An air conditioning condenser unit is the outdoor half of a split air conditioning system: the box on a wall bracket or on the ground outside that houses the compressor, the condenser coil and a large fan. It takes the heat your indoor unit has removed from the room and releases it into the outside air, and on a system that also heats, it runs the process in reverse to collect heat from outside and bring it in.

What the condenser unit does

Air conditioning does not make cold. It moves heat. A refrigerant circulates in a sealed loop between the indoor and outdoor units, picking up heat in one place and giving it off in the other. The cycle in cooling mode runs like this:

1. Indoors, the refrigerant absorbs heat. Low pressure refrigerant passes through the indoor coil (the evaporator). Room air blown across the coil gives up its heat, so the air leaving the indoor unit is cooler and drier.
2. Outdoors, the compressor squeezes it. The now warm refrigerant vapour travels to the outdoor unit, where the compressor raises its pressure and temperature well above the outdoor air temperature.
3. The condenser coil releases the heat. The hot refrigerant flows through the finned condenser coil while the fan pulls outside air across it. The refrigerant gives up its heat and condenses back into a liquid. This is why the unit is called a condenser, and why the air blowing out of it feels warm in summer.
4. The expansion valve drops the pressure. The liquid refrigerant passes through an expansion device, which drops its pressure and temperature, ready to go back indoors and absorb more heat.

Most modern systems use an inverter compressor, which speeds up and slows down to match the demand rather than switching fully on and off. That is a large part of why current systems are far more efficient and quieter than older fixed speed units.

What is inside the outdoor unit

Part	What it does	Signs it may have a problem
Compressor	Pressurises the refrigerant and drives it round the system	System runs but does not cool, loud humming or clunking, repeated tripping
Condenser coil and fins	Transfers heat between the refrigerant and outdoor air	Dirty or blocked fins, poor cooling on hot days, unit running constantly
Fan and motor	Moves outside air across the coil	Fan not turning, rattling or scraping noises
Expansion valve	Controls the flow of refrigerant and drops its pressure	Poor performance, frost in the wrong places
Reversing valve (heat pump models)	Switches the direction of the refrigerant between cooling and heating	Works in one mode but not the other
Control board and inverter	Runs the compressor and fan speeds and talks to the indoor unit	Error codes on the indoor unit or remote, no response
Service valves	Where the refrigerant pipes connect, used by engineers to test and isolate the system	Oily marks at the joints, which can mean a refrigerant leak

The outdoor unit is connected to the indoor unit by two insulated copper refrigerant pipes, a power or communication cable and, from the indoor unit, a condensate pipe that carries away the water pulled out of the air. These usually run together through a single hole in the wall.

Split, multi-split and portable units

Not every a/c unit has a separate condenser. Which type you have affects where the outdoor unit goes and how much work installation involves.

Type	Outdoor unit	Best suited to
Single split	One outdoor unit serving one indoor unit	A bedroom, home office, lounge or conservatory
Multi-split	One outdoor unit serving several indoor units	Several rooms where there is only space for one outdoor unit
Ducted	An outdoor unit plus an indoor unit hidden in a ceiling void, feeding grilles	Larger rooms or new builds where you want nothing on the walls
Portable	No separate outdoor unit. Hot air goes out through a hose in the window	Short-term use. Less efficient and noisier indoors than a split

A portable unit contains its own compressor and condenser inside the room, which is why it is louder and has to vent hot air through a window. A split system moves the noisy, hot part outside.

For a permanent system, our air conditioning installation service covers wall-mounted splits, multi-splits and ducted systems.

Cooling and heating: why the outdoor unit steams in winter

Most split systems sold for homes are heat pumps, sometimes called air-to-air heat pumps. A reversing valve in the outdoor unit swaps the roles of the two coils, so in winter the outdoor coil collects heat from the outside air and the indoor unit blows warm air into the room.

When the outdoor coil works in heating mode it runs colder than the outside air, so moisture freezes on it in cold, damp weather. The system deals with this by running a defrost cycle: the fan stops, the unit briefly reverses to warm the coil, and the ice melts. You may see steam rising from the unit and water dripping from underneath, and the indoor unit may pause its warm air for a few minutes. That is normal. It is also why the outdoor unit needs a sensible route for the meltwater, so it does not freeze on a path.

For more on what a system uses, see our guide to air conditioning running costs.

Where the outdoor unit can go

Siting the condenser unit well matters as much as choosing the right system. A good installer will look at:

• Airflow. The unit needs free air around it. Each manufacturer sets minimum clearances from walls and obstructions, and a unit boxed in by fencing or plants recirculates its own hot air and struggles.
• Mounting. A wall bracket fixed to solid masonry or a level ground base, with anti-vibration mounts to limit noise carrying into the building.
• Pipe run. Every system has a maximum pipe length and height difference between the indoor and outdoor units. Shorter runs are usually better.
• Neighbours and noise. Modern inverter units are quiet, but a unit under a neighbour’s bedroom window is still a poor choice.
• Drainage. Somewhere for condensate and defrost water to go.
• Access. The engineer needs to reach it safely for servicing.

Planning rules for outdoor units depend on the property and its location, and listed buildings and conservation areas need extra care. We check this as part of the survey. Our guide on choosing an air conditioning installer covers what a good survey should include.

Looking after your outdoor unit

There are a few simple things a homeowner can do:

1. Keep leaves, grass cuttings and litter clear of the unit, especially the back and sides where air is drawn in.
2. Keep plants and climbers trimmed back well clear of it.
3. Check that condensate and defrost water is draining away and not pooling or icing over.
4. Clean or rinse the filters in the indoor unit as your user manual describes. Blocked filters make the whole system work harder.
5. Book a regular air conditioning service, where the coils are cleaned, the electrics checked and the system’s performance measured.

Common outdoor unit problems

What you notice	Likely cause	What to do
Fan not spinning, system not cooling	Fan motor, capacitor or control fault	Switch off and book a repair
Ice on the outdoor unit in cooling mode	Low refrigerant, blocked filters or restricted airflow	Clean the indoor filters. If it continues, it needs an engineer
Steam and dripping in winter	Normal defrost cycle	Nothing, unless water is pooling or freezing on a path
Oily stains on pipe joints	Possible refrigerant leak	Book a repair. Do not run it until checked
Loud buzzing, rattling or clanking	Loose panel, debris, fan or compressor fault	Switch off and book a repair
Unit trips the electrics	Electrical fault in the unit or supply	Leave it off and book a repair

If you see any of these, our air conditioning repair team can diagnose it. We are Refcom F-Gas registered (REF1024550) and repair and service all the major brands.

How we price it

Installations are quoted as a fixed price after a free survey, so the price you agree is the price you pay. You can start with our online air conditioning quote.

For repairs, there is no call-out fee in normal working hours. After the first hour, time is charged in half-hour steps, and parts are quoted before we fit them. Evening, weekend and overnight visits are charged at the out-of-hours rates. Every rate is published on our prices page, so you know what you will pay before we arrive.

Frequently asked questions

Does every air conditioner need an outdoor unit?

No. Portable units and some through-wall units have everything in one box indoors. Split systems, which are quieter and more efficient, always have an outdoor condenser unit connected by refrigerant pipes.

How far can the outdoor unit be from the indoor unit?

It depends on the model. Every manufacturer sets a maximum pipe length and height difference, and some systems need extra refrigerant beyond a certain length. Your installer should confirm this at the survey.

Is it normal for the outdoor unit to drip water?

Yes, in heating mode. The outdoor coil collects moisture that freezes and is then melted by the defrost cycle. In cooling mode, water should come from the indoor unit’s condensate pipe, not the outdoor unit.

Can I put a cover on my outdoor unit in winter?

Not while the system is in use, because it needs air to flow through it and many systems are used for heating in winter. A small canopy designed for the unit that leaves the sides and back open is fine if the manufacturer allows it.

How long does an outdoor unit last?

It varies with the model, how much it is used and how well it is maintained. Regular servicing, clean filters and clear airflow around the outdoor unit all help it last longer.

Air conditioning help in Hertfordshire, Bedfordshire and Cambridgeshire

Whether you are planning a new system or your outdoor unit is not behaving, our F-gas registered engineers can help. See our air conditioning installation service or call 01462 506374, answered 24 hours a day, 7 days a week.