Decarbonising an economy is a big job. Coal- and gas-fired power plants must be swapped for wind, solar or nuclear ones. Petrol-driven cars must be replaced by electric versions. Less attention is paid to heating. But in cold countries such as Britain, warming houses, offices and the like consumes more fossil fuel than either electricity generation or transport.

The fuel involved is usually natural gas. This is burned in a central boiler in order to heat water that flows to radiators elsewhere in the building. Britain's government would like to change this. From 2025 gas-fired boilers will be banned in newly built homes. By the mid-2030s installing new gas boilers in existing houses will be banned, too...