Many people are surprised to learn that the air conditioning they use in summer is, in fact, a heat pump. In heating mode the system simply runs in reverse: instead of carrying heat out of your home, it extracts free heat from the outside air and moves it inside. Even on a cold London morning, there is plenty of heat in the outdoor air for the unit to harvest.
Why heat pumps are so efficient
A heat pump does not create heat by burning fuel or running an electric element — it moves existing heat. That makes it fundamentally different from electric panel heaters, fan heaters and oil-filled radiators, which convert one unit of electricity into at most one unit of heat.
- Electric panel heater: 1 kWh of electricity produces about 1 kWh of heat (100% efficient)
- Modern gas boiler: roughly 0.9 kWh of usable heat per kWh of gas burned
- Air conditioning in heating mode: typically 3 to 4 kWh of heat per kWh of electricity (300–400% efficient)
The technical term for this ratio is the coefficient of performance (COP). Across a whole UK heating season, manufacturers rate systems using SCOP (seasonal COP), and modern wall-mounted units commonly achieve SCOP figures in the region of 3.5 to 4.5. In plain terms, for every £1 of electricity you use, you get around £3.50 to £4.50 worth of heat.
“An air conditioner in heating mode is the most efficient form of heating most London homes can install without changing their entire system.”
What does it cost to run in winter?
Running costs depend on your electricity tariff, how well insulated the room is and the temperature you set. As an illustration, at typical UK electricity prices a modern unit with a SCOP of around 3.5 costs roughly the same per unit of useful heat as a condensing gas boiler — while being far cheaper than running an electric heater in the same room.
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It is worth remembering that these systems are usually used to heat individual rooms for part of the day, rather than a whole house all day. Heating a home office from 9am to 6pm, or a bedroom overnight, is a very different — and much smaller — job than running central heating throughout the property.
Does it still work when it is freezing outside?
Yes. Modern units from brands such as Daikin, Mitsubishi Electric and Fujitsu are designed to keep delivering heat well below freezing — commonly down to around -15°C to -25°C depending on the model. The efficiency does drop as the outdoor temperature falls, which is normal physics: the colder the air, the harder the unit works to extract heat.
- Mild winter days (8–12°C): the system runs at or near its peak efficiency
- Cold snaps (around 0°C): efficiency drops but remains well above an electric heater
- Defrost cycles: in freezing conditions the outdoor unit periodically defrosts itself; this is normal and the system resumes heating automatically
How to get the most from heating mode
- Set the thermostat to a steady 18–21°C rather than blasting it high and turning it off
- Use the fan on auto rather than high constant speed
- Keep indoor filters clean — a blocked filter cuts heating output and efficiency
- Direct the louvres downwards, since warm air rises and collects at the ceiling
- Book an annual service so refrigerant charge, coils and airflow stay at spec
Heating and cooling in one system
The real value of a reversible air conditioning system is that one installation does two jobs. You are not paying twice for separate cooling and heating equipment, and the units are far more efficient at heating than any plug-in electric alternative. For London homes with hard-to-heat extensions, loft conversions or garden rooms, it is often the most practical way to add comfortable, controllable warmth.
If you are considering a system — or want yours checked before the cold months — our team offers free, no-obligation site surveys across London.
