Air conditioning glossary

127 terms you’ll meet in quotes, manuals, energy labels and our guides, explained in plain English.

A

A2L: A refrigerant safety class meaning lower toxicity and mild flammability. R32 is in this class, and safety standards set how systems using it are installed, including limits on how much refrigerant can be used for a room of a given size.Guide: R32 refrigerant
A3: A refrigerant safety class meaning lower toxicity and higher flammability. R290 (propane) is in this class, which limits how much can be used in room-sized systems.Guide: Natural refrigerants and R290
Acoustic barrier: A solid screen, such as a wall or solid fence, between an outdoor unit and a neighbour's window. In the MCS 020(a) noise calculation, a barrier that completely hides the unit from the window takes 10 dB off, and one that partly hides it takes 5 dB off.Guide: Noise rules and your neighbours
Air filter: A fine mesh behind the front panel of an indoor unit that catches dust from the air passing through. It usually needs cleaning every few weeks while the system is in regular use. Some units have extra filters designed to catch finer particles.Guide: What air conditioning filters do
Air source heat pump: A heat pump that takes heat from the outdoor air. It can deliver that heat to the air inside (an air-to-air system, such as a split air conditioner that heats) or to water for radiators and hot water (an air-to-water system).Guide: Air-to-air heat pumps explained
Air-to-air heat pump: A heat pump that moves heat between the outdoor air and the air inside your home. A split air conditioner that can heat as well as cool is an air-to-air heat pump. It doesn't provide hot water.Guide: Air-to-air heat pumps explained
Air-to-water heat pump: A heat pump that takes heat from the outdoor air and uses it to heat water for radiators or underfloor heating, and often a hot water cylinder, rather than blowing warm air into rooms. It is usually fitted in place of a boiler.Guide: Air-to-air heat pumps explained
Article 4 direction: A council order that removes some permitted development rights in a particular area, so work that would normally not need planning permission does. Your local council can tell you whether one applies to your home.Guide: Do you need planning permission?
Auto mode: A mode in which the system chooses between cooling and heating to hold the temperature you set. How and when it switches varies by model.Guide: The modes explained: cool, heat, dry, fan and auto
Auto restart: A feature that switches the system back on with its previous settings after a power cut.Guide: Features worth having

B

Boost mode: A setting, called powerful or turbo mode on some models, that runs the system at full output for a short time to reach the set temperature faster. It is louder and uses more electricity while it runs.Guide: Features worth having
BTU: British thermal unit, an older unit of heat. Air conditioner sizes are often quoted in BTU per hour, and 1 kW is about 3,412 BTU per hour, so a 2.5 kW unit is usually described as about 9,000 BTU.Guide: kW and BTU explained
Building control: The service that checks building work meets the Building Regulations, provided by the local council or, in some cases, an approved private body. In England and Wales, a new electrical circuit that isn't done by an electrician registered with a competent person scheme has to be notified to building control.Guide: Building regulations
Building Regulations: Legal minimum standards for building work, covering things such as structure, fire safety, energy efficiency and electrical safety. England, Wales, Scotland and Northern Ireland each have their own.Guide: Building regulations

C

Capacity: How much heat a system can move, given in kilowatts (kW) for cooling and for heating. The rated heating capacity is often a little higher than the cooling capacity, though real heating output falls as it gets colder outside.Guide: What size air conditioner do you need?
Ceiling cassette: An indoor unit set into the ceiling, with only a flat panel showing, that usually blows air out in several directions. It needs enough depth in the ceiling void to fit.Guide: Ceiling cassettes
Circuit breaker: A switch in the consumer unit that turns a circuit off automatically if too much current flows, for example because of a fault. It is often called an MCB (miniature circuit breaker).Guide: Electrical requirements
CO2 equivalent: A way of expressing a refrigerant's warming effect as the weight of carbon dioxide that would have the same effect. It is the refrigerant's weight multiplied by its GWP, so 1 kg of R32 is 0.675 tonnes CO2 equivalent.Guide: Global warming potential explained
Commissioning: The checks an installer carries out once a system is connected and before handing it over, including testing for leaks, removing air and moisture from the pipes, and confirming it cools, heats and drains properly.Guide: Commissioning and handover
Competent person scheme: A government-authorised scheme in England and Wales that lets registered tradespeople certify that their own work meets the Building Regulations. Electricians use one to self-certify notifiable electrical work, such as a new circuit.Guide: Electrical requirements
Compressor: The pump that squeezes refrigerant gas, raising its pressure and temperature so it can give off heat. In a split system it sits in the outdoor unit, and it usually uses most of the electricity the system draws.Guide: How air conditioning works
Condensate: Water that forms when humid air meets a cold coil, the way droplets form on a cold glass. The indoor unit collects it when cooling, and the outdoor unit produces it when heating, especially when defrosting.Guide: Where the water goes: condensate drainage
Condensate pump: A small pump that carries condensate away where the water can't drain downhill by gravity, for example from an indoor unit on an inside wall. It is often hidden inside the unit, in trunking or in a nearby void.Guide: Where the water goes: condensate drainage
Condenser: The coil where refrigerant gives off heat and condenses from a gas into a liquid. When cooling, this is the outdoor unit's coil; when heating, the roles swap and the indoor coil acts as the condenser.Guide: The refrigeration cycle, step by step
Conservation area: An area the council has designated for its special architectural or historic interest. In England, permitted development rules for heat pumps are stricter there, including for walls and roofs facing a road.Guide: Listed buildings and conservation areas
Consumer unit: The box holding your home's main switch, circuit breakers and other safety devices, sometimes still called a fuse box. Most air conditioning systems need their own circuit from it.Guide: Electrical requirements
COP: Coefficient of performance, the heat a system delivers divided by the electricity it uses, measured at one set of conditions. A COP of 4 means 4 kW of heat for every 1 kW of electricity. Small air-to-air systems commonly reach 3 to 5 in mild weather, and less as it gets colder.Guide: COP and EER: what efficiency means
Core hole: The round hole drilled through an outside wall to take the refrigerant pipes, cable and drain between the indoor and outdoor units. It is usually a few centimetres across and is sealed once everything is through.Guide: What happens on installation day

D

dB(A): Decibels measured with A-weighting, which adjusts the reading to reflect how human hearing responds to different pitches. Noise figures for air conditioning are almost always given in dB(A).Guide: Decibels explained
Decibel: The unit used to measure sound level, written dB. The scale is logarithmic, so a rise of 10 dB is roughly what people hear as twice as loud.Guide: Decibels explained
Dedicated circuit: A circuit that runs from the consumer unit to one appliance only. Most fixed air conditioning systems need one, and in England and Wales adding a new circuit is notifiable under Part P of the Building Regulations.Guide: Electrical requirements
Defrost cycle: A short spell in heating mode when the outdoor unit melts frost off its coil, usually by briefly reversing the flow of refrigerant. The indoor fan normally stops meanwhile, and you may see steam or water around the outdoor unit. It is normal in cold, damp weather.Guide: Why the outdoor unit defrosts in cold weather
Dew point: The temperature at which moisture in the air starts to condense into water. When air passes over a coil colder than its dew point, water forms on the coil, which is how air conditioning takes moisture out of a room.Guide: How air conditioning removes humidity
Drain pan: The tray under the indoor unit's coil that catches condensate and feeds it into the drain. A blocked or overflowing drain pan is a common cause of water dripping from an indoor unit.Guide: Water dripping from the indoor unit
Dry mode: A mode that runs the system gently to take moisture out of the air with less cooling than cool mode. The room usually still cools a little.Guide: The modes explained: cool, heat, dry, fan and auto
Ducted air conditioning: A system with the indoor unit hidden in a loft, ceiling void or cupboard, sending air through ducts to grilles in one or more rooms. Only the grilles show, but it needs space for the unit and the ductwork.Guide: Ducted air conditioning

E

Economy mode: A setting on some models that limits how hard the system works, trading a slower response for a lower power draw. What it does varies by model, so check the manual.Guide: The modes explained: cool, heat, dry, fan and auto
EER: Energy efficiency ratio, the cooling a system delivers divided by the electricity it uses, measured at one set of conditions. Higher is better. It is the cooling counterpart of COP.Guide: COP and EER: what efficiency means
Energy label: The label showing a split air conditioner's energy efficiency class, its seasonal efficiency figures (SEER for cooling and SCOP for heating) and other details such as noise levels. Higher SEER and SCOP figures mean a more efficient unit.Guide: Energy labels: SEER and SCOP explained
Error code: A short code of letters and numbers, or a pattern of flashing lights, that a unit shows when it detects a problem. Codes differ between manufacturers, so the manual or an engineer is needed to interpret one.Guide: Error codes and flashing lights
Evacuation: Pumping the air and moisture out of new refrigerant pipes with a vacuum pump before refrigerant is let into them. It is a standard step in commissioning a split system.Guide: Commissioning and handover
Evaporative air cooler: A portable appliance that blows air over water-soaked pads, cooling the air as the water evaporates. It has no refrigerant or outdoor unit and adds moisture to the room. It works best in dry heat and does less in humid weather.Guide: Evaporative air coolers
Evaporator: The coil where refrigerant absorbs heat and evaporates from a liquid into a gas. When cooling, this is the indoor unit's coil; when heating, it is the outdoor unit's coil.Guide: The refrigeration cycle, step by step
Expansion valve: The part that lets high-pressure liquid refrigerant through to the low-pressure side of the circuit, where it becomes very cold and can absorb heat. In inverter systems it is usually electronically controlled.Guide: The refrigeration cycle, step by step

F

F-gas: Short for fluorinated greenhouse gas, the group that includes HFC refrigerants such as R32 and R410A. In Great Britain, F-gas rules cover how equipment containing them is installed, serviced and dismantled, and who may do that work.Guide: F-gas rules for homeowners
F-gas certificate: Proof that a person or business may work on equipment containing F-gas refrigerants. In Great Britain, engineers need the right F-gas qualification, and a business doing the work needs a company certificate from a certification body such as REFCOM.Guide: Why installation needs a certified engineer
Fan mode: A mode that only moves air around the room, with no cooling or heating. It uses much less electricity than cooling but doesn't lower the temperature.Guide: The modes explained: cool, heat, dry, fan and auto
Fan speed: How fast the indoor unit's fan runs, usually selectable as low, medium, high or auto. Lower speeds are quieter, while higher speeds move more air and reach the set temperature faster.Guide: Directing the airflow
Fins: The thin metal plates on a coil that give it more surface for moving heat. They bend easily, and flattened fins block the airflow, so cleaning a coil is best left to an engineer.Guide: Looking after the outdoor unit
Flare connection: A joint made by widening the end of a copper refrigerant pipe into a cone and tightening it onto a fitting with a nut. It is the usual way pipes connect to a split system's units, and it has to be made carefully to avoid leaks.Guide: Refrigerant pipework explained
Floor-standing unit: An indoor unit that sits at floor level rather than high on a wall, most often a low console unit about the height of a radiator. It suits rooms without space high on the wall, and it releases warm air low down, which helps when heating.Guide: Floor-standing and console units
Frost protection mode: A setting on some models that holds a room at a low temperature, often around 8 to 10°C, to stop it getting too cold while the home is empty.Guide: Features worth having

G

Global warming potential: GWP, a measure of how much a gas warms the planet over a set period, usually 100 years, compared with the same weight of carbon dioxide, which has a GWP of 1. The GB F-gas rules use a value of 675 for R32 and 2,088 for R410A.Guide: Global warming potential explained
Ground stand: A support that holds an outdoor unit up off the ground, from low rubber or plastic blocks to a taller steel frame, on a firm, level base such as paving slabs or a concrete pad. It keeps the unit clear of standing water and snow and lets defrost water drain away.Guide: Wall brackets and ground stands

H

Habitable room: In the MCS 020(a) noise standard, a room used for living in, such as a living room, bedroom, dining room, study or kitchen-diner. Bathrooms, toilets, halls, landings, utility rooms and kitchens used only for cooking don't count.Guide: Noise rules and your neighbours
HCFC: Hydrochlorofluorocarbon, an older type of refrigerant that damages the ozone layer. R22 is an HCFC, and it has not been allowed for servicing equipment since 2015.Guide: Older refrigerants: R22 and R410A
Heat exchanger: A part that moves heat between refrigerant and air, usually a coil of copper pipe with thin metal fins. A split system has one in the indoor unit and one in the outdoor unit.Guide: How air conditioning works
Heat gain: Heat added to a room by sunshine, warm outside air, people, lighting and equipment. It is what a system has to remove when cooling, so it largely decides the size needed for cooling.Guide: What size air conditioner do you need?
Heat loss: The rate at which a room loses heat to the outside through its walls, windows, roof and draughts. It is what a system has to replace when heating, and it rises as the weather gets colder.Guide: What size air conditioner do you need?
Heat mode: The mode in which an air conditioner runs its refrigeration cycle in reverse, taking heat from the outdoor air and releasing it indoors. The outdoor unit gets cold and may need to defrost in cold, damp weather.Guide: How air conditioning heats a room
Heat pump: A machine that moves heat from one place to another using a refrigeration cycle, rather than making heat by burning fuel. Air conditioners, fridges and air source heat pumps all work this way.Guide: Air-to-air heat pumps explained
HFC: Hydrofluorocarbon, the group of refrigerants that includes R32 and R410A. HFCs don't damage the ozone layer, but many have a high GWP, and rules on high-GWP refrigerants are tightening.Guide: Refrigerants explained
Hot start: A feature that holds back the indoor fan when heating starts, or after a defrost, until the coil is warm, so the unit doesn't blow cold air into the room.Guide: How air conditioning heats a room

I

Indoor unit: The part of a split system inside your home. It holds a coil, a fan and a filter, blows cooled or heated air into the room and collects condensate when cooling.Guide: What the indoor and outdoor units do
Inverter: Electronics that vary the compressor's speed, so a system can run gently to hold a temperature instead of switching fully on and off. Most split systems sold in the UK use one.Guide: What an inverter does
Isolator switch: A switch that cuts the power to one piece of equipment so it can be worked on safely. One is commonly fitted near the outdoor unit.Guide: Electrical requirements

K

kW: Kilowatt, a unit of power equal to 1,000 watts. Air conditioner specifications use kW both for capacity, the heat moved, and for power input, the electricity used, which are different figures.Guide: Power and energy: kW and kWh
kWh: Kilowatt-hour, the energy used by drawing 1 kW for one hour. Electricity is billed per kWh.Guide: Power and energy: kW and kWh

L

Latent cooling: The part of an air conditioner's work that takes moisture out of the air, as opposed to lowering its temperature. It helps a cooled room feel less clammy.Guide: How air conditioning removes humidity
Lawful Development Certificate: A certificate from the local council confirming that a project is lawful, for example that it falls within permitted development and doesn't need planning permission. It isn't compulsory, but it gives certainty.Guide: Do you need planning permission?
Leak check: An inspection of a system for refrigerant leaks. Under the GB F-gas rules, regular leak checks are mandatory only for systems holding 5 tonnes CO2 equivalent or more, far more than a typical home split holds, though a service may still include one.Guide: F-gas rules for homeowners
Line set: The pair of insulated copper pipes that carry refrigerant between the indoor and outdoor units, one for liquid and one for gas. It is usually run together with the cable and drain.Guide: Refrigerant pipework explained
Listed building consent: Permission from the council for work to a listed building, inside or out, that affects its character. Permitted development doesn't cover listed buildings, so talk to the council before fitting air conditioning to one.Guide: Listed buildings and conservation areas
Louvre: The adjustable blade across the outlet of an indoor unit that directs the airflow up or down. Many units also have vertical vanes that steer it left or right.Guide: Directing the airflow

M

Maximum pipe length: The longest refrigerant pipe run a manufacturer allows between the indoor and outdoor units. Every model also has a limit on the height difference between them.Guide: Refrigerant pipework explained
MCS 020(a): The noise standard an air source heat pump installation must meet to count as permitted development in England. Noise from the outdoor unit must be no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home.Guide: Noise rules and your neighbours
Multi-split system: A system with one outdoor unit serving two or more indoor units in different rooms. Each room can usually be set and switched separately, but units running at the same time normally all have to be heating or all cooling.Guide: Multi-split systems

N

Natural refrigerant: A refrigerant that occurs in nature, such as propane (R290), carbon dioxide or ammonia. They have very low GWPs, but some are flammable or toxic, which limits where and how much they can be used.Guide: Natural refrigerants and R290

O

Occupancy sensor: A sensor on some indoor units that detects whether people are in the room, so the unit can turn its output down when the room is empty or steer air towards or away from people.Guide: Features worth having
Operating range: The outdoor temperatures a system is designed to cool or heat in, listed in its specifications. The lowest temperature it can heat in varies by model, and heating output and efficiency fall as it gets colder.Guide: Using air conditioning to heat in winter
Outdoor unit: The part of a split system outside, holding the compressor, a coil and a fan. When cooling, it releases heat from indoors into the outside air; when heating, it collects heat from the outside air.Guide: What the indoor and outdoor units do

P

Part P: The part of the Building Regulations in England and Wales that covers electrical safety in homes. A new circuit, which most air conditioning needs, is notifiable, so it must be done by an electrician registered with a competent person scheme or notified to building control.Guide: Electrical requirements
Permitted development: Rights that allow some building work without a planning application, as long as set conditions are met. In England, air source heat pumps, including air-to-air systems, can usually be fitted this way, but an air-to-air system must not be used solely for cooling.Guide: Do you need planning permission?
Pipe insulation: The foam sleeve around each refrigerant pipe, which limits heat gain and loss and stops cold pipes dripping with condensation. It should be intact along the whole run, and outdoors it is usually covered or wrapped to protect it from sunlight and weather.Guide: Refrigerant pipework explained
Planning permission: Formal approval from the local council for building work. In England, an air-to-air heat pump can usually be fitted without it if it meets the permitted development conditions, which include not being used solely for cooling. Scotland, Wales and Northern Ireland have their own rules.Guide: Do you need planning permission?
Portable air conditioner: A self-contained unit that stands in the room and sends hot air outside through a hose, usually via a window. It needs no installation, but its compressor is in the room with you, so it is usually louder than a split system.Guide: Portable air conditioners
Pre-charged: Supplied with enough refrigerant in the outdoor unit for a set length of pipe, often somewhere between 5 and 15 metres depending on the model. Longer runs need extra refrigerant added on site.Guide: Refrigerant pipework explained
Pressure test: A test in which new pipework is filled with a dry gas, usually nitrogen, under pressure and watched for any drop, to show there are no leaks before refrigerant is let in.Guide: Commissioning and handover
Pump down: Moving a system's refrigerant into the outdoor unit and closing its valves, so the pipes can be disconnected without releasing it. It is used, for example, when a unit is being moved to be used again, and it is a job for a qualified engineer.Guide: Removing and disposing of an old system

Q

Quiet mode: A setting that reduces noise by running the indoor fan at its lowest speed or, on some models, by limiting the outdoor unit's fan and compressor speeds. The system may cool or heat less while it is on.Guide: Sleep and quiet modes

R

R22: An older HCFC refrigerant that damages the ozone layer, with a GWP of 1,810. It has not been allowed for servicing equipment since 2015, so a system that uses it can't be topped up with R22.Guide: Older refrigerants: R22 and R410A
R290: Propane used as a refrigerant, with a GWP of 3. It is flammable (safety class A3), which limits how much can be used in room-sized systems.Guide: Natural refrigerants and R290
R32: A refrigerant widely used in split air conditioners, with a GWP of 675. It is mildly flammable (safety class A2L), and a typical home split holds roughly 0.5 to 1.5 kg of it.Guide: R32 refrigerant
R410A: An HFC refrigerant used in many older split systems, with a GWP of 2,088. Newer split systems have largely moved to refrigerants with a lower GWP, such as R32.Guide: Older refrigerants: R22 and R410A
Rating plate: The label on a unit, usually on the side of the outdoor unit, showing its model, electrical supply, refrigerant type and how much refrigerant it holds.Guide: Refrigerants explained
Recirculated air: Air drawn from a room, cooled or heated, and blown back into the same room. Most wall-mounted split systems work this way and don't bring in fresh air from outside, so they don't replace ventilation.Guide: Common air conditioning myths
Refrigerant: The fluid sealed inside an air conditioner that carries heat, absorbing it as it evaporates and releasing it as it condenses. It must never be released into the air.Guide: Refrigerants explained
Refrigerant charge: The amount of refrigerant in a system, given in kilograms on the rating plate. Extra refrigerant may be added on site where the pipe run is longer than the pre-charge covers.Guide: Refrigerant pipework explained
Refrigerant recovery: Removing refrigerant from a system into a sealed cylinder with special equipment, so none escapes. A qualified engineer must do it before a system is removed or scrapped.Guide: Removing and disposing of an old system
Refrigeration cycle: The continuous loop in which refrigerant is compressed, gives off heat as it condenses, passes through an expansion valve and absorbs heat as it evaporates. It is how air conditioners, heat pumps and fridges move heat.Guide: The refrigeration cycle, step by step
Relative humidity: How much moisture the air holds compared with the most it could hold at that temperature, given as a percentage. Warm air can hold more moisture than cold air.Guide: Humidity and comfort
Remote control: The handset that sends settings to the indoor unit, usually by infrared. Some systems use a wired controller on the wall instead or as well, and many can also be run from a phone app.Guide: Wi-Fi, apps and smart controls
Reverse cycle: Describes air conditioning that can run its refrigeration cycle backwards to heat as well as cool. Most split systems sold for UK homes are reverse cycle.Guide: How air conditioning heats a room
Reversing valve: The valve in the outdoor unit that changes the direction of refrigerant flow, switching the system between cooling and heating. It also reverses the flow briefly when the outdoor unit defrosts.Guide: How air conditioning heats a room

S

SCOP: Seasonal coefficient of performance, a unit's heating efficiency averaged over a typical heating season and shown on its energy label. Higher is better.Guide: Energy labels: SEER and SCOP explained
SEER: Seasonal energy efficiency ratio, a unit's cooling efficiency averaged over a typical cooling season and shown on its energy label. Higher is better.Guide: Energy labels: SEER and SCOP explained
Self-clean function: A setting on some models that dries the inside of the indoor unit after cooling, usually by running the fan or a little heat. What it does varies by model, and it doesn't replace cleaning the filters or servicing.Guide: Musty or unusual smells
Sensible cooling: The part of an air conditioner's work that lowers the air temperature, as opposed to removing moisture.Guide: How air conditioning removes humidity
Service: A routine visit in which an engineer cleans a system and checks it is working safely and efficiently. Manufacturers typically recommend regular servicing, often yearly, and some warranties depend on it.Guide: What a service involves
Set temperature: The temperature you choose on the controller for the system to aim for. The room temperature is usually measured at the indoor unit, high on the wall, so the reading can differ from what you feel where you sit.Guide: What temperature to set
Short cycling: A system switching on and off frequently instead of running steadily. It can be a sign that a unit is too big for the room, and it wastes energy and adds wear.Guide: What size air conditioner do you need?
Sleep mode: A setting that gradually adjusts the set temperature through the night and keeps the fan quiet, so the room doesn't get too cold or too warm while you sleep.Guide: Sleep and quiet modes
Solar gain: Heat from sunshine entering a room, mainly through windows and roof lights. Rooms with large south- or west-facing windows and rooms under the roof often get the most.Guide: Why some rooms overheat
Sound power level: The total sound energy a unit gives out, a property of the unit itself that doesn't depend on distance. It is a higher number than the sound pressure level, and it is what planning noise calculations use.Guide: Decibels explained
Sound pressure level: The loudness of a sound at a particular place, such as 1 metre from a unit, which falls as you move further away. Outdoor units are typically 45 to 50 dB(A) at 1 metre.Guide: Decibels explained
Split system: Air conditioning in two parts, an indoor unit in the room and an outdoor unit outside, joined by refrigerant pipes and a cable. The indoor unit is often mounted high on a wall.Guide: Wall-mounted split systems
Standby power: The small amount of electricity a system uses while switched off at the remote but still powered, to run its electronics and, in some units, to keep the compressor warm in cold weather.Guide: Does air conditioning use a lot of electricity?
Swing: A setting that moves the louvre up and down, or the vanes side to side, continuously to spread air around the room.Guide: Directing the airflow

T

Thermostat: The sensor and control that adjusts cooling or heating to hold a set temperature. In a split system it is usually built into the indoor unit, and some remote controls have their own sensor.Guide: What temperature to set
Through-the-wall unit: An air conditioner fixed to the inside of an outside wall with no separate outdoor unit, drawing in and venting outside air through holes in the wall. Its compressor is in the room, so it is usually louder indoors than a split system.Guide: Through-the-wall units without an outdoor unit
Time-of-use tariff: An electricity tariff with different prices at different times of day, often lower overnight and higher in the early evening.Guide: Time-of-use tariffs
Timer: A setting that switches a system on or off at a time you choose. Many systems also offer weekly schedules, set on the controller or in an app.Guide: Timers and schedules
Trunking: Plastic casing fixed to a wall to cover and protect refrigerant pipes, cables and the drain where they run along the surface.Guide: Refrigerant pipework explained

W

Wall bracket: A pair of metal brackets that holds an outdoor unit on an outside wall, usually with rubber mounts to reduce vibration passing into the wall. The wall must be sound enough to take the unit's weight.Guide: Wall brackets and ground stands
WEEE: Waste electrical and electronic equipment. An old air conditioner counts as WEEE and must not go in general waste. Its refrigerant has to be recovered, never released, and for a split system a qualified engineer does this before the units are removed.Guide: Removing and disposing of an old system
Wi-Fi control: A connection, built in or added with a small module, that lets you control a system from a phone app, including when you are away from home. Features vary by model.Guide: Wi-Fi, apps and smart controls
Window kit: The panel or fabric seal that fits a window around a portable air conditioner's exhaust hose, so the hot air goes outside. Gaps around it let warm air back in, which reduces how well the unit cools.Guide: Portable air conditioners