Your Air Conditioning Is a Heat Pump: How It Heats Too

Yes - almost every wall-mounted or multi-split air conditioner we install in Surrey homes is a reverse-cycle unit, which means it heats as well as cools. The same machine that keeps a bedroom comfortable in July will warm it in January, at the touch of a button on the remote.

This often surprises people. The phrase "air conditioning" has come to mean cooling, but technically an air conditioner is an air-to-air heat pump. It doesn't burn anything or glow red like an electric bar fire - it simply moves heat from one place to another, which is why it can run in both directions.

An air conditioner contains a refrigerant circuit and a component called a reversing valve. In cooling mode it absorbs heat from inside your room and dumps it outside. Flip the reversing valve and the whole cycle runs the other way: the outdoor unit absorbs warmth from the outside air and the indoor unit releases it into your room.

The clever part is that there is usable heat in outside air even when it feels cold to us - a well-designed unit will still extract heat at 0°C and below. Because the system is moving existing heat rather than creating it from electricity, it delivers far more heat energy than the electrical energy it consumes. That is the whole reason heat pumps are central to the UK's plan to decarbonise heating.

COP (Coefficient of Performance) is the ratio of heat delivered to electricity used at a single point in time. A COP of 3.5 means the unit produces 3.5kW of heat for every 1kW of electricity it draws. A plug-in electric heater, by contrast, has a COP of 1 - it can never beat the electricity going in.

SCOP (Seasonal COP) is the more honest figure for the UK, because it averages performance across a full heating season including colder spells. Quality units from brands such as Daikin and Mitsubishi Electric typically advertise SCOPs around 4 to 5 in heating mode. In plain terms: for every £1 of electricity, you get roughly £3.50-£5 worth of heat. The colder it gets outside, the harder the unit works and the lower the efficiency, but even on a frosty Surrey morning a good system comfortably beats a 1:1 electric heater.

Here's where the reverse-cycle story gets genuinely interesting for running costs. Direct electric heating (panel heaters, fan heaters, immersion) is the most expensive way to heat at roughly 1 unit of heat per unit of electricity. An air conditioner running in heating mode at a SCOP of around 4 produces roughly four times the heat for the same electricity - so it can be three to four times cheaper to run than a plug-in electric heater.

Against mains gas central heating the picture is closer and depends on the price gap between gas and electricity. Gas is cheaper per unit than electricity in the UK, but a modern gas boiler is only around 90% efficient, while a heat pump's 350-450% effective efficiency narrows the gap considerably - and for a single room or a property without gas, AC heating is often the cheaper, faster option. As a rough guide, heating a typical room with a reverse-cycle unit costs in the region of 10-30p per hour, depending on the unit, the weather and your electricity tariff.

The real argument for air conditioning in a UK home isn't summer alone - it's that you get two appliances in one. A single installed system cools you through heatwaves (which the South East now sees most years) and provides fast, controllable, room-by-room heating the rest of the time. There's no waiting for radiators to warm up; a unit delivers warm air within minutes and lets you heat only the rooms you're using.

For a home office, a converted loft, a garden room or an extension that the central heating struggles to reach, a reverse-cycle unit is frequently the most sensible heating and cooling solution full stop. It's also worth being clear on one point of accuracy: while an air conditioner is genuinely a heat pump, an air-to-air heat pump is not eligible for the Boiler Upgrade Scheme - that grant covers air-to-water, ground-source and biomass systems only. Be wary of anyone claiming a government grant for standard air conditioning.

A small number of units are sold as "cooling only", and some cheap portable air conditioners don't heat. But virtually every fixed split-system we'd specify for a home or office is a reverse-cycle heat-and-cool model as standard - it costs the manufacturer almost nothing to add the heating function, so it's now the norm rather than a premium extra.

If you already have air conditioning and have only ever used it for cooling, check the remote for a sun or flame symbol (heat mode), usually alongside the snowflake (cool). The odds are very good that you've been sitting on a highly efficient heater all along.