Multi-split air conditioning systems

A multi-split runs several indoor units — typically two to five — from a single outdoor condenser. For a house where you want cooling in three bedrooms and the living room, it is usually the right answer, and it is cheaper and tidier than four separate systems. But it carries one design constraint that catches people out after installation, and it is not in most sales conversations: every indoor unit on the same outdoor unit must be in the same mode. You cannot heat the nursery while cooling the home office. Understanding that before you commit is the difference between a system you like and one you argue with every October.

The mode constraint

A standard multi-split shares one refrigerant circuit and one reversing valve. That valve is what decides whether the system is heating or cooling, and there is only one of it. So all the indoor units follow whichever mode the system is in.

In practice this means:

• Different temperatures, yes. Each room can be set to its own temperature, and each indoor unit can be off entirely.
• Different modes, no. If one room calls for heat while another calls for cooling, the system cannot do both. Depending on the manufacturer, the later request is either refused, or it forces the other room to follow — which is how people discover this in November.

For most homes this is genuinely fine, because in July everything wants cooling and in January everything wants heat. It becomes a real problem in two situations: a house with a room that runs hot all year (a south-facing home office with equipment in it, a conservatory), and shoulder seasons where the north and south sides of a house genuinely disagree.

The diversity trap in sizing

The second thing to know is that a multi-split outdoor unit is usually not rated to run every indoor unit at full output simultaneously.

Manufacturers size these systems on diversity — the reasonable assumption that you will not demand maximum capacity in all five rooms at the same moment. Add up the nameplate capacity of the indoor units on a typical multi-split and it will exceed the outdoor unit's rating, often substantially.

That assumption holds for most of the year and fails on exactly the day you care about: the hottest afternoon of the summer, when every room is calling at once. A system sized on paper can then underdeliver everywhere at the same time.

Ask your installer two questions:

1. What is the combined indoor capacity against the outdoor unit's rating?
2. What happens on a design day when every room calls at once?

A good answer involves a heat-load calculation per room and an honest statement about simultaneous demand. A bad answer is "it will be fine."

Where a multi-split genuinely wins

That last point is a genuine improvement. Under the old 0.6 m³ limit, a condenser big enough to serve four rooms could fall outside permitted development on size alone. At 1.5 m³ most domestic multi-split outdoor units fit comfortably — see our planning permission guide for the rest of the conditions.

Where separate splits win

• Redundancy. A multi-split concentrates risk. If the outdoor unit fails, every room goes off at once. Four separate systems fail one at a time.
• Mode independence. As above — separate systems can heat and cool independently because each has its own circuit.
• Phasing. You can install one room now and another next year. A multi-split is designed and sized as one project.
• Pipe run length. Rooms far from the condenser position may push pipe runs past what is efficient. Long runs cost capacity as well as money.
• Future changes. Adding a fifth room to a four-port outdoor unit is not possible; replacing the outdoor unit is.

The honest rule of thumb: two rooms is a close call, three or more usually favours a multi-split, unless one of those rooms has a genuinely different seasonal profile.

Refrigerant charge and the checks that follow

More indoor units and longer pipe runs mean more refrigerant in the system, and refrigerant charge is what triggers statutory leak checking.

For domestic systems this rarely bites — most home multi-splits sit below the threshold. But it is worth knowing where the line is: equipment containing 5 tonnes of CO₂ equivalent or more needs a leak check at least every 12 months, and for R-410A that is 2.4kg of refrigerant [2]. A large multi-split with long pipe runs can approach it, and a system serving a home office or a let property may sit in scope where a single bedroom split never would. Ask for the charge in kg and tCO₂e on the commissioning paperwork either way — you will want it if you ever sell the house.

What a multi-split costs

Multi-split pricing follows room count more closely than anything else:

Our cost guide covers what moves these figures and the extras that appear late — a dedicated electrical circuit and access equipment being the two most common. For running cost, a multi-split is no less efficient per room than separate systems; see running costs for how to work out your own figure.

Specifying one properly

Five things belong in a multi-split quote:

1. A heat-load calculation per room, not a capacity guessed from floor area.
2. The combined indoor capacity against the outdoor rating, with the diversity position stated.
3. The condenser position on a drawing, including how an engineer will reach it for servicing in ten years.
4. Pipe run lengths, since long runs affect both cost and delivered capacity.
5. The mode constraint acknowledged in writing if any room has an unusual profile.

Frequently asked questions

Can a multi-split heat one room and cool another at the same time? No. A standard multi-split shares one refrigerant circuit and one reversing valve, so all indoor units must be in the same mode. Simultaneous heating and cooling requires three-pipe heat-recovery equipment.

How many rooms can one outdoor unit serve? Domestic multi-splits commonly serve two to five indoor units. The limit is the specific outdoor unit's port count and capacity, not a general rule.

Is a multi-split cheaper than separate systems? Past two rooms, usually yes — you buy one condenser, find one position, and pay access costs once. Below that the saving is marginal.

What happens if the outdoor unit breaks? Every room served by it stops. This is the main argument for separate systems in a house where one room is critical.

Can I add another room later? Only if the outdoor unit has a spare port and spare capacity. Otherwise it means a new or larger condenser. If you may extend later, say so at the design stage.

Do I need planning permission? For a house in England, usually not, provided the system is reversible and the outdoor unit is within 1.5 m³ and meets the other Class G conditions [1]. Flats and listed buildings are different — see our planning guide.

Get it sized for your actual rooms

The mode constraint and the diversity question are both answered at survey, not from a brochure. Tell us which rooms you want covered and how you use them, and we will match you with one vetted installer who will calculate the loads and design around them: get an installer match.