Air con in a UK autumn: it heats, too

If you bought a fitted air conditioner in the last five years, you already own a heat pump. That is not marketing. It is how the machine works.

The compressor moves heat from one side to the other. In July it takes heat out of your bedroom. In October it takes heat out of the outside air and puts it into your living room. Same box, same buttons, opposite direction.

Why this matters in autumn

British autumns are damp, mild, and long. Gas central heating usually goes on in October and burns through the winter. It heats the whole house even if you only use the living room.

An air conditioner on heat mode does the opposite. It heats one room, quickly, and switches off. On a typical mild October evening it costs about 15p an hour to run at a comfortable setting. A gas boiler running the whole house for the same evening costs three to four times that.

When it stops making sense

Heat pumps get less effective as it gets colder. Modern units perform down to about minus 15 degrees, but efficiency drops the colder it gets. In a UK winter that means:

• Above 5 degrees outside, air con heats a room for pennies.
• Between minus 5 and 5, it still beats an electric radiator on a per-hour basis.
• Below minus 5 on a big draughty house, gas central heating wins.

Most UK autumn nights sit above 5 degrees. So for September, October, and much of November, air con is the cheapest heating you own.

The two ways households actually use it

The first pattern: heat the room you are in. Living room 6pm to 10pm, bedroom 10pm to 11pm, done. The rest of the house stays cool. The bill is small.

The second pattern: shoulder-season top-up. Gas heating stays off until December. Air con handles the mild months. Households we speak to report knocking £150 to £300 off their yearly gas bill this way.

Both work. The right one depends on the house, the household, and how many rooms you actually use.

What to check on your unit

Not every split system is a good heater. Look for:

• Heating capacity roughly equal to cooling capacity. Cheaper units skimp on the heating side.
• A COP (coefficient of performance) of 3.5 or higher at 7 degrees outside. This tells you 1 kWh of electricity gives you 3.5 kWh of heat.
• An inverter compressor, not a fixed-speed one. Every modern unit worth buying has this.

If your installer cannot tell you the COP figure, that is a red flag.

Not sure what you have?

Send us the model number of your unit on WhatsApp and we will tell you whether it is worth using as a heater or whether you would be better upgrading. If you do not have air con yet, autumn is the calmest time of year to get a survey - installers have short waiting lists between September and February.

Get up to 3 free quotes from vetted local installers. Takes about a minute.