
• This study could draw on feasibility type studies – i.e. feasibility review of the conversion of a limited number of real MOBs
• Overall recommendation for the suitability of Hydrogen versus alternatives with potential split between different categories of building
• Further recommendations for transition NIA to SIF project:
o Trials
o Further confirmatory evidential work
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
SNG Safety Management Framework: Managament Procedure for Converting Natural Gas Installations in Multi-occupancy Buildings to Hydrogen
Mar 2026
Publication
This procedure details the process of how riser and lateral installations currently supplying Natural Gas to domestic accommodation within multi-occupancy buildings (MOBs) are selected and converted to Hydrogen or identified for decommissioning and the provision of an alternative option.
This procedure describes the requirements for conversion of existing Natural Gas installations in multi-occupancy buildings to Hydrogen operating at pressures not exceeding 75 mbar. For gas installations deemed unsuitable for conversion it provides guidance for decommissioning and alternative energy solutions. The gas supplied is assumed to be 100% Hydrogen as specified by IGEM/H/1 and therefore does not cover blends of Hydrogen and Natural Gas.
This procedure covers guidance for:
• The identification selection and commissioning of network risers and laterals installed in existing multi-storey buildings deemed suitable for repurposing to Hydrogen as they currently are.
• The requirements to identify buildings that are not suitable for repurposing in their current form and outlines the necessary replacement or partial replacement renovation alteration or repair of existing riser and lateral installations from the pipeline isolation valve (PIV) through to the supplier’s emergency control valve (ECV) to enable Hydrogen conversion.
Note: Where an existing Natural Gas installation is to be repurposed for Hydrogen it will have been subjected to a pre-conversion survey and risk assessment to ensure components are suitable for use or otherwise replaced.
• The identification isolation and decommissioning of network risers and laterals installed in existing multi-storey buildings not deemed suitable for conversion to Hydrogen.
• The provision of alternative arrangements in the form of an energy centre or an alternative option such as heat electrification heat pump or heat network.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This procedure describes the requirements for conversion of existing Natural Gas installations in multi-occupancy buildings to Hydrogen operating at pressures not exceeding 75 mbar. For gas installations deemed unsuitable for conversion it provides guidance for decommissioning and alternative energy solutions. The gas supplied is assumed to be 100% Hydrogen as specified by IGEM/H/1 and therefore does not cover blends of Hydrogen and Natural Gas.
This procedure covers guidance for:
• The identification selection and commissioning of network risers and laterals installed in existing multi-storey buildings deemed suitable for repurposing to Hydrogen as they currently are.
• The requirements to identify buildings that are not suitable for repurposing in their current form and outlines the necessary replacement or partial replacement renovation alteration or repair of existing riser and lateral installations from the pipeline isolation valve (PIV) through to the supplier’s emergency control valve (ECV) to enable Hydrogen conversion.
Note: Where an existing Natural Gas installation is to be repurposed for Hydrogen it will have been subjected to a pre-conversion survey and risk assessment to ensure components are suitable for use or otherwise replaced.
• The identification isolation and decommissioning of network risers and laterals installed in existing multi-storey buildings not deemed suitable for conversion to Hydrogen.
• The provision of alternative arrangements in the form of an energy centre or an alternative option such as heat electrification heat pump or heat network.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Draft Standard for Converting Natural Gas Installations in Multi-occupancy Buildings to Hydrogen
Mar 2026
Publication
This supplement gives additional requirements and qualifications for the conversion of Natural Gas installations in multi-occupancy buildings to Hydrogen and is only to be used in conjunction with IGEM/G/5 Edition 3. This supplement outlines the principles required for the conversion of existing Natural Gas installations in multi-occupancy buildings to 100% Hydrogen. A conversion to Hydrogen should consider the following options:
a) Repurposing existing installations
b) Renovating existing installations; this may involve for example an internal lining being applied to the existing network pipelines.
c) Replacing existing network pipelines with either:
a. New network pipelines or
b. An energy centre.
If following a risk assessment and cost benefit assessment none of the above options are considered suitable the remaining option would be to:
d) Decommission the existing gas installation and install a suitable alternative decarbonisation option (electrification heat pump heat network etc.).
This supplement covers the principles required for the repurposing renovating and replacement of existing gas installations for Hydrogen service and the decommissioning of existing gas installations.
This supplement provides the principles required to identify buildings and gas installations that are not suitable for repurposing and outlines remedial work that may be required prior to repurposing.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
a) Repurposing existing installations
b) Renovating existing installations; this may involve for example an internal lining being applied to the existing network pipelines.
c) Replacing existing network pipelines with either:
a. New network pipelines or
b. An energy centre.
If following a risk assessment and cost benefit assessment none of the above options are considered suitable the remaining option would be to:
d) Decommission the existing gas installation and install a suitable alternative decarbonisation option (electrification heat pump heat network etc.).
This supplement covers the principles required for the repurposing renovating and replacement of existing gas installations for Hydrogen service and the decommissioning of existing gas installations.
This supplement provides the principles required to identify buildings and gas installations that are not suitable for repurposing and outlines remedial work that may be required prior to repurposing.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Inclusive Innovation: Safeguarding the Switch to Domestic Hydrogen
Mar 2026
Publication
The UK Government is exploring the possibility of switching some homes from natural gas to hydrogen to help the country reach net zero carbon emissions. Wales & West Utilities commissioned the Energy Systems Catapult to run a project to understand how to enable this transition to work well for vulnerable consumers if it is undertaken.
To do this the Energy Systems Catapult engaged with a series of industry experts and reviewed the available evidence. This helped to understand what a transition of domestic properties from natural gas to hydrogen is likely to entail for consumers and what steps should be considered to enable the transition to go well. Once the process was defined we then engaged with consumers in a variety of vulnerable situations to discuss any challenges the proposed process may present to them. In understanding these challenges the project then sought to further engage with both a range of experts and vulnerable consumers to co-create appropriate packages of support to preserve wellbeing during a transition to hydrogen.
The project was able to successfully map out the key consumer touch points during a transition and identify what demands could be placed on consumers. This enabled appropriate packages of support to be designed that should be considered when seeking to support affected residents during each of the following eventualities - that could occur during a transition to domestic hydrogen.
• Enabling a surveyor to access and conduct a survey at the property prior to the conversion
• What to do if an unsafe gas appliance is discovered during the property survey and must be disconnected until it can be repaired or replaced
• How best to capture important information about the personal circumstances of those who live in the home (e.g. health finances) to understand how best to support them during the conversion
• Occupant expectation if a gas appliance in the home is deemed incompatible with hydrogen so cannot be adapted and must be replaced
• What support is needed if an occupant does not have gas for cooking heating or hot water for a few days
• How best to navigate and safeguard residents if more complex works at required at the property e.g. upgrading the existing pipework
Many concerns were identified when discussing the different consumer touch points. These included issues with increased anxiety the impact of any disruption to important daily routines a range of potential cost implications for residents and the heightened risk of accident or injury due to a variety of impairments. For each consumer touch point three tiers of support were co-created in order to mitigate these issues. The lower tier being offered to everyone with the other tiers used for people with more specific needs. The support offered included the provision of temporary equipment help to access local community support involving 3rd parties to support consumers at certain points and in some instances the offer to take the occupant away from the property whilst the work is ongoing.
The project was further able to establish some of the challenges that may arise if seeking to deploy hydrogen on a larger scale across the UK as well as identifying several key factors that should be considered in parallel to the conversion to ensure it goes as smoothly as possible.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
To do this the Energy Systems Catapult engaged with a series of industry experts and reviewed the available evidence. This helped to understand what a transition of domestic properties from natural gas to hydrogen is likely to entail for consumers and what steps should be considered to enable the transition to go well. Once the process was defined we then engaged with consumers in a variety of vulnerable situations to discuss any challenges the proposed process may present to them. In understanding these challenges the project then sought to further engage with both a range of experts and vulnerable consumers to co-create appropriate packages of support to preserve wellbeing during a transition to hydrogen.
The project was able to successfully map out the key consumer touch points during a transition and identify what demands could be placed on consumers. This enabled appropriate packages of support to be designed that should be considered when seeking to support affected residents during each of the following eventualities - that could occur during a transition to domestic hydrogen.
• Enabling a surveyor to access and conduct a survey at the property prior to the conversion
• What to do if an unsafe gas appliance is discovered during the property survey and must be disconnected until it can be repaired or replaced
• How best to capture important information about the personal circumstances of those who live in the home (e.g. health finances) to understand how best to support them during the conversion
• Occupant expectation if a gas appliance in the home is deemed incompatible with hydrogen so cannot be adapted and must be replaced
• What support is needed if an occupant does not have gas for cooking heating or hot water for a few days
• How best to navigate and safeguard residents if more complex works at required at the property e.g. upgrading the existing pipework
Many concerns were identified when discussing the different consumer touch points. These included issues with increased anxiety the impact of any disruption to important daily routines a range of potential cost implications for residents and the heightened risk of accident or injury due to a variety of impairments. For each consumer touch point three tiers of support were co-created in order to mitigate these issues. The lower tier being offered to everyone with the other tiers used for people with more specific needs. The support offered included the provision of temporary equipment help to access local community support involving 3rd parties to support consumers at certain points and in some instances the offer to take the occupant away from the property whilst the work is ongoing.
The project was further able to establish some of the challenges that may arise if seeking to deploy hydrogen on a larger scale across the UK as well as identifying several key factors that should be considered in parallel to the conversion to ensure it goes as smoothly as possible.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Kiwa Energy Hazardous Areas Report
Mar 2026
Publication
As part of the UK’s transition towards net zero carbon emissions by 2050 the possibility of using hydrogen to replace natural gas for heating domestic and small commercial properties is being investigated. One of the areas that needs to be considered is whether the conversion to hydrogen would bring about new issues regarding hazardous areas.
This report presents the results of a literature review which looks at the existing natural gas standards and hazardous area procedures. A comparison is then made to relevant work considering hydrogen including review of the IGEM SR/25 Hydrogen Supplement published by the HSE. Discussion and recommendations are made as to whether existing industry guidance continues to be suitable if the gas being used is hydrogen and not natural gas.
The focus of this work is on domestic and small commercial installations (e.g. single units with simple boiler and hot water installations) operating at gas pressures below 25 mbarg. It is important to note however that most available guidance and standards are for larger commercial environments. Although these are not directly applicable to domestic installations they have been considered as a reference point particularly for estimation of potential leak sizes.
Zoning of hazardous areas is not applicable in a domestic setting. However separation distances between electrical and gas installations within a domestic property are advised within BS 6891. Should a property be converted to hydrogen no changes are recommended to these distances. This is because in a low-pressure domestic situation fires do occasionally occur with copper pipe rubbing on electric cables but as both natural gas and hydrogen will ignite immediately the risk from the leak is essentially independent of the gas being carried. On many (probably most) occasions causing secondary fires. It is also advised that pliable meter connections should be wrapped to reduce the risk of fire from accidental electrical contact and this will be included within PAS4441.
Any workplace (including the small commercial properties discussed within) that has a natural gasinstallation and where explosive atmospheres may occur is required under DSEAR to classify hazardous andnon-hazardous areas within the facility. IGEM SR/25 and UP/16 are available to aid installers to complete hazardous area classification and design safe gas installations. It is recommended that UP/16 should be updated for hydrogen and this is currently underway (April 2023).
From experimental findings at low gas pressures it has been shown that the flammable zone for hydrogen in a vertical direction is 130% that of natural gas. However the values proposed within the SR/25 hydrogen supplement give flammable zones that are between 200% and 350% greater at low pressure. It may be that these are driven by an assumption that general deflagration can occur at the hydrogen published lower explosive limit (LEL) of 4%. In practice such deflagration does not occur below about 6% and does not propagate downwards until 8%. The implications of the move from wind driven ventilation (for natural gas) to buoyancy driven ventilation (for hydrogen) also needs thoroughly exploring. It could be argued (for low and medium pressure releases) that the ‘motive force’ provided by the low density of hydrogen is more reliable at clearing hydrogen concentrations than the wind-driven ventilation used for natural gas. In terms of general compliance with DSEAR acknowledgement should be made that Hy4Heat recommends the use of hydrogen detection and AIV’s in properties with a demand in excess of 20m3/h.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This report presents the results of a literature review which looks at the existing natural gas standards and hazardous area procedures. A comparison is then made to relevant work considering hydrogen including review of the IGEM SR/25 Hydrogen Supplement published by the HSE. Discussion and recommendations are made as to whether existing industry guidance continues to be suitable if the gas being used is hydrogen and not natural gas.
The focus of this work is on domestic and small commercial installations (e.g. single units with simple boiler and hot water installations) operating at gas pressures below 25 mbarg. It is important to note however that most available guidance and standards are for larger commercial environments. Although these are not directly applicable to domestic installations they have been considered as a reference point particularly for estimation of potential leak sizes.
Zoning of hazardous areas is not applicable in a domestic setting. However separation distances between electrical and gas installations within a domestic property are advised within BS 6891. Should a property be converted to hydrogen no changes are recommended to these distances. This is because in a low-pressure domestic situation fires do occasionally occur with copper pipe rubbing on electric cables but as both natural gas and hydrogen will ignite immediately the risk from the leak is essentially independent of the gas being carried. On many (probably most) occasions causing secondary fires. It is also advised that pliable meter connections should be wrapped to reduce the risk of fire from accidental electrical contact and this will be included within PAS4441.
Any workplace (including the small commercial properties discussed within) that has a natural gasinstallation and where explosive atmospheres may occur is required under DSEAR to classify hazardous andnon-hazardous areas within the facility. IGEM SR/25 and UP/16 are available to aid installers to complete hazardous area classification and design safe gas installations. It is recommended that UP/16 should be updated for hydrogen and this is currently underway (April 2023).
From experimental findings at low gas pressures it has been shown that the flammable zone for hydrogen in a vertical direction is 130% that of natural gas. However the values proposed within the SR/25 hydrogen supplement give flammable zones that are between 200% and 350% greater at low pressure. It may be that these are driven by an assumption that general deflagration can occur at the hydrogen published lower explosive limit (LEL) of 4%. In practice such deflagration does not occur below about 6% and does not propagate downwards until 8%. The implications of the move from wind driven ventilation (for natural gas) to buoyancy driven ventilation (for hydrogen) also needs thoroughly exploring. It could be argued (for low and medium pressure releases) that the ‘motive force’ provided by the low density of hydrogen is more reliable at clearing hydrogen concentrations than the wind-driven ventilation used for natural gas. In terms of general compliance with DSEAR acknowledgement should be made that Hy4Heat recommends the use of hydrogen detection and AIV’s in properties with a demand in excess of 20m3/h.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
MOBs Phase 3: Task 10 - Pressure Testing
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project ‘MOBs Work Pack 2 Asset Information Review’ considered the requirements for pressure testing commissioning and decommissioning of MOBs following a conversion to Hydrogen and identified the following gap in technical evidence.
A desktop study and testing are required to confirm the applicability of test pressures test criteria and purge velocity with hydrogen to MOBs and the suitability of existing equipment for purging operation in MOBs. SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen including any testing required to address any evidence gaps.
This report focuses on the pressure testing requirements associated with the conversion of MOBs from Natural Gas to Hydrogen. The purging aspects of evidence gap are being addressed by Steer Energy as part of a separate task. It was proposed that ROSEN review the gas leakage and dispersion modelling of pure Hydrogen undertaken as part of the H21 H100 HyDeploy and Hy4Heat projects to determine the maximum permitted leak rate / pass criterion for pressure testing of network pipelines intended for Hydrogen operation.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
A desktop study and testing are required to confirm the applicability of test pressures test criteria and purge velocity with hydrogen to MOBs and the suitability of existing equipment for purging operation in MOBs. SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen including any testing required to address any evidence gaps.
This report focuses on the pressure testing requirements associated with the conversion of MOBs from Natural Gas to Hydrogen. The purging aspects of evidence gap are being addressed by Steer Energy as part of a separate task. It was proposed that ROSEN review the gas leakage and dispersion modelling of pure Hydrogen undertaken as part of the H21 H100 HyDeploy and Hy4Heat projects to determine the maximum permitted leak rate / pass criterion for pressure testing of network pipelines intended for Hydrogen operation.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
MOBs Phase 3: Task 7 - Combined Effect of Hydrogen and Thermal Loading on Material Integrity
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). SGN is leading a feasibility project with some applied testing to understand the steps needed to convert to Hydrogen. This report is part of Work Pack 3 and summarises Task 7: combined effect of hydrogen and thermal loading on material integrity.
The aim of this task is to fill the following evidence gaps identified in ROSEN report entitled ‘16357-1 Document Landscape Review Report Issue 1-0’:
♦ Validity of current diameter height lateral length and material limitations and permitted jointing methods.
♦ Susceptibility of low strength steel to hydrogen cracking when subjected to stresses resulting from expansion and contraction and effect of hydrogen on likelihood of failure of risers which do not have the required allowance for expansion and contraction.
♦ Applicability of existing thresholds including minimum permitted wall thickness before isolation and corrosion damage categories for pipe designed to operate at stress levels not greater than 40% SMYS with hydrogen.
Finite Element Analysis (FEA) has been performed to assess the performance of existing carbon steel gas riser configurations when subjected to thermal loading to understand the suitability of converting the existing pipework to hydrogen.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
The aim of this task is to fill the following evidence gaps identified in ROSEN report entitled ‘16357-1 Document Landscape Review Report Issue 1-0’:
♦ Validity of current diameter height lateral length and material limitations and permitted jointing methods.
♦ Susceptibility of low strength steel to hydrogen cracking when subjected to stresses resulting from expansion and contraction and effect of hydrogen on likelihood of failure of risers which do not have the required allowance for expansion and contraction.
♦ Applicability of existing thresholds including minimum permitted wall thickness before isolation and corrosion damage categories for pipe designed to operate at stress levels not greater than 40% SMYS with hydrogen.
Finite Element Analysis (FEA) has been performed to assess the performance of existing carbon steel gas riser configurations when subjected to thermal loading to understand the suitability of converting the existing pipework to hydrogen.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
MOBs Phase 3: Task 9 - Fire Protection Measures
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas (NG) to Hydrogen in multi-occupancy buildings (MOBs). SGN is leading a feasibility project with some applied testing to understand the steps needed to convert to Hydrogen.
This report is for work pack 3 Task 9: Fire Protection Measures. The objective of this task is to review the applicability of current requirements for fittings and components relating to the permissible leakage rate during and after a high temperature event where Hydrogen is the gas being distributed.
The objectives of this task are to:
• Confirm the applicability of the test temperature of 650°C (currently specified in BS EN 1775 Procedure B and which corresponds to the self-ignition temperature of a Natural Gas / air mixture) with Hydrogen self-ignition temperature.
• Confirm the applicability of the specified leakage rate of 150 dm3/h for pipework components during and after a test in accordance with the requirements of BS EN 1775 Procedure A.
• Confirm the applicability of the specified leakage rate of 150 dm3/h for pipework components following a resistance to high temperature test in accordance with the requirements of BS EN 1775 Procedure B.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
This report is for work pack 3 Task 9: Fire Protection Measures. The objective of this task is to review the applicability of current requirements for fittings and components relating to the permissible leakage rate during and after a high temperature event where Hydrogen is the gas being distributed.
The objectives of this task are to:
• Confirm the applicability of the test temperature of 650°C (currently specified in BS EN 1775 Procedure B and which corresponds to the self-ignition temperature of a Natural Gas / air mixture) with Hydrogen self-ignition temperature.
• Confirm the applicability of the specified leakage rate of 150 dm3/h for pipework components during and after a test in accordance with the requirements of BS EN 1775 Procedure A.
• Confirm the applicability of the specified leakage rate of 150 dm3/h for pipework components following a resistance to high temperature test in accordance with the requirements of BS EN 1775 Procedure B.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
MOBs Phase 3: Task 8 - Electrical Safety Report
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from natural gas to hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project “MOBs Work Pack 2 Asset Information Review” [1] considered the requirements for domestic electrical safety in MOBs following a conversion to hydrogen and identified gaps in technical evidence.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen. Task 8 relates to potential electrical safety requirements associated with the conversion of MOBs from natural gas to hydrogen. It was proposed that ROSEN review the projects “EUSE – Hazardous Areas Within Buildings” [2] and “ATEX Equipment & SR/25 Modification Assessment” [3] to confirm their applicability to MOBs. The objectives of this review are to:
1. Confirm whether the standards to which electrical equipment is currently specified relates to natural gas only or to any flammable gas
2. Determine whether requirements for electrical equipment are impacted by hydrogen being gas group IIC versus gas group IIA for natural gas
3. Investigate whether the existing separation distance between natural gas pipes/meter and electrical equipment remains the same with hydrogen.
ROSEN has reviewed the outputs from these research projects as well as relevant standards to determine electrical safety requirements for the conversion of MOBs from natural gas to hydrogen.
The review of standards and research projects undertaken has drawn the following conclusions in regards to electrical safety.
1. To comply with the requirements of DSEAR meter banks energy centres and common areas within MOBs must be risk assessed to determine the location and extent of explosive atmospheres and their classification once repurposed for use with hydrogen. The risk assessment includes hazardous area classification of the pipework and components.
2. For pure hydrogen the necessary air change rate per hour to allow classification as Zone 2 NE is increased from 0.5 (for natural gas) to 1.5 air changes per hour (ACH). Where this cannot be achieved uncertified electric lighting and other electrical equipment will need to be relocated or replaced with certified gas group IIC equipment.
3. There will be no requirement for a hazardous area classification for hydrogen in a domestic environment (individual dwellings) as DSEAR does not apply.
4. The minimum separation distances between electrical equipment and gas equipment are independent of the gas being transported and research projects have concluded that these do not need to change.
5. Lightning protection requirements are independent of the gas being transported and are in place to protect the pipe structure from damage and existing IGEM/G/5 guidance can remain unchanged.
6. Research indicates that most incidents relating to electrical safety are due to non-compliance with current standards.
7. Any electrical equipment located in the vicinity of gas installations should be assessed for compliance by a competent person prior to the conversion of MOBs to hydrogen.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen. Task 8 relates to potential electrical safety requirements associated with the conversion of MOBs from natural gas to hydrogen. It was proposed that ROSEN review the projects “EUSE – Hazardous Areas Within Buildings” [2] and “ATEX Equipment & SR/25 Modification Assessment” [3] to confirm their applicability to MOBs. The objectives of this review are to:
1. Confirm whether the standards to which electrical equipment is currently specified relates to natural gas only or to any flammable gas
2. Determine whether requirements for electrical equipment are impacted by hydrogen being gas group IIC versus gas group IIA for natural gas
3. Investigate whether the existing separation distance between natural gas pipes/meter and electrical equipment remains the same with hydrogen.
ROSEN has reviewed the outputs from these research projects as well as relevant standards to determine electrical safety requirements for the conversion of MOBs from natural gas to hydrogen.
The review of standards and research projects undertaken has drawn the following conclusions in regards to electrical safety.
1. To comply with the requirements of DSEAR meter banks energy centres and common areas within MOBs must be risk assessed to determine the location and extent of explosive atmospheres and their classification once repurposed for use with hydrogen. The risk assessment includes hazardous area classification of the pipework and components.
2. For pure hydrogen the necessary air change rate per hour to allow classification as Zone 2 NE is increased from 0.5 (for natural gas) to 1.5 air changes per hour (ACH). Where this cannot be achieved uncertified electric lighting and other electrical equipment will need to be relocated or replaced with certified gas group IIC equipment.
3. There will be no requirement for a hazardous area classification for hydrogen in a domestic environment (individual dwellings) as DSEAR does not apply.
4. The minimum separation distances between electrical equipment and gas equipment are independent of the gas being transported and research projects have concluded that these do not need to change.
5. Lightning protection requirements are independent of the gas being transported and are in place to protect the pipe structure from damage and existing IGEM/G/5 guidance can remain unchanged.
6. Research indicates that most incidents relating to electrical safety are due to non-compliance with current standards.
7. Any electrical equipment located in the vicinity of gas installations should be assessed for compliance by a competent person prior to the conversion of MOBs to hydrogen.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Great Britain Distribution QRA Risk Predictions
Mar 2026
Publication
The natural gas distribution system has operated safely across Great Britain for around 50 years. It is an established means of supplying fuel for heating and cooking to residential dwellings and non-domestic buildings and is also used in some industrial processes. In order to meet environmental targets set by the UK Government multiple gas industry projects have been completed or are in progress to investigate the conversion of the existing natural gas system to carry hydrogen. This would make use of a valuable national asset and would help to meet carbon emissions targets.
As part of this programme of work across the gas industry DNV has carried out a Quantitative Risk Assessment (QRA) of the gas distribution system across Great Britain. Note that the ‘distribution system’ in this report covers a wider scope than the common meaning of the ‘distribution network’. Here the system includes the gas distribution network operating at up to 7 bar upstream of the Emergency Control Valve (ECV) and gas use within customer properties downstream of the ECV. The primary aim of the assessment is to quantify the risks posed by a repurposed hydrogen system and the benefits of risk mitigation measures where necessary. To put these results in context the risk associated with the equivalent natural gas distribution system is also quantified.
This report contains the results of the risk predictions. Separate reports gives details of the following:
• A DNV report describes the methodology reviews and modelling updates that were carried out as part of this QRA of the Great Britain distribution system.
• A DNV report describes a separate project carried out to investigate the risks associated with hydrogen use in Multi Occupancy Buildings (MOBs).
• A report produced by the Gas Distribution Networks (GDNs) evaluates each potential risk mitigation measure and justifies their adoption or rejection.
The risk predictions cover the Great Britain gas distribution system and end use within buildings for heating and cooking. The study does not consider hydrogen production or storage gas transmission at pressures above 7 bar industrial use of gas or vehicle refuelling stations.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating:HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
As part of this programme of work across the gas industry DNV has carried out a Quantitative Risk Assessment (QRA) of the gas distribution system across Great Britain. Note that the ‘distribution system’ in this report covers a wider scope than the common meaning of the ‘distribution network’. Here the system includes the gas distribution network operating at up to 7 bar upstream of the Emergency Control Valve (ECV) and gas use within customer properties downstream of the ECV. The primary aim of the assessment is to quantify the risks posed by a repurposed hydrogen system and the benefits of risk mitigation measures where necessary. To put these results in context the risk associated with the equivalent natural gas distribution system is also quantified.
This report contains the results of the risk predictions. Separate reports gives details of the following:
• A DNV report describes the methodology reviews and modelling updates that were carried out as part of this QRA of the Great Britain distribution system.
• A DNV report describes a separate project carried out to investigate the risks associated with hydrogen use in Multi Occupancy Buildings (MOBs).
• A report produced by the Gas Distribution Networks (GDNs) evaluates each potential risk mitigation measure and justifies their adoption or rejection.
The risk predictions cover the Great Britain gas distribution system and end use within buildings for heating and cooking. The study does not consider hydrogen production or storage gas transmission at pressures above 7 bar industrial use of gas or vehicle refuelling stations.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating:HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Great Britain Distribution QRA Methodology Updates in CONIFER
Mar 2026
Publication
The natural gas distribution system has operated safely across Great Britain for around 50 years. It is an established means of supplying fuel for heating and cooking to residential dwellings and non-domestic buildings and is also used in some industrial processes. In order to meet environmental targets set by the UK Government multiple gas industry projects have been completed or are in progress to investigate the conversion of the existing natural gas system to carry hydrogen. This would make use of a valuable national asset and would help to meet carbon emissions targets.
As part of this programme of work across the gas industry DNV has carried out a Quantitative Risk Assessment (QRA) of the gas distribution system across Great Britain. Note that the ‘distribution system’ in this report covers a wider scope than the common meaning of the ‘distribution network’. Here the system includes the gas distribution network upstream of the Emergency Control Valve (ECV) and gas use within customer properties downstream of the ECV. The primary aim of the assessment is to quantify the risks posed by a repurposed hydrogen system and the benefits of risk mitigation measures where necessary. To put these results in context the risk associated with the equivalent natural gas distribution system is also quantified.
This work has been carried out in partnership with Cadent Northern Gas Networks and SGN. Additional information has been supplied by Wales and West Utilities and several of the Independent Gas Transporters thereby covering almost all the Great Britain distribution system.
This report contains the details of methodology reviews and modelling updates that were carried out as part of the Great Britain distribution system QRA. A separate report details the risk predictions. The updates include some made in response to review by the Evidence Review Group within the HSE and some that take make use of outputs from other gas industry projects. Major updates made as part of this work include:
• Introduction of non-domestic buildings and Multi Occupancy Buildings (MOBs) into the risk predictions. This report is primarily concerned with defining their characteristics and modifying the risk assessment models to represent their features that are not found in houses.
• Updates to the failure frequencies applied to all parts of the system from governor kiosks through the distribution network to end use.
• Modifications to the model that predicts gas movement through the ground following a leak from a main or service.
• Changes to the methodology used to predict gas dispersion movement and accumulation within a building.
• Updates to the treatment of gas explosions inside buildings.
• Several other minor changes and investigations into the sensitivity of various inputs.
This report also contains the risk calculation methodology for governors and benchmarking of the risk predictions against natural gas operational experience.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
As part of this programme of work across the gas industry DNV has carried out a Quantitative Risk Assessment (QRA) of the gas distribution system across Great Britain. Note that the ‘distribution system’ in this report covers a wider scope than the common meaning of the ‘distribution network’. Here the system includes the gas distribution network upstream of the Emergency Control Valve (ECV) and gas use within customer properties downstream of the ECV. The primary aim of the assessment is to quantify the risks posed by a repurposed hydrogen system and the benefits of risk mitigation measures where necessary. To put these results in context the risk associated with the equivalent natural gas distribution system is also quantified.
This work has been carried out in partnership with Cadent Northern Gas Networks and SGN. Additional information has been supplied by Wales and West Utilities and several of the Independent Gas Transporters thereby covering almost all the Great Britain distribution system.
This report contains the details of methodology reviews and modelling updates that were carried out as part of the Great Britain distribution system QRA. A separate report details the risk predictions. The updates include some made in response to review by the Evidence Review Group within the HSE and some that take make use of outputs from other gas industry projects. Major updates made as part of this work include:
• Introduction of non-domestic buildings and Multi Occupancy Buildings (MOBs) into the risk predictions. This report is primarily concerned with defining their characteristics and modifying the risk assessment models to represent their features that are not found in houses.
• Updates to the failure frequencies applied to all parts of the system from governor kiosks through the distribution network to end use.
• Modifications to the model that predicts gas movement through the ground following a leak from a main or service.
• Changes to the methodology used to predict gas dispersion movement and accumulation within a building.
• Updates to the treatment of gas explosions inside buildings.
• Several other minor changes and investigations into the sensitivity of various inputs.
This report also contains the risk calculation methodology for governors and benchmarking of the risk predictions against natural gas operational experience.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
H21 Phase 2: Ignition Considerations for Network Procedures
Mar 2026
Publication
The objective of the H21 programme is to reach the point whereby it is feasible to convert the existing natural gas (NG) distribution network to 100% hydrogen (H₂) and provide a contribution to decarbonising GB’s heat and power sectors with the focus on decarbonised fuel at point of use. This report is one of a series of reports that look to address the work areas previously identified following a review of network procedures. It looks at the difference between the ignition sensitivities of natural gas and hydrogen primarily regarding electrostatics and friction and how they impact on network procedures. Equipment currently used by NGN was reviewed for its suitability for use with hydrogen. A number of further precautions have been identified for hydrogen and the main findings are summarised below.
Ignition topics that should not require alteration with hydrogen.
The LEL based criteria (20% LEL for the use of non-ATEX equipment when deciding upon evacuation and other safety precautions 70% LEL for switching off electrics under emergency conditions and 5% LEL and falling for reoccupation of buildings) remain fit for purpose with hydrogen; however these do require accurate hydrogen gas detection. Damp rag earthing of PE pipe was demonstrated experimentally to be effective in preventing electrostatic discharges and is suitable for use with hydrogen. Protection against electrostatic discharges extends 10 cm beyond the damp rag. The continuity bonds used for natural gas remain fit for use with hydrogen. The cathodic protection remains suitable for hydrogen from an electrostatics point of view although the location of impressed current cathodic protection (ICCP) systems should be in Zone 2 NE (Negligible Extent) areas. There is little change in the risk of pyrophoric ignition and electrostatic discharge remains the greatest ignition risk with “black dust”.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
Ignition topics that should not require alteration with hydrogen.
The LEL based criteria (20% LEL for the use of non-ATEX equipment when deciding upon evacuation and other safety precautions 70% LEL for switching off electrics under emergency conditions and 5% LEL and falling for reoccupation of buildings) remain fit for purpose with hydrogen; however these do require accurate hydrogen gas detection. Damp rag earthing of PE pipe was demonstrated experimentally to be effective in preventing electrostatic discharges and is suitable for use with hydrogen. Protection against electrostatic discharges extends 10 cm beyond the damp rag. The continuity bonds used for natural gas remain fit for use with hydrogen. The cathodic protection remains suitable for hydrogen from an electrostatics point of view although the location of impressed current cathodic protection (ICCP) systems should be in Zone 2 NE (Negligible Extent) areas. There is little change in the risk of pyrophoric ignition and electrostatic discharge remains the greatest ignition risk with “black dust”.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.
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