AIR SOURCE HEAT PUMPS

An air source heat pump is a renewable heating source that heats your home by harnessing warmth from the outside air. David Hilton looks at the pros and cons

An air source heat pump is a low-carbon heating system. 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 as they deliver lower temperatures than a boiler, they need to be carefully designed into well insulated, airtight homes to ensure optimum efficiency.

HOW DO THEY WORK?

Air source heat pumps work in a very different way from boilers. They don’t create heat, they move 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.

Electricity is needed to power the pump, but this could be from solar PV, for example, meaning that it’s possible to use an air source heat pump off grid (make sure they are compatible for this purpose though).

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.

WHAT TO CONSIDER

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

● Do you have enough space? 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 ASHP 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 ASHP is generally smaller than a gas boiler, so you should have no issue fitting this in its place, though if you’re continuing to use a gas boiler in tandem, you’ll need to consider this internal space too.

● What emitters will you use? Air source heat pumps work best with underfloor heating, but low-flow temperature radiators, such as oversized radiators, multi-finned aluminium radiators or fan convectors, will work as well.

● How well insulated is your home? Air source heat pumps work more efficiently when producing heat at a lower temperature, so a home that requires less space heating because it is better insulated is key to getting the most from this technology.

● What system are you replacing? The initial expenditure of an ASHP is high. When replacing a gas boiler, an ASHP may actually cost more to run (though much more sustainable), so when it comes to retrofitting a heat pump, it’s often more attractive when replacing a more expensive heating system such as electric heating, or including within a new home.

WHAT ARE THE COSTS?

The cost to supply and install an air source heat pump starts from £11,000. The cost to install in a self-build will be lower than in an existing property, where allowances to upgrade the building ’s fabric and potentially replace existing radiators could see costs of up to £27,000.

IS AN ASHP WORTH IT?

In a new build, especially one that has been built with fabric first principles at the forefront, equipping your home with an air source heat pump is definitely worth considering. Not only do they offer low carbon heating for your home, they’re reliable and last much longer than a gas boiler.