Heat pumps work on the same principle as fridges and freezers – but in reverse. They cool a refrigerator to a low temperature and then circulate it through a set of coils that are either buried in the ground (ground source heat pump) or have a fan that circulates air over them (air source heat pump). The refrigerant warms up, picking up the heat from either the ground or the air. The heat is then extracted from the refrigerant and the cycle starts again. The heat from the refrigerant is used to drive a central heating system as well as domestic hot water.
Ground source heat pumps tend to be more effective (as the ground temperature does not vary as much as the air temperature), but they tend to be more expensive as they need coils to be buried in the ground.
Because of the way these pumps work, you need to put energy in (to reduce the temperature of the refrigerant), but you get energy back from the ground/air. For every unit of energy put in, ground source heat pumps will generate 3-5 units of energy. This ratio is called “Coefficient of performance” (COP) and you will find it quoted on most of the ground source heat pumps on the market. For example, for the NIBE F1345 unit that we are considering has a COP of 4.79.
To calculate the size of heat pump required, you need to provide the size of the rooms, the use of the rooms (bedrooms tend to be cooler than other rooms) and insulation levels. Where you live in the UK will also affect the amount of energy that these pumps will be able to extract. You don’t need to worry about all of this as the ground source heat pump suppliers will do the calculations for you.
The calculations for our property indicate that we need a pump capable of generating around 22Kw. The nearest size heat pump is 24Kw. This is fairly large heat pump and unfortunately this requires a 3 phase electricity supply as the load is too great for a single phase supply (which is the “normal” electricity supply in most domestic properties). I have had to get a quote from our electricity power supplier (Northern Power Grid) for installing a 3 phase supply. I will cover this elsewhere on the blog.
Heat pumps are not cheap. A ground source heat pump tends to be more expensive that air source, if only because of the coils that need to be buried into the ground. A 24Kw unit is going to be around £10K and all the additional bits and pieces will roughly double that cost. However, there is some good news as there is a Renewable Heat Incentive (RHI) available from the Government. RHI is intended to encourage people to install renewable heat sources and from the calculations that I have done, I suspect that the RHI will cover the cost of the ground source heat pump over a 7 year period. I will cover RHI in more detail elsewhere on this blog.