Multi-head split systems explained

A single split pairs one indoor head with one outdoor unit. A multi-head keeps one outdoor unit, with one compressor and one outdoor coil, and connects two or more indoor heads to it. Each head has its own pair of refrigerant pipes, its own drain and its own controller, and valves inside the outdoor unit share refrigerant between whichever heads are calling.

Most home multi-heads run each head's pipes straight back to the outdoor unit, so its side carries a row of connections, one pair per room. Larger systems may use a branch box part-way. Either way, the manufacturer limits the number of heads, the length of each run, the total pipe length and how far above or below the outdoor unit a head can sit.

The compressor is an inverter, so it speeds up and slows down to match demand. With one bedroom head on, it ticks over. With every room on during a hot evening, it runs flat out and shares that output around.

The outdoor coil can absorb heat or release it, not both, so every head must be in the same mode. If the lounge is cooling and someone switches a bedroom head to heat, one of them stops and shows a mode-conflict light or code until the modes match. Many brands let you nominate a priority head, or give priority to whichever head was switched on first.

In midwinter and midsummer this rarely matters. In spring and autumn it can. A west-facing lounge in a Williamstown cottage can want cooling at 4pm while a shaded south bedroom wants heat after dark. Auto mode on one head makes clashes more likely, so on a multi-head it's best to choose heat or cool on each controller yourself.

Systems that heat and cool different rooms at once do exist, but they're commercial equipment. A residential multi-head works one way at a time.

Add up the heads on a typical multi-head quote and the total is often larger than the outdoor unit's rated output. That's deliberate. Rooms rarely peak together: the lounge in the evening, bedrooms overnight, the study by day. An outdoor unit sized to the sum of every head would mean paying for capacity that almost never gets used.

The catch comes when everything does run together, such as a heatwave evening with everyone home. Each head gets a share of the outdoor unit's output rather than its full rating, and the far rooms usually feel it first. If several rooms genuinely need full output at the same time, a larger outdoor unit or a different layout is the answer.

The opposite problem exists too. One small bedroom head running alone on a big outdoor unit can be less efficient than a matched single split, because the compressor can only slow so far before it starts cycling on and off.

Every head depends on the same compressor, fan, circuit board and power supply. A fault in the outdoor unit or a tripped circuit takes out every room at once. Laverton averages 27 days a year at or above 30 °C, and July's mean minimum is 5.2 °C, so a breakdown at the wrong moment matters.

With separate splits, a breakdown costs you one room. With a multi-head it costs you the lot. That's the trade-off to weigh if one room is critical, such as a bedroom for someone who struggles in heat or a home office you rely on. Some households put most rooms on a multi-head and give the critical room its own split.

Defrost works the same way. On a frosty morning the single outdoor coil pauses to clear its ice, and every head that's heating waits while it does.

Comparing multi-heads is harder than comparing single splits. In stores, the Zoned Energy Rating Label is mandatory on unitary and single-split units, and multi-head combinations aren't named in that rule, so you often won't see a star label for your exact set of heads. Ask for the manufacturer's figures for the specific combination. Melbourne is in the label's Cold zone, where performance in frosting conditions below 5.5 °C is critical, so look closely at heating output in cold weather.

Noise is often in a multi-head's favour. One outdoor unit makes one noise in one spot, which can be easier on neighbours than three units along a side fence. Indoors, each head is about as quiet as a single split of the same size.

A multi-head makes most sense when the site, not the rooms, is the problem: a townhouse with one small courtyard, an apartment allowed one unit on the balcony, a narrow side path, or a period frontage in Newport or Williamstown you'd rather keep clear of boxes. It also suits households whose rooms are used at different times.

It suits less well a house whose rooms pull in opposite directions in the shoulder seasons, a double storey where heads would be a long pipe run from the only outdoor spot, or a home where one room must always stay comfortable. Where most of the house needs covering and the roof space allows, ducted reverse-cycle is the other comparison worth pricing.

Each pipe run is pressure tested and vacuumed before refrigerant is released into it, and every head is run in both modes, alone and together. The Australian Refrigeration Council says a holder of only the restricted split system licence may not install multi-head systems, while a full refrigeration and air conditioning licence covers any system. It's a fair question to ask whoever quotes.

The outdoor unit gets its own circuit, and the Wiring Rules call for a lockable isolating switch next to it, not on it. Because the heads are usually powered through the outdoor unit, that one switch shuts down every room for servicing. A head can only be added later if the outdoor unit has a spare connection and capacity, so mention any future room at the start.

Servicing covers every head and the outdoor unit. The Victorian Government suggests cleaning dust filters about twice a year, more often with heavy use. If a warning light appears, check every controller is in the same mode before booking a repair.