18 CONTEXT 182 : DECEMBER 2024 of the energy in the gas will be lost in the flue gasses. This compares with a COP of four for a typical heat-pump (meaning that it produces four times as much thermal output as its electrical input). However, as air temperature varies widely, the COP for an air-source heat‑pump may fall to 1.5 on a frosty morning. Where the heat source is the ground or a body of water, the COP can remain closer to four. For retrofits, one disadvantage of heat-pump technology is that the temperature produced is typically around 40 degrees C, which is too low for a conventional radiator system. They work best with underfloor heating systems as the heat source is spread over a much wider surface area. For solid ground floors this can be both expensive to install and disruptive, but finned pipework may be used where the floors are of timber, achieving the same result with much less alteration. The running cost of an air-source heat‑pump is generally competitive with an oil-fired central heating system, but more expensive than gasfired systems, so uptake in towns and cities where gas is available remains low. Burning biomass such as wood-pellets rather than fossil fuels is another low-carbon option if the wood is grown sustainably and harvested locally, since carbon dioxide is captured and stored as the plants grow. Unlike a conventional boiler which can be turned on and off instantly on demand, solid fuel has to be burnt continually until the fuel has been spent, so a large thermal store is required such as a hot water tank. A wood-pellet stove also requires more maintenance than a conventional boiler, so biomass often works best when used to heat large buildings or small neighbourhoods. The finned, flexible pipework of this underfloor heating system is laid between the floor joists, providing a broad surface area that is ideally suited to the low temperatures of an air-source heat-pump. (Both photos: Tom DixonSmith, Ultra-Fin) Traditional cast iron radiators, including rare Victorian ones like this, can often be retained in a lowtemperature system if supplemented by radiators with a larger surface area.
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