Methane leaks from boilers could be 100 times greater than estimated, research finds

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EMISSIONS of unburnt methane from domestic boilers could be more than 100 times greater than previous estimates, according to UK government research.

Laboratory testing by the Department of Energy Security and Net Zero (DESNZ) found that the 20.2m boilers in England could be releasing up to 66,500 t/y of unburnt methane into the atmosphere, amounting to 1.86 MtCO2e. Official figures for 2023 put methane leakage from domestic space and water heating at 600 t. DESNZ recommended the Inventory Agency, which compiles UK greenhouse gas emissions data, “should consider reviewing” their data and that “further research is merited”. 

In collaboration with Kiwa Energy, DESNZ measured unburnt methane emissions from five boilers over several central heating and hot water cycles. It then extrapolated the data according to average usage figures for all boilers in England, calculating a minimum of 16,000 t/y, amounting to 0.45 MtCO2e. 

DESNZ said boilers emit unburnt natural gas, predominantly methane, at startup and shutdown of a heating cycle, with virtually no emissions during ignition. Methane escapes at startup as the gas valve is opened and “a small pulse of unburnt gas is forced out of the vent prior to ignition”, the report said. There is then another short release through the vent immediately after the gas valve is closed on shutdown. 

The boiler that emitted the greatest amount of unburnt methane was a combination boiler with a cylindrical burner and electronic gas/air control, with a central heating power rating of 30 kW – the highest-rated of all five boilers DESNZ tested. The lowest emissions came from a similar boiler but with pneumatic gas/air control and with a power rating of 21 kW. Leakage from boilers that burned a fuel blend comprising 80% methane and 20% hydrogen was lower than boilers burning natural gas, although measurements were “limited”. 

DESNZ said the research addressed a gap in domestic emissions data that usually does not consider leaks from end-use appliances. “As a result, methane emissions from end users have been potentially underestimated or excluded”, the report said. Domestic boilers are already one of the largest contributors to UK emissions, accounting for 14% of the country’s total, but this does not include leaks of unburnt methane. 

The latest figures could also be an underestimate, as the testing did not accurately “replicate real life usage”, DESNZ said. Hot water operation typically released more methane when the boilers were started up from cold, but DESNZ’s tests were carried out in relatively quick succession, meaning they were starting fairly warm. “This might not reflect real-life operation, where hot water may be required infrequently and therefore more of the boiler starts will be cold starts”, they wrote.