The short answer
The indoor unit pulls room air through a filter and across a coil that cools or warms it. The outdoor unit holds the compressor that pumps the refrigerant, plus a second coil and fan that release heat outside when cooling and collect it when heating.
On this page
A split system gets its name from being divided in two: the heavier, noisier machinery sits outside, and only the parts that handle room air sit inside. The two halves are joined by a bundle of pipes and cable that usually passes through a single hole in the wall. Knowing what each casing holds makes the noises, the water and the upkeep easier to understand.
Inside the indoor unit
On a typical wall-mounted unit, air is drawn in through the grille on top, passes through the filters and the coil, and is pushed back out through the slot along the bottom.
| Part | What it does |
|---|---|
| Filters | Mesh screens that catch dust before it reaches the coil |
| Coil | Copper pipes and thin aluminium fins that cool or warm the air |
| Fan | A long, drum-shaped fan running the width of the unit |
| Louvres | Flaps that steer the air up, down and side to side |
| Drip tray | Catches water from the coil and feeds the drain hose |
| Sensors and circuit board | Read temperatures and take signals from the remote |
The coil is also called a heat exchanger. In cooling it runs cold, and moisture from the room condenses on its fins. The room temperature sensor usually sits near the air intake, high on the wall, so its reading can differ a little from what you feel at chair height.
Some units have extra filters designed to catch finer particles, and many can connect to Wi-Fi for app control. The mesh filters are the part you look after yourself: they usually need cleaning every few weeks while the system is in regular use, as described in how to clean the filters.
Inside the outdoor unit
The outdoor unit is larger and much heavier than the indoor one, mostly because of the compressor. In most home systems the valves that control the refrigerant are housed here too.
| Part | What it does |
|---|---|
| Compressor | Pumps the refrigerant round and raises its pressure |
| Coil | Releases heat to the outside air, or collects it when heating |
| Fan | Draws outdoor air through the coil |
| Expansion valve | Drops the refrigerant's pressure so it can boil and absorb heat |
| Reversing valve | Switches the system between cooling and heating |
| Control board | Sets the compressor and fan speeds |
| Service valves | Where the pipes connect and an engineer attaches gauges |
A new outdoor unit usually arrives holding the system's refrigerant, sealed in by the service valves. Once the pipework is connected, pressure-tested and cleared of air and moisture, the engineer opens the valves to let the refrigerant into the rest of the circuit.
What connects them
The bundle running between the two units usually contains:
- Two copper refrigerant pipes. The thinner one is known as the liquid line and the thicker one as the gas line. Both are insulated because, depending on the mode, either can run cold enough to drip with condensation or warm enough to lose useful heat.
- A drain hose that carries water from the indoor unit's drip tray to a drain or outside, helped by a small pump where it can't run away by gravity.
- A cable that carries power and control signals between the units.
Power usually comes from a dedicated circuit from the consumer unit, and an isolator switch near the outdoor unit is common.
Many systems are pre-charged with refrigerant for a set pipe length, often somewhere between 5 and 15 metres depending on the model. Longer runs need refrigerant added on site, and every model has limits on total length and on the height difference between the units. See refrigerant pipework explained.
Where each one sits
The indoor unit usually goes high on a wall, with space above for air to enter and nothing blocking the airflow in front. On or near an outside wall, the pipe run stays short and the drain has an easy route out. See where to put the indoor unit.
The outdoor unit needs air moving freely around it, a solid bracket or stand, room for an engineer to work and somewhere for water to drain when it is heating. In England its position also affects whether it counts as permitted development, which includes meeting the MCS 020(a) noise limit for neighbouring homes. See where the outdoor unit can go.
Normal sounds and signs
- Indoor unit: a soft rush of air, an occasional trickle or gurgle as refrigerant and water move, and light clicks as the plastic casing warms and cools.
- Outdoor unit: a steady hum from the compressor and the sound of the fan, both rising and falling with demand.
- Drain hose in summer: water dripping from its outside end while the system cools.
- Outdoor unit in winter: water running from its base, and now and then a cloud of steam while it clears ice from the coil.
Loud rattles, grinding, a strong smell or water dripping from the indoor unit itself are not part of normal running and are signs that something needs attention.
Related guides
- How air conditioning worksAir conditioning cools a room by moving heat out of it, not by making cold. In a typical split system, a refrigerant picks up heat from the room at the indoor unit and a compressor pumps it outside, where the outdoor unit releases it.
- Refrigerant pipework explainedTwo insulated copper pipes carry refrigerant between the indoor and outdoor units. They are joined at each unit, tested for leaks and cleared of air before use, and their length and route affect how the job looks and whether extra refrigerant is needed.
- Where the outdoor unit can goAn outdoor unit needs a firm fixing, free airflow, service access and somewhere for defrost water to drain. On a house in England it can usually go on a wall, the ground or a flat roof without planning permission if set conditions are met.
- Where to put the indoor unitA wall-mounted indoor unit usually goes high on a wall with clear space around it, where its airflow can reach the whole room without blowing straight at people. The best spot also depends on the route for the pipes and where the condensate can drain.