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Heat pumps are often the cheapest way to heat a property.  They must be carefully sized to the meet the needs of your building and you will get the most benefit from them if your property is well insulated.

The way heat pumps work is best explained using water as an analogy.  Water flows downwards from a high point to a low point, but can be made to move the other way using a pump.  In the same way, heat flows from a hot body to a cold body, but can be driven in the other direction by a heat pump.  Like a water pump, a heat pump needs some electrical energy to do its work.  Much more energy is needed to pump the heat when the temperature difference between the hot and cold bodies is very large.  (In the same way, pumping water up a very high hill is more work!)

Air source heat pumps are so-called because the “cold body” that the heat is extracted from is in fact the outside air.  Large volumes of air are blown through the system by fans and the heat is extracted from the air as it passes.

Heat pump efficiency is measured by the Coefficient of Performance, or “CoP”.  The CoP refers to the amount of heat energy pumped to the hot space by one unit of electrical energy.  The reason heat pumps give a large energy benefit is that one unit of electricity can put out several units of heat, giving a high CoP.  (The higher the CoP, the more efficient the pump.)  The average CoP over the whole year, sometimes called the Seasonal Performance Factor, is the key figure to look at when comparing heat pumps.

A good CoP for an air source heat pump is between 3 and 4, which means that the pump produces 3 to 4 times as much heat energy as the electrical energy that it uses.

How does this actually work?
Most heat pumps work on a “vapour compression cycle”, which is very similar to a fridge being run in reverse.  The refrigerant inside the pipes goes through a 4-stage loop, passing through an evaporator, a compressor, a condenser and a throttle.  The key point to grasp in order to understand how a heat pump works is that a liquid at high pressure will evaporate at a higher temperature than normal.

The 4 stages of the loop are as follows:

Stage 1 - cold, liquid refrigerant is passed through the evaporator.  Fans take air from the outside (which is warmer than the refrigerant) and drive it past the coils.  Here, the air heats up the refrigerant enough for it to evaporate, turning it into a gas.

Stage 2 - A compressor is then used to force the gas into a smaller space at high pressure, heating it up.  This is the part of the process that uses the electrical energy.

Stage 3 - The gas then passes into the condenser where it changes back into a liquid, giving back all the energy it absorbed from the condenser and the compressor.  But, because it is at high pressure, the gas gives back its energy at a much higher temperature!  This energy is collected by the heat pump and used to heat the air or water in your house.

Stage 4 - Finally, the high-pressure liquid is passed through a little nozzle (or throttle) to allow its pressure to drop back to where it started.  This also cools the liquid, making it colder than the outside air and ready to start absorbing heat again in the evaporator. 

Types of heat pump
Air source heat pumps come in two varieties:

• Air-to-air - This type of pump transfers heat from the condenser directly to the air in your home, using fans to provide an even distribution of heat throughout the building.

• Air-to-water - This type of pump heats water, which can then be used for underfloor heating and everyday hot water use.  It is possible to run normal radiators with an air source heat pump, but because these run at high temperatures the heat pump is less efficient.

What could possibly go wrong?
The air leaving the heat pump has had some heat extracted from it, so it is very cold and often below freezing temperature.  In the moist British climate, this can mean that water vapour in the air freezes onto the heat exchanger.  This cuts down the rate of heat transfer and makes the heat pump much less efficient. Modern air source heat pumps have a “defrost cycle”, where they periodically run in reverse for a short period of time to heat up the coils and get rid of the ice.

The main problem with air source heat pumps is that on the coldest days of the year – when the heating is needed most – the temperature difference between the house and the air is largest, so the heat pumps are the least efficient!  Good systems are designed with an electrical heating element that kicks in to help the heat pump on the few coldest days of the year.