Why are they installing new boilers?

Here’s some news that is gonna cause some shitty takes, whether you’re passionate about decarbonisation or a “heat pumps don’t work” denier: for larger buildings, it’s increasingly common to install gas boilers alongside heat pumps (ASHPs) for primary space and water heating, especially when retrofitting ASHPs into a building that already had a gas boiler — even if it means replacing an old boiler with a brand-new one! Depending on your priors, you might think that’s evidence that ① ASHPs don’t work, or ② building owners don’t care about climate change, or ③ Big Oil is desperate to keep people buying fossil fuels. But why are they doing this?

Boilers have limited turndown, and they’re very inefficient when turned on and off quickly (short-cycling). That’s why we tend to buffer their heat output into a tank of hot water or oil, and then circulate that (through radiators) for a long time while the boiler is off.

ASHPs prefer to run continuously, so we don’t always pair them with a thermal store, but they still only achieve their max efficiency at full power, and short-cycling is bad for them too.

When you design a building, you have to size the heating system for the coldest day of the year. In southern England, you might be designing for a 25–30 ℃ temperature difference between inside and outside. Say you put in one big ASHP to supply that heat. But for all the spring and autumn, it might only be supplying a 10 degree temperature difference: it’s going to be running at half power for half the year, and that’s inefficient.

Now instead imagine you halve the capacity of the ASHP, but you put in a boiler (or keep your old boiler) that you’re only going to run for maybe 1 month per year. The gas boiler is burning fossil fuels, but for most of the year, your ASHP is working a lot more efficiently. Depending on the usage pattern and how much grid decarbonisation (i.e. reducing the carbon footprint of National Grid electricity) improves in future, this building might have a lower carbon footprint than the first one!

Why would you use gas boilers for the booster instead of just another ASHP? Well, some buildings do use multiple ASHPs to improve the available turndown — they can just turn one ASHP off completely when they don’t need it — but don’t forget that ASHPs are at their least efficient when the outside air is coldest. An ASHP that runs only in the middle of winter is never going to achieve its quoted seasonal coefficient of performance. Many of these buildings are retrofitting ASHPs into an existing system where there’s already plant space for a boiler, and keeping the existing system design makes the work a lot cheaper and reduces embodied carbon.