The short answer
A multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.
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Multi-splits are a common choice when several rooms need air conditioning but there's space, or permission, for only one outdoor unit. They work on the same principle as a single wall-mounted split, with more pipework and an outdoor unit that shares its output between rooms.
How a multi-split works
Each indoor unit has its own pair of refrigerant pipes, its own cable and its own condensate drain. On most home systems, each indoor unit's pipes run all the way back to their own connection, or port, on the outdoor unit. Outdoor units for homes commonly have between two and five ports, and some larger systems use a branch box to split one pipe run into several.
The outdoor unit's compressor is usually inverter-driven, speeding up and slowing down to match whichever rooms are calling for heating or cooling. Depending on the range, the indoor units don't all have to be the same type: a wall-mounted unit in a bedroom can share an outdoor unit with a floor console or a ceiling cassette elsewhere.
Sharing capacity
The outdoor unit has a total capacity that the indoor units share. Because rooms don't usually all need full output at the same moment, manufacturers often allow the indoor units' combined sizes to add up to more than the outdoor unit's, within limits they set. On the hottest or coldest days, when every room is calling at once, each room may get less than its unit's full output.
Each room is still sized in the usual way, as explained in what size air conditioner you need. A good installer will size the indoor units room by room, then check that the combination suits the outdoor unit.
Controls and modes
Each indoor unit has its own remote control or wall controller and its own temperature setting, and it switches on and off independently. You can cool a bedroom at night without running the living room unit.
The main limit is the mode. On a standard multi-split, all the indoor units running at the same time must be in the same mode, either all heating or all cooling. A unit asked to do the opposite typically waits in standby until the others stop or change mode. This matters most in spring and autumn, when a sunny room may want cooling while a shaded one wants heat.
Where a multi-split helps
- One outdoor unit instead of several. There's less equipment on the outside of your home, and only one spot to find that suits the unit and meets the noise limit.
- Planning. Permitted development in England allows one heat pump on a house that isn't detached, or two on a detached house, and a multi-split serves several rooms from a single outdoor unit. The other conditions still apply, and Scotland, Wales and Northern Ireland have their own rules. See the planning rules.
- Room-by-room control, with a mix of indoor unit types if the range allows it.
The trade-offs
- More pipework. Every indoor unit needs its own run, which can mean longer routes through lofts, cupboards or along outside walls in trunking, and more holes. The manufacturer sets maximum lengths for each run and for the system as a whole, and longer runs may need extra refrigerant. Refrigerant pipework explains how runs are planned.
- One outdoor unit for every room. If it develops a fault, all the rooms it serves lose heating and cooling together.
- Efficiency. Single splits often have higher efficiency ratings than multi-splits, and a large outdoor unit may not run at its best when only one small room needs it.
- Longer installation, because there's more pipe to run and more units to connect and test.
- Planning ahead. Adding a room to the system later is only possible if the outdoor unit has a spare port and enough capacity, within the manufacturer's limits.
Separate splits or a multi-split
Several separate single splits and one multi-split can cover the same rooms. The main differences:
| Separate single splits | Multi-split | |
|---|---|---|
| Outdoor units | One per room | One for all rooms |
| Pipework | A short run per room | A run from each room to one unit |
| If an outdoor unit fails | One room affected | All rooms affected |
| Heating one room, cooling another | Possible | Not on a standard system |
| Efficiency ratings | Often higher | Often lower |
| Adding a room later | Add another system | Needs a spare port and capacity |
Related guides
- Wall-mounted split systemsA wall-mounted split system pairs an indoor unit fixed high on a wall with an outdoor unit outside, joined by refrigerant pipes. It is a common choice for fixed air conditioning in UK homes, and most models heat as well as cool.
- What size air conditioner do you need?Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.
- Do you need planning permission?Usually not in England, where air-to-air heat pumps that aren't used only for cooling count as permitted development if conditions on number, size, position and noise are met. Listed buildings, flats and protected areas follow different rules.
- 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.