The short answer
Many beliefs about air conditioning don't match how modern systems work. Most wall-mounted systems heat as well as cool, don't bring in outdoor air, don't need regular regassing, and won't cool a room much faster if you set a very low temperature.
Air conditioning is less common in UK homes than in hotter countries, so many people know it mainly from offices, holiday apartments or portable units. Some of those beliefs were never true, and others describe equipment quite different from a wall-mounted split system. The common ones fall into four groups: how systems work, how they're used, what they cost, and what fitting one involves.
How systems work
"Air conditioning only cools." Most wall-mounted split systems are heat pumps that can run in reverse, moving heat into the room in winter as well as out of it in summer. Cooling-only models exist but are the exception. How air conditioning heats a room explains how.
"It brings in fresh air from outside." A split system moves heat, not air. The pipes between the units carry refrigerant, and the indoor unit recirculates the air already in the room. Most don't ventilate at all, so ventilation still comes from windows, trickle vents and extractor fans.
"It doesn't work when it's cold outside." Air-to-air heat pumps keep heating in cold weather, although their efficiency falls as the temperature drops. Many models are designed to heat well below freezing, and each has a stated operating range. In cold, damp weather the outdoor unit defrosts itself from time to time, which is normal.
"Bigger is better." An oversized system reaches the set temperature quickly, then tends to switch off and on more often. That makes the temperature less even and takes less moisture out of the air. A system sized for the room and its heat gains is more comfortable.
Settings and everyday use
"A very low setting cools the room faster." When a room is well above the setting, the system is already running at or near full output. A lower setting doesn't make it cool much faster; it just keeps cooling past the temperature you wanted and uses more electricity doing so. Many systems have a powerful or turbo mode for a quick boost.
"Opening a window helps it." Open windows and doors let warm, humid air in, so the system works harder for less effect. Keep them closed while it's running and ventilate at other times.
"It's too noisy for a bedroom." With a split system the compressor is in the outdoor unit, and quiet indoor units are often around 19–26 dB(A) on their lowest fan speed. Portable units have the compressor in the room with you, so they're usually louder.
"Filters only need cleaning at a service." Filters usually need cleaning every few weeks while the system is in regular use, and it's a quick job you can do yourself. Clogged filters reduce airflow and make the system work harder. Servicing, often recommended yearly, is separate.
Running costs and upkeep
"It always costs a fortune to run." The cost depends on how much electricity the system uses and what you pay for it, and you can work it out:
- energy used (kWh) = average power (kW) × hours running
- cost = energy used × your rate per kWh
An inverter system spends much of its time running at reduced output, so its average power is usually well below its maximum rating. The figures below are illustrative examples, not measurements of any particular system, at an example rate of 25p per kWh — check your own tariff.
| Example average power | Hours | Energy used | Cost at 25p per kWh |
|---|---|---|---|
| 0.3 kW | 8 | 2.4 kWh | 60p |
| 0.6 kW | 8 | 4.8 kWh | £1.20 |
| 1.0 kW | 8 | 8 kWh | £2.00 |
How much air conditioning costs to run goes into what affects the average.
"Leaving it on all day always uses less electricity than switching it off." Sometimes it does and sometimes it doesn't. It depends on how long you're out, how well the room holds its temperature and the weather. Leave it on or switch it off goes through the trade-offs.
"It needs regassing every year." Refrigerant circulates in a sealed circuit and isn't used up. If a system needs topping up, refrigerant is escaping, and a qualified engineer should find and repair the leak. With an F-gas refrigerant such as R32, that means one holding the right F-gas qualification.
Fitting
"It means a big hole in the wall." A split system usually needs one hole for each indoor unit, a few centimetres across, for the refrigerant pipes, cable and drain. Through-the-wall units without an outdoor unit need two much larger holes, and portable units need a window gap or a vent for their hose.
"You always need planning permission." In England, air-to-air heat pumps can usually be fitted under permitted development without planning permission, as long as the conditions are met. These include limits on where the outdoor unit goes, a noise standard, and not using an air-to-air system solely for cooling. Listed buildings, conservation areas and flats have different rules, and Scotland, Wales and Northern Ireland have their own. The planning permission guide sets out the conditions.
Related guides
- How air conditioning worksAir conditioning cools a room by moving heat out of it, not by making cold. In a typical split system, a refrigerant picks up heat from the room at the indoor unit and a compressor pumps it outside, where the outdoor unit releases it.
- Leave it on or switch it off?It depends on how long the room will be empty. For an hour or two the difference is small, so choose on comfort; for a day or longer, switching off or turning the setting back usually means less heat for the system to move.
- How much does air conditioning cost to run?Multiply the electricity it uses, in kWh, by your unit rate. How much a unit uses depends on the room, the weather, the temperature you set and its efficiency, so estimate it from its average power draw and the hours it runs.
- What size air conditioner do you need?Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.