Heat pumps explained: Oxford Brookes professor on why they are the future of sustainable heating

Professor Rajat Gupta of Oxford Brookes University
Professor Rajat Gupta

As homeowners and businesses in the UK and globally are encouraged to adopt more energy-efficient and eco-friendly energy sources, including air source and ground source heat pumps, Rajat Gupta, Professor of Sustainable Architecture and Climate Change at Oxford Brookes University, explains how heat pumps work, their benefits, eligibility for installation, and running costs.

What are heat pumps and how do they work?
Heat pumps are devices that absorb naturally occurring heat from the surrounding environment, either the ground or the air, and then use electricity to convert it into heat and hot water for buildings, both commercial and residential. They work similarly to a fridge or freezer, but in reverse.
Heat pumps come in two different types, air source (ASHP) which are often visibly positioned on the outside of a building, and ground source (GSHP), the bores for which are located underground. Air source heat pumps are usually cheaper and easier to install than ground source heat pumps, which are typically used in larger rural properties or for groups of buildings/blocks.

Why are heat pumps more popular in other European countries? 
Low carbon heat pump take-up in the UK is among the lowest in Europe, possibly due to higher upfront cost even with the Government grant. The UK has begun to show signs of interest after the Government boosted its grant scheme by 50%. Norway has by far the highest penetration of heat pumps per capita in the world, at 60% of all households, and its Scandinavian neighbours are not far behind. There are over a million installed heat pump units in Sweden, where more than 40% of households have one. The popularity of heat pumps has been on the rise since the 1970s, when the global oil crisis triggered a rethink of energy policy in many countries without domestic supplies of oil and gas.
France has installed heat pumps at ten times the rate of the UK. The success in heat pump rollout is due to longstanding preference for electric heating over gas in the country, while the market benefited from “clear and decisive policy by the French Government”, according to Charlotte Lee, head of the UK’s Heat Pump Association [1]. The key success elements included strict new-build standards, heat pump grants, relatively low electricity prices, and a growing base of heat pump installers. In France, households can get a grant of up to €15,000 (£13,000) if they buy a ground source heat pump for an existing home, and up to €9,000 for an air source heat pump.

How sustainable/green are heat pumps?
Heat pumps use electricity to power their system, and with every single unit of electricity they consume they can generate up to three to four units of heat in return, making them more energy efficient than gas boilers.
Heat pumps do not need to be turned off manually. They operate continuously with low output temperature, automatically adjusting to outside weather and indoor temperatures, for example under warm weather conditions they will adjust accordingly.
Heat pumps, even if run 100% on non-renewable electricity, use about one-third less energy than a gas boiler to generate the same amount of heat. There are substantial amounts of renewables added to the grid in the UK every year, therefore heat pumps can be responsible for 75% less carbon emissions than a gas boiler over 15 years.

Are heat pumps more expensive to run than gas boilers?
Heat pumps can have similar running costs to gas boilers, even though electricity is more expensive per unit than gas. This is because heat pumps generate more heat than the amount of electricity used to power them, therefore they use three to four times less energy compared with a gas boiler. 
Additionally, many energy suppliers now offer specific electricity tariffs - which determines how a provider charges customers for energy use - that cater for heat pump owners. For example, this can include specific hours in a day where the price of an electricity unit falls below the average rate, giving home or business owners the opportunity to take advantage of the drop in price to power their heat pump at a cheaper price. 
Overall, heat pumps can deliver cost savings over a gas boiler if the system runs efficiently and because of the available grants and energy tariffs.

Are heat pumps suitable for all types of homes?
Heat pumps can be installed in the majority of new and existing UK homes, even those that suffer a bit of heat loss in the winter season. Improving the energy efficiency of such homes before heat pump installation can increase the efficiency of the heat pumps, reducing the heat loss and heating cost. This can involve upgrading loft and cavity wall insulation and installing double glazed windows.
Ground source heat pumps can also be installed in high-rise flats and tower blocks. Large heat pumps can fuel district heating networks connecting apartments.

Would I need to replace the entire system in my property?
Heat pumps are usually ideal for underfloor heating systems, but they work well with radiators as well. Since heat pumps function best at a slightly lower temperature than traditional boilers, it may be required to install a couple more radiators or replace them with slightly bigger radiators to maintain indoor temperature, without affecting the efficiency of heat pumps.

What is the average cost of installing a heat pump system?
With the Government £7,500 Boiler Upgrade Scheme grants [2], heat pump installation cost is comparable to a boiler, making it an affordable choice. Despite this, the installation cost of heat pumps can be up to four to five times higher than the cost of gas boilers without using such schemes.

Is it true that heat pumps will only work sufficiently in well insulated homes, while not working so efficiently in existing buildings?
Although heat pumps work more efficiently in well-insulated homes, they can also work well in homes with less insulation including existing dwellings, depending on the heating needs and property type. In less energy efficient homes, heat pumps may require larger radiators to work well, and may need to work longer to maintain the desired indoor temperature. 

Is it true that heat pumps don’t perform in cold climates, or when it is cold?
The majority of heat pumps have been deployed in buildings located in the coldest climates. Additionally, heat pumps still work well under cold weather although a hybrid system may be required for very cold temperatures below freezing.

Do I need a lot of outside space to accommodate a heat pump for my property?
While ground source heat pumps require a garden for the fluid-filled pipes to absorb heat from the ground, air source heat pumps need much less outside space due to being more compact. Air source heat pump units are about one-metre-tall and one and a half metres wide, and can be fitted to a wall or placed on the ground in an outdoor space. In other words, they require a space as big as two washing machines located side by side, with some space around it to allow a good flow of air.

Are heat pumps noisy?
If heat pumps are installed properly and in the right place, they usually generate little noise. Air source heat pumps emit some noise, slightly higher than a low whisper. Ground source heat pumps are usually quieter than air source heat pumps since they work without fans. The new generation of air source heat pumps are even quieter. If a heat pump starts to make loud noises, it is usually a sign that there is something wrong.

Rajat Gupta, Professor of Sustainable Architecture and Climate Change at Oxford Brookes University, is an expert in indoor air quality and sustainable building solutions, including heat pumps. He is also the Director of the Oxford Institute for Sustainable Development (OISD) and the Low Carbon Building Research Group.


[1] https://www.theguardian.com/environment/2023/dec/23/heat-pumps-are-hot-property-in-europe-does-britain-have-cold-feet
[2] https://www.gov.uk/apply-boiler-upgrade-scheme/what-you-can-get