A split system is two boxes and a pipe. The indoor head takes heat out of the room, the outdoor unit rejects it, and insulated refrigerant pipe with a drain and a cable joins them. Everything else in the domestic market is a variation on that arrangement, and the main variation is how many indoor heads share one outdoor unit.
Single split: one room, one system, the highest split AC installation cost per room
One head, one condenser, one pipe run. It is the simplest installation, the easiest to price and the easiest to fault find, and each room is completely independent of the others. The cost per room is highest, and the visual cost is real too: three single splits means three condensers on the elevation.
Multi room air conditioning: several heads, one condenser
A multi split runs two, three or more indoor heads from a single outdoor unit, with a separate pipe pair to each head. It saves a condenser position and usually saves money against repeating whole systems, at the price of a much longer first fix and a shared point of failure. Get the outdoor unit position right, because now several rooms depend on it.
What the choice really turns on
Where the condensers could go, and how many of them the elevation and the neighbours can take. On a terrace with one small yard, a multi split is often the only arrangement that works. On a detached house with a side return nobody looks at, three singles may be cheaper overall and are certainly simpler to service and to replace one at a time.
Sizing a multi split correctly
The outdoor unit is sized for a realistic simultaneous load rather than the sum of every head at full output, because rooms are rarely all at peak at the same moment. That assumption is the installer's judgement, and it is worth asking what they have assumed, because a condenser sized for a household that is never all home at once will struggle for a household that is.
Questions people ask about split air conditioner
Is a multi split system air conditioner cheaper than separate systems?
Usually on equipment and condenser positions, not always on labour, because the pipe runs are longer and the first fix is bigger. The saving grows with the number of rooms and shrinks where the rooms are far apart in the building.
Can I add a room to a multi split later?
Only if the outdoor unit has a spare port and the capacity headroom for it, which is a decision taken when it is first specified. If you think a second or third room is coming, say so at survey; the extra cost of a larger condenser now is much smaller than replacing it later.
Do all the heads have to be the same?
No. Heads of different capacities and even different types can normally be mixed on one outdoor unit within the manufacturer's rules, which is how a large living room and a small bedroom end up on the same system. The rules are the manufacturer's, so it is their compatibility table that decides.
What happens if the outdoor unit fails?
On a multi split, every room served by it stops. That is the trade off for one condenser instead of three, and it is a fair argument for singles where continuity matters, such as a room holding equipment that needs cooling.