AIR SOURCE HEAT PUMPS

An Air Source Heat Pump uses an electric powered compressor that forces cold air through a heat exchanger. Using compressed refrigerant the pressure of the refrigerant gas increases along with the temperature. When the pressure decreases, the temperature decreases. This relationship between pressure and temperature is the key to how a heat pump works. As the refrigerant’s heat is transferred through the heat exchanger, it cools down. As the refrigerant then expands, it cools down even further and is now cold enough to absorb more heat from outside to begin the process again. The residual heat delivered to the heat exchanger is then used to heat your home. This is normally done through a conventional central heating system – but it could also be done using warm air in either an Air-to-Air heat pump or an Exhaust Air Heat Pump.

The compressor in a heat pump works harder when there is a larger temperature difference between the outside source temperature and the water temperature needed in your radiators or underfloor heating. The less the compressor needs to work, the less electricity the heat pump uses. Whilst we are unable to control the outdoor source temperature, it is possible to design a heating systems that uses low temperature water indoors where the heat pump can use less electricity and still heat your home comfortably. This is achieved by using radiators with a larger surface area or underfloor heating. More heat can be delivered into the room without increasing the water temperature as well as running the heating system for longer to provide a continuous ambient heat.

Conversely, if you have radiators with a smaller surface area, the heat pump will have to run at a higher temperature. This will result in the compressor working harder to deliver the same amount of heat as it would with larger radiators, or if it had a longer time to run. When the compressor works harder, it uses more electricity, which makes the system less efficient and more expensive to run. The aim of a well-designed system is to reduce the heating water temperature as much as possible. The closer the required temperature is to the source temperature (ie the outside air or ground temperature), the more efficient the heat pump will be, and therefore the lower the running costs. Heat pumps are more efficient than other heating systems because the amount of heat they produce is more than the amount of electricity they use. 

The amount of heat produced for every unit of electricity used is known as the Coefficient of Performance (CoP). So, if a heat pump has a CoP of 3.0, then it will give out three units of heat for every unit of electricity consumed. Every heat pump has a published datasheet telling you what its measured CoP is. The CoP is measured at a single point in time, under specific test conditions. However, in real life the heat pump experiences seasonal temperature variations throughout the year, so the CoP is not always a true reflection of the efficiency. Instead, the Seasonal Coefficient of Performance (SCoP) or Seasonal Performance Factor (SPF) is used to show the efficiency of the heat pump across a whole year. To help advise you better, our surveyors will be able provide an illustration of the SPF for an ideal system for your home. This calculation will demonstrate how the heat pump should perform given the average temperatures at your location and other factors such as the size of your radiators. The SPF will give you a better indication of what to expect in terms of running costs and efficiency than the CoP figure.