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Are heat pumps the answer to cheaper, greener heating?

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If you’ve been paying attention to discussions about how to make your home ‘greener’ and reduce your carbon footprint, then you’ve probably heard about heat pumps, writes Jeremy Green.

They hold out the promise of heating your home more efficiently and with less fossil-fuel based energy. You might even be aware that there are grants of £5,000 available to help make them cheaper to install – the government’s Boiler Upgrade Scheme.

But what’s it like replacing your conventional gas boiler with a heat pump? Does it save you money on your energy bills? Is it really more efficient, and is it really greener? The answer is not entirely straightforward.

A heat pump transfers heat into your house, just like a fridge’s mechanism transfers heat outside the metal box that makes up your refrigerator. It distributes that heat around the house using heated water, pipes and radiators, just like a gas boiler. But it doesn’t make the water as hot as a gas boiler does, so it needs a bigger surface area to transfer the same amount of heat. You will probably need new, bigger radiators.

And you’ll also have to get used to a new way of running their heating, without radiators that feel very hot to the touch. The house is going to take longer to heat up. If you’ve got a well-insulated house that holds its heat that might not be too much of a challenge. If you’re in a Victorian semi with single-layer brick walls, an uninsulated floor and roof, and draughty single-glazed windows, then it’s going to feel colder.

The same goes for hot water. The heat pump will heat water in a tank, just like many conventional gas boilers. But there are no combi-style heat pumps, so you are going to need a hot water tank; sizing that tank, and deciding when you want to heat and use the water, is going to be important.

It all adds up to a need to learn how to operate the pump-based system, and getting used to some differences in how it’s going to work.

Most of the people interviewed were on the whole happy with their decision to install a heat pump. For some it was a principled decision: “We didn’t want to replace our gas boiler with another one…we couldn’t go on burning fossil fuels to heat our house.”

“We both work for climate charities, we had to do the right thing.”

Several mentioned the warm green glow that they felt now: “We feel virtuous…now we don’t need to feel quite so bad about doing other things, like flying for a holiday.”

But there was a consensus that the smoothness of the installation itself was not matched by the quality of advice about how to use the new heating system: “We had to spend a lot of time fiddling with settings and finding out the best way to heat our home.”

“You can’t just turn it on. Unlike a gas boiler it takes a long time to heat up the house, so you need to plan for when you want it to be warm, and you need to not let your house cool down.”

Will your heat pump save you money? By itself, probably not. From a physicist’s point of view heat pumps are very efficient, in that they use a relatively small amount of energy to transfer a greater amount of heat. But they use energy in the form of electricity, and mains electricity is much more expensive than mains gas – about three times the price.

Sometimes the heat pump’s efficiency is enough to cancel that difference out, but often it isn’t. It has to work harder when there’s less heat in the outside air – that is, when you are most likely to want heat – and so it uses more electricity.

Heat pumps are part of the government’s climate change strategy, aiming to replace domestic heating based on burning a fossil fuel (natural gas) at the point of use with heating based on electricity derived from renewable sources. A growing proportion of UK energy is based on renewables – 43 per cent in 2020. Electricity at least has the potential of being a low carbon form of energy, while burning fossil fuel gas can never be. That’s the reason why the government provides subsidies for the replacement of gas boilers with heat pumps.

But as with so much government policy on climate, there’s an absence of joined up thinking. Some of the price differential between electricity and gas is due to the tax regime, which bizarrely loads the cost of environmental initiatives onto the price of electricity rather than gas. A White Paper in 2020 considered whether to change this, but this looks increasingly unlikely in the face of the “cost of living crisis” and rising energy bills.

The government has failed to increase the size of the electricity generation grid and has even stopped electricity companies from doing it themselves, even though a grid with more capacity is an absolute requirement if a substantial number of houses move to electricity for their domestic heating. As things stand at the moment, if the drive to replace gas boilers with electric heat pumps was successful then the grid would fall over.

And though it’s clear that one of the biggest needs is for properly trained installers who can explain to their customers how to use their new heating systems, there’s been no money going into that. Nor has there been enough money going into insulation, though it’s hard to better Caroline Lucas MP’s comment that “putting heat pumps in an uninsulated house is like pouring hot water into a cracked teapot.”

If you had to sum up the heat pump story, it would be thus; a great technology with much potential that isn’t suitable for heating much of Britain’s housing stock without big changes, and won’t be much use in decarbonising our energy system without even bigger changes.

How heat pumps work

Home heat pumps work by transferring heat from one location to another, using a small amount of energy to move a large amount of heat. It is, of course, the same technology that’s used in your fridge, or in an air conditioning unit. So they can be an energy-efficient alternative to traditional heating and cooling systems – considering efficiency in terms of the energy used to deliver a given amount of heat.

There are two main types of home heat pumps: air-source and ground-source. Air-source heat pumps extract heat from the outside air, while ground-source heat pumps extract heat from the ground. Ground-source heat pumps are more efficient than air-source heat pumps, but they are also more expensive to install.

Heat pumps work by using a refrigerant, a substance that can absorb and release heat, to transfer heat from one place to another. In heating mode, the heat pump takes heat from the outside air or ground, and transfers it to the indoor air. It does the same in reverse when it’s cooling. It’s worth noting that the chemicals used as refrigerants are not at all good for the environment; the most common fluids used in heat pumps are hydrofluorocarbons, which have a global warming potential over 1,000 times that of CO2. R410A, the most commonly used, has a  global warming potential of 2,088 times that of CO2. So it’s really important that any heat pump system doesn’t leak, and that it is correctly installed and eventually correctly disposed of.

The main components of a system include the compressor, the evaporator, the condenser, and the expansion valve.

The compressor is the heart of the system and is responsible for compressing the refrigerant and moving it through the system.

The evaporator is where the refrigerant absorbs heat from the outside air or ground in heating mode, or from the indoor air in cooling mode.

The condenser releases heat to the outside air or ground in heating mode or absorbs heat from the refrigerant in cooling mode.

The expansion valve regulates the flow of refrigerant through the system.

Heat pumps are not a new technology. In the mid-19th century physicist Lord Kelvin (William Thomson – one of the six men featured on the frieze around the Centre for Science and Arts in Lansdown) first suggested the idea of a heat pump, but there weren’t commercially available products until the 20th century. Then, in the 1920s the first heat pump was developed by the Swedish inventor Baltzar von Platen and the Swiss engineer Carl Munters. They created a heat pump that used a closed-loop refrigeration cycle to extract heat from the air or water, and transfer it to a space for heating. During World War II the use of heat pumps became more widespread due to the shortage of fuel; by the 1940s, they were being used in industrial and commercial buildings, and for district heating in Sweden and Switzerland.

Domestic heat pumps had to wait until the 1950s, when the first residential heat pump was developed in the United States. But the first models were expensive and relatively inefficient, though they received a boost during the 1970s oil shock.

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