One outdoor unit can run several indoor units, and the number depends on the model (Rinnai’s MON6H19B1TA, for example, takes up to six), but the kilowatts on the indoor heads don’t add up the way most people expect. The outdoor unit has a fixed amount of capacity to share. When every room asks for cooling at once, each head gets a slice of that, not its full rating.
That’s the whole trick of a multi-split, and it’s where a lot of disappointed owners come from. Here’s how the sharing works, what it looks like on a hot afternoon, and how to tell whether a quote has been put together with a bit too much optimism.
Why don’t the indoor units just add up?
Because the manufacturer lets you connect more indoor capacity than the outdoor unit can actually produce. They’re betting, fairly, that you won’t run every room flat out at the same time. The ratio between the two is called the combination or connection ratio, and each brand publishes its own limits for each outdoor unit.
Rinnai’s published combination table for its MON5H11B1LA outdoor unit is a good real example (Rinnai Australia, multi-split combination table). That outdoor unit is rated at 10.5kW in cooling. Hook up four heads rated 2.6kW, 2.6kW, 3.5kW and 5.0kW, and you’ve got 13.7kW of indoor units on the wall. Rinnai lists that combination at 130.5%.
Now look at what each room gets when all four are running in cooling, straight from the same table:
- Bedroom one (2.6kW head): 2.0kW
- Bedroom two (2.6kW head): 2.0kW
- Study or third bedroom (3.5kW head): 2.7kW
- Living area (5.0kW head): 3.8kW
Add those up and you’re back at 10.5kW, the outdoor unit’s rating. The heads didn’t shrink. The outdoor unit just doesn’t have more to hand out.
What actually happens when every room calls at once?
The system shares out what it has, and each room cools more slowly than it would on its own. On a mild day you won’t notice. On a 40 degree afternoon with the whole family home, the living area head that would happily hold 24 degrees by itself may struggle once three bedrooms are pulling cooling too.
Manufacturers say this plainly in their own literature. York’s multi-split brochure notes that if the combination ratio is above 130%, the indoor units should be zoned so some aren’t running during heavy demand, and that full performance with all indoors running under high demand needs proper matching at installation (York multi-split brochure, September 2017). In other words, the high ratio is fine if you live with it the right way. It’s not fine if nobody told you.
So ask yourself honestly: how will you use it? A house where the living area runs during the day and the bedrooms run at night suits a high ratio well. A house where everyone retreats to their own room on the hottest afternoons does not.
How to spot a multi-split that’s been specified optimistically
Watch for a quote that sells you the sum of the heads instead of what the outdoor unit can deliver. These are the signs:
- The quote only lists indoor kW: if you can’t see the outdoor unit’s model and rated capacity, ask for it before you sign anything.
- No combination table: a good installer can show you the manufacturer’s figures for your exact combination, including what each room gets with everything running.
- Every room sized for its worst case: if each head was sized as if it were a standalone split, then joined to one outdoor unit, the ratio will be high and nobody may have checked the shared output.
- The big room is on the shared unit: a west facing living area that needs its full output on summer afternoons is often better on its own system.
- No conversation about how you use the house: if nobody asked which rooms run together, the sizing was a guess.
Room sizing still matters underneath all this. Our guide to sizing an air conditioner in kW covers how to work out what each room needs before you start sharing capacity between them.
The electrical side of one bigger outdoor unit
A multi-split swaps several small outdoor units for one larger one, so the electrical load lands on a single supply. Whether it needs its own circuit depends on the unit and your board, and that’s for the electrician to assess, not the salesperson. Energy Safe Victoria is clear that electrical installation work must go through a registered electrical contractor, and that doing your own electrical work is illegal (Energy Safe Victoria, electrical workers).
Older homes are where this bites. If your switchboard still has ceramic fuses, get it looked at before the aircon goes in. In Melbourne’s western suburbs, Electrx Electricians do switchboard upgrades, including swapping old ceramic fuses for circuit breakers and safety switches. For the circuit itself, see our piece on aircon circuits and switchboard capacity.
The refrigerant side is a separate licence again. Only licensed technicians can handle refrigerants, so the pipework and gas charge on a multi-split need an ARC-licensed tech (DCCEEW, refrigeration and air conditioning for consumers).
Is a multi-split even the right call?
Sometimes it isn’t. A multi-split earns its place when you’ve got limited wall space outside, rooms with different use patterns, and a sensible ratio. If you want the whole house cooled at once, a ducted system or separate splits may serve you better. We compare those options in ducted vs multi-split aircon.
Either way, ask the installer one question before you sign: “With every head running on a hot day, what does each room actually get?” If they can answer that from the manufacturer’s table, you’re in good hands.
