How Does Air Conditioning Work, And Can It Heat Your Home?

The Refrigerant Loop, Step By Step

A refrigerant circulates in a sealed loop through the pipes between the two units. It boils at a low temperature, so it can take in heat from room air that feels cool to you. The whole machine works by changing the refrigerant’s pressure so it boils in one place and condenses in another (Penn State, EGEE 102, How Air Conditioners Work).

In cooling mode the loop runs in this order:

1. 1

   Evaporator, the indoor coil. Cold, low-pressure liquid refrigerant runs through the coil inside the wall unit. A fan pushes room air across it, the refrigerant takes in the room’s heat and boils into a gas, and cooler air blows back into the room.

2. 2

   Compressor, in the outdoor unit. The compressor squeezes that gas, which raises its pressure and pushes its temperature above the outdoor air. This is the step your electricity pays for.

3. 3

   Condenser, the outdoor coil. The outdoor fan blows air over the hot coil. Heat passes from the refrigerant into the outside air, and the refrigerant condenses back into a high-pressure liquid.

4. 4

   Expansion valve. The liquid forces its way through a narrow valve, its pressure drops, and it comes out cold, ready to take in more heat indoors.

Then it goes round again, for as long as the unit runs. Moisture in the room air condenses on the cold indoor coil, so a running unit dries the air as it cools it, and the water leaves through a small drain pipe.

Two Boxes And A Pipe: The Split Unit

The type fitted to a house wall is a split system. The outdoor unit holds the compressor, the outdoor coil and a fan; the indoor unit holds the indoor coil and a fan that spreads the air around the room (Daikin, How air conditioning works). Refrigerant pipes, a power cable and the condensate drain connect them through one small hole in the wall, so the unit treats the air already in the room and brings none in from outside. In a multi-split system, one outdoor unit serves indoor units in several rooms.

The Mitsubishi units we fit measure 46 to 52 dB at the outdoor unit and 19 to 21 dB indoors on quiet mode. Our guide to air-to-air heat pumps and home air conditioning covers where the outdoor unit can go and how a survey sizes the indoor units to each room.

Inverter Control And The Steady Hum

A fixed-speed compressor has two settings, flat out and off, so the room swings around its set point as the unit stops and starts. An inverter drive varies the speed of the compressor motor to match the demand: it slows as the room nears its target and ticks over to hold it (Daikin, How air conditioning works). You get a steadier temperature, less noise from outside at light load, and a compressor spared the wear of starting all day. Makers advertise energy savings for inverter units as well; the figure on your bill depends on the room and the hours you run it.

Reverse Cycle: Air Conditioning That Heats

A reverse cycle unit adds a four-way valve, called the reversing valve, that sends the refrigerant round the loop the other way. In heating mode the outdoor coil becomes the evaporator and takes heat out of the outside air, and the indoor coil becomes the condenser and gives that heat to your room. Air at 0°C still holds heat, and the refrigerant in the outdoor coil runs colder than the air around it, so heat keeps flowing into the loop on a frosty morning.

Frost builds up on the outdoor coil on cold, damp days. The unit clears it with a defrost cycle: it switches the loop to cooling flow for a short spell so hot refrigerant runs through the outdoor coil and melts the ice. The warm air from the indoor unit pauses while that happens.

That makes a reverse cycle air conditioner an air-to-air heat pump, one machine under two names. HMRC’s VAT notice on energy-saving materials describes most air conditioning units as air source heat pumps (VAT Notice 708/6, section 2.15), and the government’s Boiler Upgrade Scheme page lists the category as air-to-air heat pumps. Australian sellers market the same machine as reverse cycle air conditioning, which is the phrase you will meet on many review sites. A cooling-only unit has no reversing valve, so it pumps heat one way and stops there.

SCOP And SEER: The Efficiency Numbers On The Label

An electric panel heater turns one kilowatt hour of electricity into one kilowatt hour of heat. A heat pump moves heat it did not make, so it can deliver several units of heat for each unit of electricity it draws. COP is that ratio under one set of test conditions. SCOP, the seasonal coefficient of performance, measures it across a whole heating season, and SEER, the seasonal energy efficiency ratio, does the same job for cooling.

The UK energy label for split air conditioners turns those seasonal figures into letters (Regulation (EU) No 626/2011 as it applies in the UK, Annex II, Table 1):

Below the label sits a legal floor. A new split unit under 6 kW has to reach a SCOP of at least 3.8 and a SEER of at least 4.6 if its refrigerant has a global warming potential above 150, or 3.42 and 4.14 if the refrigerant scores 150 or less (Regulation (EU) No 206/2012 as it applies in the UK, Annex I, Table 6).

A SCOP of 4 means about four units of heat back for each unit of electricity across the label’s heating season, against one from the panel heater. The heating figure on the label uses an average European heating season. On a hard frost in the hills the unit works harder for each unit of heat, and on a mild, wet day it works less. The question we hear on the phone, “Is air conditioning really expensive?”, gets its own answer in pence per hour in our guide to how much air conditioning costs to run.

Air Conditioning Heats Room Air, And Stops There

In heating mode the indoor coil gives its heat to the air in the room and to nothing else. A standard split unit has no water circuit, so it runs no radiators and heats no cylinder. Grand Designs magazine puts it plainly in a June 2026 guide: most air-to-air systems can’t heat your hot water, and you need a separate water heater. A few newer products pair an air-to-air system with a hot water tank, Daikin’s Multiplus among them. We have not fitted one, so we can’t tell you how it performs through a Peak District winter.

The heat also stays in the rooms with an indoor unit. A wall unit in the lounge warms the lounge and some of the hall through the open door, while the bathroom, the landing and the back bedroom stay at whatever the rest of your heating gives them.

That suits a house with a working boiler for the radiators and the taps, where you want a hot loft bedroom or a home office cooled in July and the rooms you use most warmed in spring and autumn without firing up the whole system.

When An Air-To-Water Heat Pump Is The Better Route

If you want air conditioning to replace your boiler in a house with radiators and hot water to make, the answer points to a different heat pump.

An air-to-water heat pump runs the same refrigerant loop as your air conditioner with one change: the coil that gives out heat warms water instead of room air. That water runs round your radiators or underfloor pipes. The government’s Energy Technology List names heat for domestic hot water and cooling through a water loop as functions some of these units also provide (Energy Technology List, air-to-water heat pumps), with the hot water stored in a cylinder.

Four things point to the whole-house route:

Radiators decide whether the whole-house route works in your home. A radiator sized for a boiler running hot gives out less at heat pump temperatures, and our guide to heat pump radiators shows the sum room by room. The design also sets the flow temperature, which drives the running cost: our systems are designed to a target SCOP of 3.5 to 4.0 where the house allows it, with the figure your design reaches written into your quote. Install prices from government data, and what pushes a quote up, sit in our guide to heat pump costs, install and running.

The heat pump installation page sets out the full process. In short:

Air conditioning stays the better buy for a lot of homes. If your boiler has years left and your problem is a top-floor room that hits 28°C in August, a split unit fixes that for a fraction of the disruption.

Air Conditioning That Heats, Within About 20 Miles Of New Mills

We fit air conditioning from our base in New Mills to homes and workspaces within about 20 miles: Manchester and Stockport, the High Peak and its edges (Glossop, Buxton, Whaley Bridge, Marple and the villages between), and the Hope Valley toward Sheffield. Our engineer quotes each job after a survey. The air conditioning in the High Peak and air conditioning in Stockport pages cover each area.

We fit Mitsubishi units, and we are F-Gas certified. Installs take one day for a single room up to a week for a whole house, and a 2-year workmanship guarantee covers the installation alongside the manufacturer’s warranty. Fixed air conditioning systems we supply and install in a home carry 0% VAT under the energy-saving materials relief until 31 March 2027, so the quoted price is the price.