The UK’s September mini heatwave may have come a little late this year, but there is still time to renew the usual debate about air conditioning – what impact it has on the climate, whether the electricity grid will be overloaded by people pumping cool air into their homes and so on. But what this debate often overlooks is the role air conditioning units – which are actually a type of heat pump – might play in reducing carbon emissions from home heating. In some homes, they could replace gas boilers altogether, removing one of the UK’s biggest sources of carbon emissions in doing so.
The white boxes that we call air conditioning units are, in fact, usually a kind of heat pump. More accurately known as an air-to-air heat pump, these machines can be set to temperatures anywhere between the mid-teens to over 30 degrees Celsius. They can heat as well as cool. Crucially, they also enjoy the energy efficiency bonus common to all heat pumps, typically generating at least twice as much heat as the energy they consume. That makes these air-to-air heat pumps good candidates for replacing boilers.
But these air-to-air heat pumps are surprisingly rare in UK homes. Most British homes have central heating, which means using water, rather than air, to heat a home. The water is typically heated by a boiler and sent through pipes and radiators around the home. The UK’s plan to decarbonise home heating is to replace these boilers with heat pumps, while keeping the central heating system intact.
This kind of wet heating system is popular – adverts for homes often include the phrase “gas central heating” as a selling point. But central heating – with a network of pipes and radiators around the home – is generally a more complicated system, and is therefore more expensive to switch over to a heat pump. Air-to-air systems, by contrast, are often more straightforward, in some cases just requiring a few units on the wall.
This complexity is reflected in the cost of getting a heat pump. Switching to an air-to-water heat pump system – which is the most common kind in the UK – typically costs upwards of £10,000 (although subsidies are available to cover some of this cost). Air-to-air heat pumps, by contrast, typically cost around £2,000 to install per unit. In a small home or flat, which may only require one or two units, this can be a significant cost saving. The central heating variety of heat pump is often more efficient – reducing energy bills – but usually not by enough to offset the higher upfront cost.
On top of that, air-to-air heat pumps can cool homes as well as they heat them. Water-based heat pumps can also cool homes in principle, but are much less commonly set up for this purpose. In a world where heat waves happen more often – especially in cities, where flats are more common – joint heating and cooling could be a very attractive proposition.
One drawback of air con heat pumps is that they don’t provide hot water. But this downside is relatively easy to address via a separate direct electric heating system or alternative system. And because we usually use far less energy for hot water than to heat our homes, having a less efficient system for hot water isn’t such a problem.
Air-to-air heat pumps are not a niche technology. They outnumber water-based systems by around 4 to 1 in Europe. In Scandinavia, often seen as the holy grail of heat pump adoption, air-to-air systems dominate. Even in the UK, they are extremely common in commercial premises – there’s a high chance your office or local supermarket is heated by one. The UK has led the way on commercial air-to-air heat pumps since the 1990s.
There are around 50,000 engineers with the F-gas accreditation required to install air-to-air heat pumps, compared to around 3,000-4,000 engineers able to install air-to-water heat pumps. Expanding the workforce on heat pumps is already proving a significant challenge, and drawing on a much wider pool of experienced engineers could help expand the industry more quickly.
Despite all of this, the UK government remains focused almost exclusively on central heating-based heat pumps in homes. Air-to-air heat pumps are barely mentioned in the government’s strategies around low-carbon heating. There are no subsidies available for installing them, and they are excluded from the proposed Clean Heat Market Mechanism, which will oblige boiler manufacturers to install heat pumps. While a lot of this policy makes sense – most UK houses will likely stick with central heating in future – a little more flexibility on other kinds of heat pump might ease the transition.
The fight against climate change is full of technology choices. On home heating, the heat pump finally seems to have won the argument, but perhaps the debate on what type of heat pump has been shut down too early. Brits are unlikely to give up their central heating systems en masse, but being a little more open to blowing hot air around our homes could help us stop blowing carbon into the atmosphere.
Andrew Sissons, Nesta