Using air conditioning in a heatwave

Many UK homes are designed to keep heat in rather than out, so in a heatwave they warm up through the day and stay warm long into the night. That's when air conditioning works hardest. A few habits help the system keep up, and help you stay comfortable in the rooms it doesn't reach.

Getting ready before it arrives

• Clean the filters. Dusty filters restrict the airflow and reduce the cooling you get. How to clean the filters shows how.
• Check the outdoor unit. Clear leaves, plants and anything stored around it, so air can flow freely through it.
• Try cooling on a warm day first. If something isn't working, you'll find out before the hottest days, when engineers tend to be busiest.

A day in a heatwave

Timing matters. Walls, floors and furniture soak up heat through the day and give it back for hours, so a room that has been allowed to get hot takes far longer to bring down than one kept cool from the morning.

Blinds matter most on east-facing windows in the morning, south-facing ones around the middle of the day and west-facing ones in the afternoon and evening. There's more in doors, windows and blinds.

Settings for very hot days

• Choose a moderate temperature. A setting far below what you want won't cool the room much faster, and a big gap between indoors and out makes going outside feel harsher. What temperature to set has starting points.
• Use cool mode. Dry mode only cools gently, and suits muggy, mild days rather than real heat.
• Use powerful mode briefly if your model has it, for example when you come home to a hot room, then go back to normal.
• Turn off eco mode if the room isn't keeping up. On many models it limits how much power the system draws, and so how much cooling it gives.
• Leave the fan on auto or a higher speed while the room cools down. Quiet settings move less air, and so less heat.
• Let it run steadily. Switching off in the afternoon lets heat build up in the room, and the system then has to remove it.

The outdoor unit in the heat

The outdoor unit gets rid of the heat taken from your rooms, so it blows out hot air. Keep the space around it clear, and never cover it or box it in. If you want to shade it, leave plenty of space all round for air to flow, because a shade that traps hot air around the unit does more harm than good.

On very hot days the outdoor unit may sound louder than usual, because it's running at a higher speed. Humid heat also means more water from the indoor unit, so if your condensate pipe ends outside, expect a steady drip while the system runs.

When the system can't keep up

Cooling output falls as the outdoor temperature rises, and the manual gives the range of outdoor temperatures the system is designed to work in. Even a well-sized unit may not hold your setting on the hottest days in a room facing the afternoon sun, or in a loft room. Running flat out for hours without quite reaching the setting isn't necessarily a fault.

Get it checked if the air coming out isn't noticeably cool, ice forms on the unit or pipes, or the outdoor unit keeps stopping and starting while the room is still warm. Not cooling or heating properly goes through the likely causes.

To help it along:

• Make one room your cool room. Keep its door shut, spend the hottest part of the day there, and sleep there if you can.
• Cut other sources of heat. Switch off lights and equipment you're not using, and cook at cooler times of day.
• Keep sun off the glass. Shading outside a window stops more heat than a blind inside it.
• Use fans in other rooms. They don't lower the temperature, but moving air feels cooler on your skin.

The NHS has advice on coping in hot weather, including who is most at risk from the heat.