One of the main benefits to heat pumps is that they’re incredibly efficient. They’re capable of running at efficiencies of over 300%, since they transfer heat rather than generate it. However, maintaining this level of efficiency is essential for heat pumps to be a worthwhile investment.
The efficiency of heat pumps depends on how hard they must work to maintain a comfortable room temperature in your home – the lower the flow temperature (the temperature water flows through your radiators), the greater the efficiency. That’s why having a bigger emitter system makes such a difference – with larger radiators and underfloor heating, the heat pump doesn’t have to operate at such a high temperature to heat your house, so the efficiency increases.
Heat pump efficiency is measured by its Coefficient of Performance (CoP), and it shows how efficiently the air source and ground source systems can heat your home under the best possible conditions. Commercial air source heat pumps can operate at a maximum CoP of 4, whereas ground source heat pumps can reach up to 5. This means that for every unit of electricity you put in, the heat pumps have the potential to produce 4 and 5 units of heat respectively.
To put this into perspective, electric heaters operate at around 100% efficiency (1 unit of electricity produces 1 unit of heat), and even brand-new oil and gas boilers only function at an efficiency of around 90%. However, given that gas is a third of the cost of electricity, heat pumps are required to work above an average efficiency of 300% to compete with conventional gas systems.
|Type of heat generator||Typical flow temperature (°C)||Fuel||Typical seasonal efficiency (%)||Capital cost (£)|
|Air to air heat pump||40||Electricity||250||Medium|
|Air to water heat pump||40||Electricity||350||Medium|
|Ground source heat pump||40||Electricity||350||High|
|High efficiency boiler, LTHW||80||Gas, LPG, oil, bio-gas and bio-oil||85||Low|
|Condensing boiler, LTHW||70||Gas, LPG, oil, bio-gas and bio-oil||90||Low|
|Combined heat and power||90||Gas, LPG, oil, bio-gas and bio-oil||Not applicable||High|
|Solar thermal||80||Solar radiation||Not applicable||Medium|
|Biomass boiler, LTHW||80||Wood chip, pellets||80||High|
|Steam boiler||170||Gas, oil, biofuels||85||High|
|M/HTHW boiler||>90||Gas, oil, biofuels||85||High|
Based on gross calorific value of fuel
Sourced from CIBSE Guide B1 : 2016
The issue with comparing heat pumps based on their CoP values is that they give unrealistic expectations of heat pump performance, as they only show the efficiency in peak conditions. During the winter when the temperature outside drops, the efficiency of air source heat pumps decreases (sometimes to around 1.5 CoP), as they must use more electricity to maintain your normal room temperature.
It’s much more beneficial to compare heat pumps based on their Seasonal Coefficient of Performance (SCoP). The SCoP gives a true indication of heat pump efficiency because it takes seasonal change into account, giving you an average value over the whole year. The Seasonal Coefficient of Performance is also used to determine how much you’ll be receiving in RHI payments.
Seasonal change doesn’t affect the efficiency of ground source heat pumps to the same degree, as the temperature underground remains constant throughout the year (roughly 10°C), but the porousness of the ground does. If the ground underneath your garden is a thick clay-like substance, heat will not conduct very well to replace that which is absorbed through the heat pump’s pipes, so the efficiency will be reduced. That’s why the quality of the ground is taken into consideration during the survey process.
The Seasonal Coefficient of Performance does has its limits though. For instance, an immersion heater is used for supplying the house with domestic hot water (usually at an efficiency of less than 100%). If the heat pump supplies the heat for domestic hot water, the SCoP will decrease. Other areas that aren’t taken into account in the SCoP include the electricity required by the circulation pump and defrost cycles for air source heat pumps.