Air conditioning guides

What’s going on inside the units, in plain English.

• 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.
• What the indoor and outdoor units doThe indoor unit pulls room air through a filter and across a coil that cools or warms it. The outdoor unit holds the compressor that pumps the refrigerant, plus a second coil and fan that release heat outside when cooling and collect it when heating.
• The refrigeration cycle, step by stepRefrigerant boils in a cold coil, taking in heat; a compressor squeezes the vapour, making it hot; it condenses in a second coil, giving the heat away; and an expansion valve drops its pressure so it can boil again.
• What an inverter doesAn inverter lets the compressor change speed. Instead of switching on and off, the system runs hard to reach the setting, then slows down to match what the room needs, holding a steadier temperature with less noise.
• How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
• Air-to-air heat pumps explainedAn air-to-air heat pump collects heat from outdoor air and delivers it to your rooms as warm air, usually through wall-mounted units. It is the same equipment as air conditioning that can heat, so it also cools in summer, but it can't heat water.
• Why the outdoor unit defrosts in cold weatherIn heating mode the outdoor coil runs colder than outdoor air, so in damp weather near or below freezing, moisture freezes onto it. Every so often the system briefly reverses to send heat to the outdoor unit and melt the ice, which is normal.
• How air conditioning removes humidityAir conditioning removes humidity as it cools. Room air passes over a coil colder than its dew point, so water vapour condenses on the fins, drips into a tray and drains away through a hose.

The systems you’ll come across, and where each one fits.

• Wall-mounted split systemsA wall-mounted split system pairs an indoor unit fixed high on a wall with an outdoor unit outside, joined by refrigerant pipes. It is a common choice for fixed air conditioning in UK homes, and most models heat as well as cool.
• Multi-split systemsA multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.
• Ducted air conditioningDucted air conditioning hides the indoor unit in a loft, ceiling void or cupboard, and ducts carry the air to grilles in the rooms. Only the grilles are visible, but it needs space and usually more building work than a wall-mounted system.
• Ceiling cassettesA ceiling cassette is a split system's indoor unit set into the ceiling, with only a flat panel showing. It suits larger and open-plan rooms with space above the ceiling, but in most houses fitting one means building work.
• Floor-standing and console unitsFloor-standing and console units are split-system indoor units that sit at floor level rather than high on a wall. They suit rooms with no suitable high wall and are well placed for heating, but they take up floor-level space.
• Portable air conditionersA portable air conditioner is a self-contained unit on wheels that cools a room and sends the heat outside through a hose, usually out of a window. It needs no professional installation, but it is noisier and less effective than a fixed split system.
• Through-the-wall units without an outdoor unitThrough-the-wall air conditioners keep all their parts in one indoor unit that draws and expels outside air through two holes in an outside wall. They need no outdoor unit, but are noisier indoors and usually less efficient than a split system.
• Evaporative air coolersAn evaporative air cooler blows air through a wet pad so that evaporating water cools it. It uses little electricity and needs no hose or installation, but it isn't air conditioning, adds humidity and does little on humid days.
• Fixed or portable: which is right for youA fixed split system cools better, runs more quietly and can heat efficiently, but needs professional installation and somewhere for the outdoor unit. A portable suits occasional hot spells, renters and homes where a fixed system isn't possible.

Sizes, ratings, noise and features, and the questions worth asking.

• 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.
• kW and BTU explainedkW and BTU per hour both measure an air conditioner's capacity, meaning how much heat it can move. One kW is about 3,412 BTU per hour, so a 3.5 kW unit is roughly a 12,000 BTU unit.
• Energy labels: SEER and SCOP explainedSEER and SCOP are the seasonal efficiency ratings on an air conditioner's energy label, for cooling and heating. Each shows roughly how much heat the unit moves over a standard season for every unit of electricity it uses, and higher is better.
• How loud is air conditioning?Quiet indoor units are often around 19–26 dB(A) on their lowest fan speed and louder at higher speeds, while outdoor units are typically 45–50 dB(A) at 1 metre. How loud it seems also depends on where the units are and how hard they're working.
• Decibels explainedA decibel (dB) is a unit on a logarithmic scale, so adding 10 dB means ten times the sound energy and sounds roughly twice as loud. Air conditioner figures are usually in dB(A), which reflects how sensitive hearing is to different pitches.
• Features worth havingThe features that make the most day-to-day difference are an inverter, heating that copes with cold weather, a genuinely quiet lowest fan speed, good airflow control, useful timers and easy-to-clean filters. Many extras are pleasant rather than essential.
• What air conditioning filters doAir conditioning filters catch dust and fibres before they reach the indoor unit's coil and fan, keeping air flowing freely and the inside of the unit cleaner. Most are washable mesh screens that need cleaning every few weeks in regular use.
• Wi-Fi, apps and smart controlsMany air conditioners can be controlled from a phone app over your home Wi-Fi, either built in or through an add-on adapter. An app lets you switch on, change settings and set schedules from anywhere, while the remote usually still works as normal.
• Which rooms benefit mostBedrooms usually benefit most, because heat makes sleeping hard, followed by top-floor and loft rooms, home offices and sunny living spaces. The best candidates are rooms you use a lot that regularly overheat, rather than every room in the house.
• Where to put the indoor unitA wall-mounted indoor unit usually goes high on a wall with clear space around it, where its airflow can reach the whole room without blowing straight at people. The best spot also depends on the route for the pipes and where the condensate can drain.
• Questions to ask an installerAsk how they sized the system, where both units and the pipes will go, whether it meets planning and noise rules, what the quote includes, which F-gas and electrical qualifications they hold, and what warranty and servicing come with it.

What it costs to run, and how to keep that down.

• 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.
• Power and energy: kW and kWhA kilowatt (kW) measures power, the rate at which energy is used or delivered at any moment. A kilowatt-hour (kWh) measures energy, the amount used over time, and it's what your electricity bill charges for.
• COP and EER: what efficiency meansCOP (heating) and EER (cooling) compare the heat a unit moves with the electricity it uses, under one set of test conditions. A COP of 3 means 3 kW of heat for every 1 kW of electricity; higher is more efficient.
• Heating with air conditioning compared with gasWhich costs less depends on your prices and equipment. Work out the cost of a kWh of heat from each: gas unit rate divided by boiler efficiency, and electricity unit rate divided by the air conditioner's COP.
• How to cut running costsSet a moderate temperature, stop heat getting in (or out, in winter), run the unit only where and when you need it, and keep the filters and outdoor unit clean. Your tariff matters too.
• Air conditioning and solar panelsSolar panels can supply some or all of an air conditioner's electricity while they're generating, which suits cooling on sunny days far better than heating in winter. Solar electricity you use yourself costs you any export payment you'd otherwise have earned.
• Time-of-use tariffsTime-of-use tariffs charge different unit rates at different times of day. Whether one suits you depends on when your air conditioning and the rest of your home use electricity, and how those rates compare with what you pay now.
• Does air conditioning use a lot of electricity?Less than its size suggests: a wall-mounted unit's kW rating is the heat it moves, and it usually draws a fraction of that in electricity. The total depends mainly on how many hours it runs and how hard it has to work.

What fitting involves, from the first hole to the handover.

• What happens on installation dayInstallers fix the indoor and outdoor units, link them with insulated pipes through a hole in the wall, test the pipework for leaks and clear it of air, then connect the power, run the system and show you how to use it.
• How long installation takesA straightforward single-room installation usually takes half a day to a full day. Longer or hidden pipe routes, extra rooms, electrical work and difficult access can each add time, sometimes a day or more.
• Preparing your home for installationClear space around where the indoor and outdoor units will go, make sure the engineers can reach the consumer unit and anything on the pipe route, and plan for the power being off for a while.
• Refrigerant pipework explainedTwo insulated copper pipes carry refrigerant between the indoor and outdoor units. They are joined at each unit, tested for leaks and cleared of air before use, and their length and route affect how the job looks and whether extra refrigerant is needed.
• Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.
• Electrical requirementsMost wall-mounted systems need their own circuit from the consumer unit, often with an isolator switch near the outdoor unit. In England and Wales a new circuit is notifiable building work, so it's done by a registered electrician or notified to building control.
• Where the outdoor unit can goAn outdoor unit needs a firm fixing, free airflow, service access and somewhere for defrost water to drain. On a house in England it can usually go on a wall, the ground or a flat roof without planning permission if set conditions are met.
• Wall brackets and ground standsWall brackets fix the outdoor unit to a sound wall and keep the ground clear, while ground stands hold it on a firm, level base. Both should raise it clear of water, snow and debris and sit it on anti-vibration mounts.
• Commissioning and handoverCommissioning is the set of tests that prove a new system is sealed, safe and working as designed. Handover is when the engineer shows you how to use and look after it and gives you the records and certificates to keep.
• Why installation needs a certified engineerA split system is joined into a sealed refrigerant circuit on site, and in Great Britain anyone doing that work needs the right F-gas qualification, with the business holding a company F-gas certificate. Poor work can cause leaks, damage and safety problems.

Modes, settings and habits that get the best from it.

• The modes explained: cool, heat, dry, fan and autoCool lowers the temperature and takes out some moisture, heat warms the room, dry mainly removes moisture with gentle cooling, fan only moves air, and auto chooses between cooling and heating for you.
• What temperature to setAround 23–25°C for cooling and 20–22°C for heating are sensible starting points, adjusted a degree at a time to suit you. A setting far lower or higher than you want won't get the room there much faster.
• Sleep and quiet modesSleep mode usually shifts your setting by a degree or two over the first hours of the night, warmer when cooling and cooler when heating, and turns the fan down. Quiet mode slows the fan to cut noise, at the cost of some output.
• Timers and schedulesMost systems have on and off timers that count down a set number of hours, and many controllers and apps add clock times and weekly schedules. Used well, they have a room comfortable when you need it and the system off when you don't.
• Directing the airflowPoint the airflow level or slightly up when cooling, because cool air sinks, and down when heating, because warm air rises. Avoid aiming it straight at people, and use swing if the air isn't reaching the whole room.
• Using air conditioning to heat in winterChoose heat mode, point the airflow down and give the room time to warm. Most systems keep heating well below freezing, though less efficiently, and pause now and then to defrost the outdoor unit, which is normal.
• Using air conditioning in a heatwaveStart cooling before your home heats up, keep windows, doors and sun-facing blinds closed, and let the system run steadily at a moderate setting. On the hottest afternoons it may run at full output for hours, which is normal.
• 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.
• Doors, windows and blindsKeep doors and windows shut while the system runs, and close blinds or curtains on sunny windows in warm weather. In winter, let the sun in by day and close curtains at dusk.

Looking after your system, and what to do when something seems wrong.

• How to clean the filtersSwitch the unit off, lift the front panel and take out the mesh filters, then vacuum or rinse them in lukewarm water and let them dry fully before refitting. In regular use, this usually needs doing every few weeks.
• What a service involvesA service is a clean and check of the whole system, indoors and out, plus tests of how it runs and a look for leaks and wear. Manufacturers typically recommend one regularly, often once a year.
• Looking after the outdoor unitKeep the space around it clear so air flows freely, brush off leaves and snow with the system switched off, and leave cleaning the coil and anything inside the casing to an engineer. Otherwise it needs little beyond its regular service.
• Water dripping from the indoor unitWater in the room usually means the moisture the unit collects isn't draining away, often because of a blocked drain or dirty filters. Clean the filters and check the drain outlet, and call an engineer if it carries on.
• Ice on the pipes or outdoor unitA thin layer of frost on the outdoor unit while it heats in cold, damp weather is normal and clears when it defrosts. Ice that stays or keeps building, or that appears while cooling or on the pipes, points to a fault.
• Musty or unusual smellsA musty smell usually comes from damp dirt on the coil, drain tray or fan, and a dusty smell when heating first starts usually fades. A burning or electrical smell means switching the system off straight away.
• Noises: what's normal and what isn'tSoft airflow, light clicks as plastic warms and cools, and hisses or gurgles as refrigerant and water move are all normal, as is a whoosh during defrost. Rattling, grinding, squealing, scraping, banging or loud buzzing usually mean something is loose, worn or failing.
• Not cooling or heating properlyStart with the simple causes: the mode and setting on the remote, timers, dirty filters, open windows and a blocked outdoor unit. If those are fine and it still struggles, it may have a fault or leak that needs an engineer.
• Error codes and flashing lightsError codes and flashing lights usually show that the system has detected a problem, but they mean different things on different makes and models. Look the code up using your model number, and give it to the engineer if you need one.
• How long air conditioning lastsA well-installed, regularly serviced wall-mounted system typically lasts around 10 to 15 years, and some keep going longer. How long yours lasts depends on installation quality, how hard it works, where the outdoor unit sits and how it's looked after.
• When to call an engineerCall an engineer when simple checks don't fix a problem, and switch off straight away for burning smells, smoke, a circuit breaker that keeps tripping or signs of a refrigerant leak. Refrigerant, electrics and anything inside the casing are work for a qualified engineer.

Planning, permissions and regulations, explained.

• Do you need planning permission?Usually not in England, where air-to-air heat pumps that aren't used only for cooling count as permitted development if conditions on number, size, position and noise are met. Listed buildings, flats and protected areas follow different rules.
• Noise rules and your neighboursIn England, a heat pump fitted without planning permission must be no louder than 37 dB(A) at 1 metre from a neighbour's nearest habitable-room window or door, worked out with the MCS 020(a) calculation. Councils can also investigate noise complaints separately.
• Listed buildings and conservation areasFitting air conditioning to a listed building needs listed building consent, and permitted development doesn't cover it. In England, conservation areas and other protected areas still have permitted development, but with stricter rules, including on walls and roofs facing a road.
• Flats and leasehold homesIn a flat or leasehold home, fitting air conditioning usually needs written consent from the freeholder under the lease, and the planning rules for flats differ from those for houses. Freeholder consent and planning are separate questions.
• Air conditioning in rented homesTenants usually need the landlord's written permission before fitting fixed air conditioning, and a flat's landlord may need the freeholder's consent too. Portable units usually don't need permission, as long as nothing is drilled or altered.
• Building regulationsThe part of the building regulations that most often affects an air conditioning installation is the electrical work. In England and Wales a new circuit is notifiable, so it's done by a registered electrician or notified to building control.
• F-gas rules for homeownersF-gas rules in Great Britain require anyone installing, servicing or removing a split system to hold the right qualification, and ban releasing refrigerant. A typical home system is well below the size that needs mandatory leak checks.
• Selling a home with air conditioningWhen you sell, buyers and their solicitors are likely to ask about planning, electrical certificates, servicing and warranties for a fixed system. Keeping the installation paperwork together lets you answer those questions quickly and accurately.

What’s in the pipes, and the bigger picture.

• Refrigerants explainedA refrigerant is the fluid that carries heat around an air conditioner, boiling to absorb it and condensing to release it. Refrigerants differ in their pressures, efficiency, flammability and effect on the climate, which is why the choice matters.
• R32 refrigerantR32 is the refrigerant in most new wall-mounted systems. It carries heat efficiently, usually needs less of it than R410A, and has about a third of R410A's global warming potential. It is mildly flammable, so it's handled with specific precautions.
• Global warming potential explainedGlobal warming potential (GWP) compares how much a gas warms the climate with the same mass of carbon dioxide, usually over 100 years. A refrigerant's GWP shows how much harm a leak would do, and it underpins the rules on refrigerants.
• Older refrigerants: R22 and R410AR22 damages the ozone layer and can no longer be used to service equipment, so a leaking R22 system usually means replacement. R410A is still widely used and can be serviced, but its high global warming potential means most new systems use R32.
• Natural refrigerants and R290Natural refrigerants such as propane (R290) and carbon dioxide occur in nature and have a very low global warming potential. R290 is efficient but highly flammable, which limits how much can be used in room-sized systems.
• Air conditioning and carbon emissionsAn air conditioner's effect on the climate comes from two places: the electricity it uses, and its refrigerant if it leaks. Using less electricity, running it when the grid is cleaner and keeping the system leak-free all reduce it.
• Removing and disposing of an old systemAn old split system has to be taken out by a qualified engineer, who recovers the refrigerant before anything is disconnected. The units then go for recycling as electrical waste, and the holes and fixings left behind need making good.

Heat, humidity, sleep, and the rooms that get too warm.

• Humidity and comfortHumidity changes how warm a room feels at the same temperature. Most people are comfortable with relative humidity of roughly 40–60%; above that, warm rooms feel close and sticky, and air conditioning helps by taking moisture out as it cools.
• Sleeping in hot weatherBedrooms stay warm at night because the building gives back the heat it soaked up during the day. Keeping sun out by day, ventilating once it's cooler outside, and quiet cooling or a fan all make a warm night more comfortable.
• Why some rooms overheatA room overheats when heat gets in or builds up faster than it can escape. The usual causes are sunshine through large or west-facing windows, a roof directly above, heat from people and equipment, and too little ventilation.
• Loft rooms and top floorsLoft rooms and top floors run hotter than the rest of the house because the roof heats up in the sun, warm air rises, and roof windows catch sunshine for much of the day. Shading, ventilation, insulation and well-planned cooling help.
• Keeping a home office comfortableA home office warms up through the day from computers, screens and you, often in a small room with the door shut. Shading, ventilation and cutting equipment heat help, and quiet air conditioning suits rooms that still get too warm.
• Keeping cool without air conditioningKeep the sun off your windows during the day, open up when the air outside is cooler than inside, usually from late evening to early morning, and cut the heat produced indoors. Fans make you feel cooler, though they don't cool the room.
• Does air conditioning dry the air?Yes, in cooling and dry modes it takes moisture out, which is part of why a cooled room feels comfortable, but it rarely leaves a UK home uncomfortably dry. Heating mode removes no moisture, although warming air lowers its relative humidity.
• Common air conditioning mythsMany 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.