It doesn’t create cold air, it moves heat. Refrigerant picks up warmth from the room and carries it to the outdoor unit, which releases it outside. What’s left behind is cooler, drier air.

Yes. Nearly every modern system you can buy runs in reverse to give heating as well as cooling. Many people use theirs through winter for a garden room, loft conversion or home office.

Less than most people expect. Modern inverter units adjust their output to what the room needs instead of running flat out, so they use far less electricity than older systems. Your actual cost depends on the unit size, how often you use it and your energy tariff.

A fitted system is much quieter than a portable one, because the noisy part (the compressor) sits outside. Indoor units are typically quiet enough to sleep next to.

It helps. The filters in the indoor unit trap dust and pollen as air passes through, and the cooling process pulls moisture out of the air, which makes a humid room feel fresher.

Floor area is only the starting point. A proper calculation also weighs up things like how much glass the room has and how many people use it. Too small and the unit struggles, too big and it wastes energy, so a survey is the reliable way to get it right.

Wall-mounted units are the most common choice for homes. Floor-level units suit lofts and rooms with limited wall space, ceiling cassettes suit offices and shops, and ducted systems hide everything out of sight. One outdoor unit can also run several indoor units in different rooms.

In almost all cases, yes. The indoor unit can’t do its job without the outdoor one, because the heat it removes has to go somewhere. A few specialist systems avoid an outside box, but they suit a narrow range of properties.

Portable units are a stopgap for short-term needs, whereas a fitted system is permanent, tidier and quieter. Fitted systems also cool more effectively and cost less to run for the same result.

No. A unit is specified for one room and will only properly cool that space. For several rooms you’d need a multi-room system.

There’s no single winner. The well-known manufacturers all make reliable equipment, and the right choice comes down to your room, your budget and the warranty on offer.

It varies with the number of rooms, the type of unit, how far the pipework has to run and how easy the access is. A survey is the only way to get an accurate figure.

Expect an engineer on site for up to a day for a single room, and two to three days for a whole house.

Yes, retrofits make up the majority of installations. The main job is finding a neat route for the pipework and drain between the indoor and outdoor units.

Often not for a house, but it depends on the property. Listed buildings, conservation areas and flats are the usual exceptions. Rules also differ by council, so it’s worth a quick check with your local planning department before work starts.

You’ll need permission first. Tenants need their landlord’s agreement, and leaseholders normally need written consent from the freeholder.

No. Fitting must be done by an engineer who holds F-Gas certification, because the system contains refrigerant gas that is regulated by law.