How do air conditioners work? The cycle step by step
Every air conditioner, from a single wall-mounted split to a rooftop VRF system, uses the same vapour-compression refrigeration cycle. Refrigerant flows round a sealed circuit and changes between liquid and gas, picking up heat indoors and dumping it outdoors.
- 1
The evaporator absorbs heat
Warm room air is blown across the cold indoor evaporator coil. Low-pressure liquid refrigerant inside the coil absorbs that heat and boils into a gas, so the air leaving the unit is cooler.
- 2
The refrigerant carries the heat away
The refrigerant, now a low-pressure gas loaded with heat, travels through insulated copper pipework to the outdoor unit. Modern systems use refrigerants such as R32 or R410A.
- 3
The compressor raises the pressure
The compressor squeezes the gas, raising its pressure and temperature well above the outdoor air temperature. This is the step that uses most of the electricity.
- 4
The condenser releases heat outside
Hot, high-pressure gas flows through the outdoor condenser coil. A fan pushes outside air across it, the refrigerant gives up its heat and condenses back into a liquid.
- 5
The expansion valve drops the pressure
The liquid passes through an expansion valve, which drops its pressure and temperature sharply. Cold refrigerant returns to the evaporator and the cycle starts again.
If any part of this loop fails — a refrigerant leak, a worn compressor or a blocked coil — the unit loses cooling. That is when an air conditioning repair visit from an F-Gas certified engineer is needed.
How AC dehumidifies and filters the air
Because the evaporator coil runs below the dew point of room air, moisture condenses on its fins, just as water forms on a cold glass. That water drips into a tray and drains away, so the air leaving the unit is drier as well as cooler. Drier air feels more comfortable at the same temperature, which is why AC helps in humid offices and kitchens.
Before air reaches the coil it passes through a filter that traps dust, lint and debris. Clean filters protect the coil and keep airflow strong. Dirty filters are one of the most common reasons for poor cooling, which is why regular AC maintenance and servicing matters.
Inverter vs fixed-speed: how does AC save energy?
Older fixed-speed units run the compressor at full power until the room reaches its set temperature, then switch off, then start again when it warms up. That constant stopping and starting wastes energy and lets the temperature swing.
Inverter units vary the compressor speed. They work hard to reach the set temperature, then slow right down and tick over to hold it. Starting is the most power-hungry moment for a compressor, so avoiding repeated starts makes inverter systems noticeably cheaper to run, quieter and longer-lasting. Almost every new system we fit during air conditioning installation is inverter-driven.
How the same system heats in winter
Most modern air conditioners are reverse-cycle heat pumps. A reversing valve changes the direction the refrigerant flows, so the outdoor coil becomes the evaporator and collects heat from outside air — even on cold days — while the indoor coil becomes the condenser and releases that heat into the room. Because it moves heat rather than generating it, a heat pump can deliver several units of heat for each unit of electricity used. See our air conditioning heat pumps page for more.
Split vs multi-split vs VRF/VRV for commercial premises
- Split system: one outdoor unit serving one indoor unit. Ideal for a single office, server room or shop floor.
- Multi-split: one outdoor unit serving two to five indoor units, saving wall and roof space where several rooms need cooling.
- VRF/VRV: variable refrigerant flow systems that serve dozens of zones from shared outdoor units, each zone with its own control. Some can heat one area while cooling another.
The right choice depends on the building, the heat load and how zones are used. Our commercial air conditioning team designs, installs and maintains all three across London.
How does AC work? Frequently asked questions
Does air conditioning bring in outside air?
No. A standard split air conditioning unit recirculates and cools the air already in the room. Only the heat is moved outside through the refrigerant pipework. Fresh air comes from windows or a separate ventilation system.
Why does my air conditioning unit leak water?
Cooling pulls moisture out of the air, which collects as condensate and drains away through a pipe. If the drain is blocked, the condensate pump has failed or the unit is not level, water can drip from the indoor unit.
Can air conditioning heat a building?
Yes. Most modern systems are reverse-cycle heat pumps. A reversing valve swaps the job of the two coils, so the outdoor unit collects heat from outside air and the indoor unit releases it into the room.
How often should air conditioning be serviced?
Commercial air conditioning should be serviced at least once a year, and every six months in busy shops, offices and kitchens. Systems with larger refrigerant charges also need regular F-Gas leak checks.
Why are inverter air conditioners cheaper to run?
An inverter compressor changes its speed to match demand instead of switching fully on and off. It runs gently most of the time, which uses less electricity and holds a steadier temperature.
What is the difference between split, multi-split and VRF systems?
A split system has one outdoor unit serving one indoor unit. A multi-split links several indoor units to one outdoor unit. VRF or VRV systems serve many zones from shared outdoor units and suit larger commercial buildings.