The energy efficiency of a boiler is usually quoted in percentage terms. e.g. If a boiler is said to be 90% efficient, then 90% of the gas energy comes out as useful heat to water, and 10% is lost as heat out of the flue.
Heat pumps require electricity to function. However, this power input is typically between 1/2 and 1/5th of the available heat output. We normally use the ‘coefficient of Performance’ (COP) as the measure of efficiency.
In the example below the COP is 3, so 3 kWatts of useful heat are delivered to the house for the cost of 1kW of electricity. This is actually 300% efficient, and only possible due to the 2kW being extracted from the garden.
The graph below gives an idea of the COP variations of a typical heat pump. This shows the difference in temperure between the source and the heat-delivery side. If you assume the heat pump’s source is zero Celsius, you can read the horizontal (X axis) scale as the heated water temperature.
This is an example of a typical product label. Every unit should have this type information on it. It shows ratings at various temperature conditions.
‘B0’ relates to ‘Brine inlet at 0ºC’, ‘W35’ relates to heated water flow outlet of 35ºC. As you can see, the COP rating varies depending on the operating temperatures, and broadly agrees with the graph above. Never read a COP figure without questioning what temperatures the figure relates to.
The input power should include power for fans and pumps within the unit. It should also include any energy required for air source defrosts (maybe 10%). Beware of optimistic ratings that relate to warmer days given that an air source heat pump will do most of its work in the winter,
Seasonal Performance Factor (SPF)
The COP is the efficiency when operating at certain temperatures. This will vary over the seasons. The SPF is a measure of the total annual heat delivered from a system compared to the total energy input to run the system. If electric supplementary back-up heaters are included, then the SPF must include this. It also includes any fans and pumps on the system.
The actual SPF of a system (heat-meter output reading divided by the electric input meter) will be greatly affected by the whole system and the way that it is operated. If you want the highest SPF, it may be more important to focus on the design and mode of operation rather than to get too hung up on the rating of the unit.