Heat pump running costs higher than expected
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Most likely cause & what to check
A heat pump should produce 3–4 units of heat for every 1 unit of electricity it consumes (a Coefficient of Performance, or COP, of 3–4). If your electricity bills are higher than expected, the heat pump may not be achieving its design efficiency.
Check the flow temperature. Running at 55°C flow temperature drops the COP to around 2.5; at 45°C it rises to 3.5 or above. If your radiators are large enough to heat rooms at lower flow temperatures, reducing the flow temperature by even 5°C can save 10–15% on running costs.
Check the hot water cylinder temperature. If the heat pump is heating domestic hot water to 60°C every day (to satisfy Legionella control), it uses significantly more electricity for hot water than for space heating. Consider reducing the immersion backup frequency to weekly (sufficient for Legionella protection) and using the heat pump to heat to 55°C on other days.
Enable weather compensation if your system has it. Weather compensation automatically reduces the flow temperature when it is warmer outside — reducing electricity consumption on mild days. It is the single biggest efficiency gain available in heat pump controls.
Check all radiators are fully open and flowing. Restrict flow through TRVs forces the heat pump to run at higher pressures and temperatures. On a heat pump system, all TRVs should ideally be fully open, with temperature controlled via the main thermostat.
Consider switching to a heat pump electricity tariff. Octopus Energy's Cosy Octopus tariff, for example, offers cheaper rates during certain hours. Running the heat pump during cheaper rate periods (and using thermal mass of the house to coast through peak periods) can cut electricity costs by 20–30%.
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Frequently asked questions
What Coefficient of Performance (COP) should I expect from my heat pump?
A well-designed and correctly installed ASHP should achieve a Seasonal COP (SCOP) of 3–4 in UK conditions — meaning 3–4 units of heat for every 1 unit of electricity consumed. A SCOP below 2.5 suggests the system is not optimised. Your installer should provide a predicted SCOP figure as part of the MCS design.
Are there cheaper electricity tariffs for heat pump owners?
Yes — several suppliers offer heat pump-specific tariffs with cheaper overnight rates. Octopus Energy's Cosy Octopus tariff offers reduced rates during certain periods. Running the heat pump to pre-heat the house during cheaper rate hours and coasting through peak periods can cut annual running costs by 20–30%.
Should I run my heat pump continuously or on a schedule?
Heat pumps are most efficient when run continuously at low output rather than cycling on and off to meet large temperature swings. Set a steady target temperature and let the heat pump maintain it — rather than programming big setbacks overnight. This approach produces better SCOP figures and is kinder to the compressor.