Air Conditioning and Home Heating Glossary

A

Air-to-air heat pump

A heat pump that heats or cools the air in individual rooms. A portable reverse-cycle unit is the smallest version; a wall-mounted split system is the more permanent one. Distinct from air-to-water, and the distinction matters for grant eligibility. See our heat pump guide.

Air-to-water heat pump

A boiler replacement. A large outdoor unit that heats a whole home’s radiators, underfloor heating and hot water cylinder. This is the category UK government schemes such as the Boiler Upgrade Scheme are designed to support, and it requires an MCS-certified installer.

Article 4 Direction

A planning tool a council can use to remove permitted development rights in a defined area. If one applies to your property, work that would normally be permitted needs a planning application. Common in conservation areas. See our listed buildings and conservation areas guide.

B

Balancing

Adjusting the lockshield valves across a heating system so that flow is distributed properly and every radiator heats evenly. The usual answer when one room never warms up despite the radiator being clear of air and the valve working.

Bleeding (a radiator)

Releasing trapped air from the top of a radiator. Fixes a radiator that’s cold at the top and warm at the bottom. A radiator that’s cold at the bottom has a different problem — sludge — which bleeding won’t touch. See our bleeding guide.

BTU (British Thermal Unit)

The measure of an air conditioner’s cooling capacity per hour. Higher means more capacity, but bigger isn’t better — an oversized unit cools the air quickly, shuts off, and doesn’t run long enough to remove much moisture, leaving a room that’s cold but clammy. See our BTU sizing guide.

C

Carbon monoxide (CO)

A colourless, odourless gas produced when fuel doesn’t burn completely. You cannot smell it. Fit a working CO alarm if you have any fuel-burning appliance, and never block ventilation serving one.

Compressor

The pump at the heart of a refrigeration circuit, and the part that makes most of the noise in a portable unit. In a mono-block portable it sits inside the unit, which is why a fixed split system is always quieter indoors.

Condensate

The water an air conditioner removes from the air as it cools it. Collected in a tank, evaporated out through the exhaust, or drained away continuously. On a boiler, condensate is the acidic water a condensing boiler produces, drained through a pipe that commonly freezes in winter.

Conservation area

An area designated for its special architectural or historic character. Permitted development rights generally still apply but with extra conditions — for air conditioning, an outdoor unit must not go on a wall or roof fronting a highway.

Continuous drainage

Running a hose from a unit’s drain port to a drain or sink, so it never fills a tank and never shuts off. Generally more useful than a “self-evaporating” claim, and the specification worth looking for. See our drainage guide.

D

Defrost cycle

On a heat pump in heating mode, a brief automatic reversal that clears ice from the outdoor-facing coil. It blows cool air for a minute or two. Designed behaviour, not a fault.

Desiccant dehumidifier

A dehumidifier that passes air over a moisture-absorbing material rather than a cold coil. Works well in cold spaces, unlike compressor models, so it’s the right choice for a garage, cellar or unheated room. See our dehumidifier guide.

Dry mode

A setting that maximises moisture removal per unit of cooling. Not a separate dehumidifier — it’s the same refrigeration cycle tuned differently, which is why it doesn’t work in a cold room. See our dry mode comparison.

E

Energy efficiency rating

How much cooling or heating a unit delivers per unit of electricity, expressed as a letter class. An A-rated unit costs less to run than a lower-rated one of the same capacity.

Evaporative cooler (air cooler)

A fan blowing air over a wet pad. No compressor, no exhaust hose, and it removes no heat from the room — it trades temperature for humidity. Works in dry climates; works poorly in a muggy British summer. See our full comparison.

Evaporator coil

The cold coil inside an air conditioner that room air passes over. Where moisture condenses out, and where ice forms if the room is too cold or airflow is restricted.

F

F-Gas certification

A qualification required by law for anyone working on a system’s refrigerant circuit. There is no householder exemption. See our installation law guide.

Flow temperature

The temperature of water leaving a boiler to the radiators. Condensing boilers run more efficiently at lower flow temperatures, and many are installed set higher than necessary.

H

Heat health alert

The yellow, amber and red warning system issued by UKHSA with the Met Office, indicating expected health impacts from hot weather. See our alerts guide.

Humidistat

The control that lets a dehumidifier hold a target humidity and cycle off, rather than running continuously. Around 40–60% relative humidity is the usual comfortable indoor range.

L

Lawful Development Certificate

A council document confirming in writing that work is permitted development. Not compulsory, but worth having in a conservation area — it removes the risk of an enforcement dispute and reassures a buyer’s solicitor later.

Listed building

A building on the national list for its special architectural or historic interest. Protection covers the whole building including the interior. Unauthorised work is a criminal offence, not merely a planning breach.

M

Mono-block

A unit where the entire refrigeration circuit, including the compressor, is in one box. All portable air conditioners are mono-block, which is why they’re noisier indoors than split systems.

O

Operative temperature

Roughly an average of air temperature and the temperature of surrounding surfaces. The reason a room can read 20°C and still feel cold — your body radiates heat to cold walls and windows regardless of what the thermostat says. See our explainer.

P

Part P

The section of the Building Regulations covering electrical safety in dwellings in England and Wales. Certain fixed electrical work is notifiable; a registered electrician can self-certify.

Permitted development

Planning rules allowing certain works without a formal application, subject to conditions on size, siting and noise. Rights are more limited for flats, removed for listed buildings, conditioned in conservation areas, and can be withdrawn by an Article 4 Direction. Check the current conditions on the Planning Portal rather than relying on a figure quoted second-hand — published sources disagree.

Purge ventilation

Opening windows wide for a short period to exchange the air in a room quickly — as opposed to leaving a window ajar all day. The right way to shed stored heat overnight, and to clear moisture after a shower.

R

R290

A refrigerant used in many modern units, with a much lower global warming potential than older alternatives. It is flammable, which is why charge sizes are limited and why you should keep a leaking unit away from ignition sources.

Reverse cycle

A unit with a reversing valve, able to move heat into a room as well as out of it. The same thing as an air-to-air heat pump. Look for a separately stated heating output — “4-in-1” often means cool, fan, dehumidify and timer rather than heating.

S

Self-evaporating

A unit that recycles condensate onto the hot condenser and expels it as vapour through the exhaust. Reduces tank emptying but doesn’t eliminate it — in humid conditions the reserve tank still fills and the unit shuts off.

Single-hose vs dual-hose

A single-hose unit draws its condenser air from inside the room, putting the room under slight negative pressure so it pulls warm outside air in through every gap. A dual-hose unit draws that air from outside instead. See our comparison.

Split system

A fixed air conditioner with an indoor unit and a separate outdoor condenser, connected by refrigerant pipework. Quieter indoors than any portable, but needs professional installation and may need permissions.

Stack effect

Warm air rising through a building, typically up a staircase, drawing cooler air along the floor below to replace it. Why upstairs is hotter in summer, and why a door at the foot of the stairs is one of the most useful doors in a house. See our explainer.

Sudden stratospheric warming (SSW)

A rapid warming high in the atmosphere that can disrupt the polar vortex and, weeks later, bring cold outbreaks to the UK. The genuine mechanism behind most severe British cold snaps. See our polar vortex guide.

T

Thermal bridging

A path where heat bypasses insulation — typically at junctions, around joists, or where a floor meets a wall. Where cold spots and condensation appear first.

Thermostatic radiator valve (TRV)

A valve letting you set each radiator independently. The single biggest heating control improvement for most households — and they seize regularly, which is the commonest cause of a room that won’t heat. Pull off the plastic head and check the pin moves freely before assuming anything is broken.

Tropical night

A night where the temperature doesn’t fall below 20°C. Increasingly common in the UK, and the reason overnight cooling matters more than daytime peaks — the body needs cooler nights to recover. See our tropical nights guide.

W

Window kit

The panel and seal that fits an open window so the exhaust hose can vent outside. Almost always fits approximately as supplied, and sealing the remaining gaps properly is the highest-value free improvement to any portable unit. See our seal kits guide.

Sources

• GOV.UK — Fluorinated gases (F-gases): rules
• Planning Portal — Do you need permission?
• GOV.UK — Approved Document P: Electrical safety
• GOV.UK — Approved Document F: Ventilation
• GOV.UK — Boiler Upgrade Scheme
• Energy Saving Trust — Thermostats and heating controls
• Met Office — Heat-Health Alerts
• Gas Safe Register — Carbon monoxide safety