An undersized pump will not heat your home or business sufficiently when you need it most – in the winter months. An oversized pump will cost you more than you need to pay for the initial installation. An inefficient pump will cost you more to run.
So what do we mean by efficient and how can you evaluate it? There are two terms that you will often see in relation to heat pump efficiency and it is essential to understand and differentiate between them – coefficient of performance and seasonal performance factor (sometimes abbreviated to COP and SPF).
The coefficient of performance (COP) is the amount of heat energy you get out of the pump in relation to the amount of electrical energy that you need to drive it. So, for example, a pump that generates 4kW of heat for every 1 kW of electrical energy used to run it will have a COP of 4/1. However, this means nothing unless you are told at what ambient temperature the COP is calculated. A heat pump will use more energy to produce heat when the ambient temperature is low (say -4°) than when it is moderate (say 10°). So a pump that has a COP of 4/1 at 10° may only have a COP of 2/1 at -4° and so use twice as much energy to run it when the weather is below freezing.
The COP is not, therefore, a very useful guide as a heat pump efficiency calculator, or how much it will cost to run the pump unless you plan to use it in a location where the temperature never changes! Be wary of comparisons between pumps that only make comparisons based on COP.
Much more useful is the seasonal performance factor (SPF). This is the average COP of the pump over an entire year for the location in which the pump is to be used given typical seasonal temperatures for that location. So a pump that has an SPF of 3/1 means that as an average over the whole year it will produce 3kW of heat energy for every kW of electrical energy used to power it. The Mitsubishi Ecodan, which is one of the most efficient pumps on the market, for example, has an SPF of 3.1 in the UK – that is it will on average throughout the year produce 3.1kW of heat for every 1kW of electricity. This makes it more than 3 times cheaper than using direct electrical heating (and upto 50% cheaper than using oil or LPG).
Sizing the Pump
Sizing a heat pump is critical and something that you should entrust only a competently trained person to do. A heat pump should be sized so as to provide 100% of the heat needed to heat the premises to the desired temperature (usually 21°C) at typical winter temperatures for that location.
Heat pump efficiency and temperature range. In order to establish the output of the pump required the heat loss of the premises at the typical ambient temperature at that location during winter months needs to be calculated. So for example, if a well insulated semi-detached house loses heat at 5 kW per hour at a typical temperature of -4°C in January then the pump needs to be able to provide at least 5kW of heat per hour to maintain the desired temperature of the house.
However, that does not mean that a pump with a declared capacity of 5kW will be big enough. Just as with the COP (see above) you need to know at what temperature the pump will deliver 5kW of heat – as temperatures fall the capacity of the pump to deliver heat will also fall. Some pumps’ capacities drop off dramatically at lower temperatures so it is important that you know the pumps actual capacity at the design ambient temperature (i.e. the typical temperature at the location in winter).
Consider this example:
The yellow box shows the heat loss of the dwelling to be 4.97KW at an outdoor temperature of -4°C but the Ecodan output at that temperature of the 5kW model (shown in the green box) is only 4.71KW – it may therefore be prudent to choose the larger 8.5kW. Note that the capacity drop off of all 3 Ecodan pumps in this example is very small – other pumps often have much greater drop offs than this.
Selecting and installing a heat pump system is a highly specialist task – make you enlist the services of an approved installer – not just your local plumber!