Split, multi-split or ducted?
This is usually presented as a question about efficiency. It is mostly a question about how much of your house you intend to heat.
The three options
Single split
One outdoor compressor, one indoor head, usually in the living area. Cheapest to buy at $1,500 to $3,700 installed, and cheapest to run — because it heats one room. If your household spends the evening in one space and sleeps under blankets, this is often all you need, and the money saved buys a lot of insulation.
Multi-split
One outdoor unit, several indoor heads, typically three to five. Costs $5,000 to $12,000 installed. Conditions most of the house without ducts, and — this is the part that matters — lets you run one head at a time. Most of its running cost advantage over ducted comes from that, not from better hardware.
Ducted reverse cycle
One system, ducts to every room, $10,000 to $21,000 installed. The most comfortable and the most expensive to run. Two separate things make it dearer, and they get conflated constantly: the ducts lose heat on the way to the room, and it is conditioning the whole house including the hallway nobody is standing in.
What they cost
| System | Does | Conditions | Typical life | Installed cost |
|---|---|---|---|---|
| Single split system | Heating and cooling | One main living area | 12 years | $2,600 |
| Multi-split system | Heating and cooling | Most of the house | 12 years | $8,500 |
| Ducted reverse cycle | Heating and cooling | The whole house | 15 years | $15,500 |
| Ducted gas heating | Heating only | The whole house | 15 years | $10,250 |
| Gas space heater | Heating only | One main living area | 15 years | $2,700 |
| Electric panel or fan heater | Heating only | One room | 8 years | $225 |
| Ducted evaporative cooling | Cooling only | The whole house | 12 years | $6,500 |
| Ceiling fans | Cooling only | The whole house | 15 years | $600 |
Installed cost is the midpoint of a supply-and-fit range before any rebate. Read the “conditions” column alongside the running costs: the cheapest systems to run are the ones that heat or cool the least of your house, which is a real saving but not the same as efficiency.
Set your own state, floor area and insulation in the calculator and every figure recalculates.
The comparison that misleads
Take the annual running costs at face value and a single split looks three times better than ducted. It is not three times more efficient. It is heating roughly a third as much house.
Per square metre actually conditioned, the three are much closer than the headline figures suggest — the real efficiency gap between a good split and a good ducted system is the duct loss, which is around 30%, not the 200% the raw comparison implies.
This matters because the decision is not "which is more efficient". It is "how much of my house do I want warm, and what am I prepared to pay for that". Framed that way the answer is usually obvious within about a minute, and it is different for a couple in a townhouse than for a family of five.
When ducted is the right answer anyway
- You actually use the whole house. Children in bedrooms doing homework in July are not warmed by a head in the living room.
- Open plan with high ceilings. A single head struggles to move air through a large connected volume, and multi-split heads in an open plan space fight each other.
- You are replacing ducted gas and the ducts are sound. Reusing existing ductwork removes a large part of the installation cost and makes the comparison much closer.
- Resale. Ducted reverse cycle is a listed feature in a way that three wall heads are not. That is not an energy argument, but it is a real one.
And the case against, plainly: over ten years a ducted system in a pre-2005 house costs thousands more than a multi-split doing most of the same job. If the budget is finite, spending part of it on insulation and the rest on a multi-split beats spending all of it on ducted, every time.
Frequently asked questions
How many indoor heads can one outdoor unit run?
Typically two to five for a residential multi-split, depending on the capacity of the outdoor unit. The constraint worth knowing is that the outdoor unit is sized for the total, so running one head on a system built for five is less efficient than running one head on a single split — the compressor is operating well below its design point.
Is one big split cheaper than several small ones?
To buy, usually yes. To run, usually no, and the reason is oversizing: a unit too large for its room reaches temperature quickly and then short-cycles, which is the least efficient way for a compressor to operate. Sizing to the room rather than buying headroom is one of the few places where the cheaper purchase is also the cheaper running cost.
Can I add ducted later if I start with a split?
Yes, and starting with a single split in the main living area is a defensible staged approach — it is the cheapest thing that solves most of the problem, and it does not foreclose anything. What it does mean is paying for installation twice, so it suits people who are unsure how long they will stay more than people settling in for twenty years.
Do the heads have to be on the same wall as the outdoor unit?
No, but pipe runs cost money and long ones lose a little performance. The practical limit for a residential system is usually around 15 to 25 metres of refrigerant line per head. This is a question for the installer at quote stage, and it is one of the reasons multi-split installation quotes vary so widely for apparently identical houses.