Air Source Heat Pump

An air source heat pump is a low-carbon heating source. It uses a refrigeration cycle to move heat from the air outside and transfers it as useable heat in your home. 

Air source heat pumps, like boilers, are a heat source, but they deliver lower temperatures than a boiler. As such, they ideally need to be carefully designed into well-insulated, airtight homes to ensure optimum efficiency.

There are two main types of an air source heat pump:

• Air to water heat pumps are used in wet central heating systems to heat radiators, underfloor heating and sometimes to generate hot water. 
• Air to air heat pumps are perhaps better known as air conditioning, and heat the air directly. This will include some level of air movement and noise as you’d expect from an air conditioning unit. 

This article will focus on air to water heat pumps.

How Does an Air Source Heat Pump Work?

If you’re wondering how do heat pumps work, then it’s first important to note that air source heat pumps work in a very different way to boilers. An air source heat pump does not create heat — it simply moves it from one place to another through the vapour compression cycle (or refrigeration process) to make it more useable. 

Heat from the air gets absorbed into a fluid, which causes it to ‘boil’ and become a gas. The gas is then compressed, raising its temperature. The higher temperature is then transferred into the heating system (i.e. the radiators or underfloor heating).  

Do Air Source Heat Pumps Need Electricity?

Yes, like ground source heat pumps, electricity is needed to power an air source heat pump.

The efficiency, or the measure of the heat energy output per kW of electricity, is stated as the COP (Coefficient of Performance) or SCOP (or Seasonal Coefficient of Performance — the SCOP is the average COP over a defined period of time such as a year). 

For example, a SCOP of 3.2 means that for every 1kW of electricity, 3.2kW of heat is generated.

However, electricity could be provided solar PV (together with a battery for storage), meaning that it’s possible to use an air source heat pump off grid.

What are the Advantages of Air Source Heat Pumps?

It is essential to design and specify the system correctly and ideally the insulation, airtightness and emitters (typically underfloor heating or carefully sized radiators) of the property should be optimised, to allow you to get the most out of your air source heat pump.

There are many benefits to installing an air source heat pump, including:

• Unlike a gas boiler, an air source heat pump does not produce carbon when operating. While they do use electricity, ASHPs can be combined with solar PV panels or a renewable/green energy tariff for clean electricity.
• In the right context, air source heat pumps have comparatively low running costs, especially when compared with off-grid fuels such as propane, oil or direct electric heating.
• The outdoor unit takes up less space as compared to an above-ground oil or LPG tank.
• Air source heat pumps partner well with underfloor heating.

Do Air Source Heat Pumps Work in Cold Weather?

An air source heat pump gets its energy from the surrounding air, so as the ambient air temperature drops, so does the efficiency. It is therefore key to understand the heat load of the property and the performance characteristics of the heat pump. 

When it gets closer to zero degrees outside, the air source heat pump will also need to perform a defrost cycle more often and this will also lower the efficiency. 

However, while the efficiency may drop, that doesn’t mean that air source heat pumps don’t work in cold weather. It’s been found that they can extract heat from the air in temperatures as low as -20°C, and are used in cold climates around the world. 

Is my Home Suitable for an Air Source Heat Pump?

There are some key criteria your home will need to meet to make installing an air source heat pump viable, including:

Do you have enough space for one to be installed?

You’ll need to have space for a unit to be attached to a wall or fitted on the ground, with space around it to ensure a good airflow. For an air source heat pump to be installed under Permitted Development, it also needs to be located at least one metre from the boundary of the house. 

The internal unit for an air source heat pump is generally smaller than a gas boiler. However, you will need space for a hot water cylinder, if you do not have one already, and controls.