What is a heat pump?
Transitioning from gas boilers to low-carbon heating systems such as heat pumps will be essential for the UK to reach its net zero goals. But despite this, most people in the UK have never seen a heat pump - with less than 1 percent using heat pumps to heat their homes. Heat pumps are more environmentally friendly than gas boilers as they produce no carbon emissions at the point of use. While you still need electricity to power a heat pump, UK grid electricity increasingly comes from renewable sources. When the grid is completely decarbonised, all electric heating, including heat pumps, will be zero carbon.
So how does a heat pump work?
An air source heat pump takes heat from the outside air and boosts it to a higher temperature using a compressor. It then transfers the heat to the heating system in your home.They work on the same principle as a fridge or freezer - moving heat from one place to another. Heat pumps use electricity to run but they are more than 100 percent efficient. This means they produce more heat energy in kilo-watt hours (kWh) than they use in electricity. This makes it an energy efficient way to warm your home. By contrast, gas boilers consume more energy than they put out into your home. Modern condensing gas boilers are labelled between 92-94 percent efficient. Boilers installed before 2005, when new regulations came in, are significantly less efficient. Air source heat pumps work even if the temperature is below freezing (zero degrees Celsius) because there is heat energy present in air at any temperature above absolute zero (-273 degrees Celsius).
Heat pump types
There are two main types: air source and ground source.
Air source is split into:
- air-to-water (most popular)
- air-to-air (very common in Europe where they're better known as an air conditioning unit)
Air source heat pumps work in different ways and are compatible with different types of heating systems.Ground source heat pumps harness heat stored underground to provide your home with space heating and hot water. They perform the same role as a gas, oil or LPG boiler in a central heating system but use ambient heat from the ground instead of burning fossil fuels.
Air source heat pump
An air source heat pump absorbs heat from the outside air and uses it to increase the temperature inside your home. The size depends on how much heat they'll need to generate for your home - the more heat (or the higher the temperature required), the bigger the heat pump. Air-to-water heat pumps (also know as hydronic heat pumps) take heat from the outside air and feed it into your wet central heating system. This type of heat pump can be a direct replacement for your gas or oil boiler. They're most suitable for homes with larger radiators or water underfloor heating because the heat they produce is cooler than that from a conventional gas or oil boiler. To be most effective, they need a large surface area (in simple terms, larger radiators) to release the heat. It is more straightforward to incorporate larger radiators or underfloor heating for a heat pump while you're extending your home or in a new-build property. It can also cost less than retrofitting underfloor heating later on. If you currently have a wet central heating system, an air-to-water heat pump will be easier to fit into your exisiting set-up than other types. They are more expensive than air-to-air heat pumps but you can get a government grant (the boiler upgrade scheme) for £7,500 towards your costs.
Air-to-air heat pump
An air-to-air heat pump takes heat from the outside air and feeds it into your home through fans. This type of heat pump is not a direct replacement for a gas or oil boiler as it doesn't heat radiators. It is an alternative to other types of electric heating (storage heaters or panel heaters) and could be a good choice for flats where an air-to-water system is not feasible or where you are hoping to heat (and cool) a small space.To heat multiple rooms, you need a warm-air circulation system to move the heat around your home through ducts. Otherwise, you need a separate unit for each room (similar to the way individual air conditioning units are often installed for each room in hotter countries). These systems cannot produce hot water so you'll need a separate immersion heater or other water-heating system. In the summer, an air-to-air heat pump can operate in reverse. In other words, you can use it like an air-conditioning unit to provide cool air for your home. Air-to-air heat pumps are cheaper than air-to-water ones but you will need to create a new circulation system in your home as they don't use a wet central heating system (radiators and pipes). They are not currently covered by the government's boiler upgrade scheme but it is being considered (June 2025).
Things to consider
When thinking about switching to a heat pump, take the following into account:
- Heat pumps are more environmentally friendly than gas boilers because they emit no carbon dioxide at the point of use. They run on electricity - if you get yours from the National Grid, more of this electricity is generated using renewable sources - such as solar and wind power - than ever before. Renewable energy's share of overall generation was 47.3 percent in 2023, up from 41.5 percent in 2022
- Heat pumps are highly efficient. They generate more heat than the electricity they consume. For every unit of energy it uses, a typical heat pump produces between 2.5 and 4 units of heat, making heat pumps around 250-400 percent efficient. Efficiency varies depending on system design, system sizing, heat loss and the weather, so it's essential that your heat pump is set up well to get these high returns
- You could save money on heating compared to using a gas boiler or other heating system if you install a suitably-sized heat pump that is set up and used efficiently, and you buy cheap electricity on a heat pump tariff
- You need suitable space outside your home for the external condenser unit, and you may need indoor space for a hot water cylinder. This is often not an issue for larger homes with a garden but could be inconvenient or even impossible in a smaller property. Some types can be mounted on an external wall rather than on the ground, if you don't have outside ground space
- You may need to replace existing pipework and/or radiators in your current wet central heating system to make sure it can deliver enough warmth with an air-to-water heat pump. This is to compensate for the lower flow temperatures supplied by a heat pump. Note: newer heat pump models are producing higher flow temperatures which will be beneficial for older pipework
- You will probably need to boost your home's insulation to get the most from your heat pump. You can install a heat pump without doing this, but you might find you need a larger model, and it won't be running as efficiently. That said, it's wise to insulate your home anyway, even if you're sticking with a gas boiler, as this will make your home more comfortable and save you money on heating bills
- It's more effort than installing a new boiler. Installing a heat pump for the first time requires a lot more effort and planning than simply replacing a gas boiler with a new one. You'll need a careful design of your heating system, based on accurate heat-loss calculations, to achieve your desired comfort levels and minimise your bills. The installation and associated works may take a number of days and could be somewhat disruptive
- Heat pumps must be set up correctly, which creates room for error. If your heat pump is unsuitable or set up incorrectly, you could find yourself spending a lot more on energy bills. You'll need to be willing to learn how to operate your heating system, as it can be different from a gas boiler
Many of the last points are one-time-only when you install your first heat pump, or can be simply addressed by getting the correct set-up and good advice from a competent installer.
Source: Which?
