Sizing ducted reverse-cycle: what a heat load assessment covers

A wall split looks after one room. A ducted system heats and cools most of the house from one indoor unit in the roof space, sending air through ducts to outlets in each room and back through a return air grille. Its size has to suit the whole house at its worst moments, the zones you'll actually run, and the ducts in between.

The Victorian Government offers a starting range for ducted systems: 1.1 to 1.4 kW of heating output for every 10 m² covered, depending on the home. That spread is the point. On 200 m² it's the difference between about 22 and 28 kW, more than one model size apart, and the details of the house decide where you land.

A heat load assessment works out, room by room, how much heat each space gains on a hot afternoon and loses on a cold morning. Add up the rooms in each zone and you have the load the unit and ducts must meet.

For a recent build, the energy rating report prepared for the house models its heating and cooling needs, and it's useful input. For older homes, it starts with a tape measure and a look in the roof.

Victorians typically use more energy for heating than cooling. Laverton's coldest month is July, with a mean minimum of 5.2 °C, and the Commonwealth's guide to the Zoned Energy Rating Label puts Melbourne in the Cold zone, where performance in frosting conditions below 5.5 °C is critical.

A reverse-cycle unit draws its heat from outside air, so its heating output drops as that air gets colder, and it pauses now and then to defrost. A system chosen on its rated heating capacity can fall short on a still, frosty morning, just when everyone wants the house warm. Manufacturers publish how output changes with outdoor temperature, and that table tells you more than the headline kW. The star label isn't mandatory on ducted systems, so ask for the figures.

Summer still counts, especially for upstairs rooms and west-facing glass. Laverton averages 27 days a year at or above 30 °C, and the worst arrive on hot northerly winds. The final size has to meet both loads, and heating is usually the tighter of the two.

Two houses with the same floor area can need very different systems. A 1970s brick veneer in Hoppers Crossing with thin ceiling batts, single glazing and a west-facing family room gains and loses heat far faster than a recent double-storey in Point Cook built to a higher energy rating. A per-square-metre figure treats them alike.

Rules of thumb also hide the spaces that drive the load. One big sliding door facing west, a raked living room ceiling or a void over the stairs can each add more load than a spare bedroom, and those are exactly the spaces where an undersized system shows first.

They ignore how you live, too. A household that runs the living zone in the evening and the bedrooms overnight needs a different system from one that heats the whole house all day.

The unit doesn't need to equal the sum of every room's peak, because not every zone runs at full load at once. The Victorian Government recommends zoning ducted systems so each area can be heated or cooled on its own. Sizing then looks at the largest combination you'll realistically run together, such as living plus bedrooms on a cold night.

The other end matters as well. A ducted unit needs a minimum airflow, so when one small zone runs alone, the surplus air has to go somewhere: a common zone that never closes, or a spill damper. An oversized unit makes this worse, with noisy outlets and a unit that cuts out on its own protection.

The roof space can run well above outside temperature on a summer afternoon and close to it on a winter night. Air loses some of its heating or cooling on the way through, so duct length, insulation and joints decide how much capacity reaches each room. YourHome says ducts should be insulated to at least R1.5 with sealed joints.

Duct sizes have to match the airflow the unit is designed to move. Undersized ducts make outlets whistle and starve the far rooms, and long runs with tight bends do the same. When a system is replaced, the old ducts need checking against the new unit's airflow rather than being assumed to fit.

Undersizing shows as a house that never quite gets there on the coldest mornings and hottest afternoons, a unit running flat out for hours, and pressure to push the set temperature further, which adds running cost without adding much comfort. The Victorian Government suggests 18 to 20 °C for heating and 24 to 26 °C for cooling, and says each degree beyond that adds around 10% to running costs.

Oversizing costs more to buy and connect, cycles on and off in mild weather, and in summer can leave the air cool but clammy because the coil hasn't run long enough to pull moisture out.

Large ducted units also draw a lot of power, so we check the switchboard and supply before quoting. Many homes run ducted reverse-cycle on single-phase. Three-phase makes sense for larger systems, some of which are only made in three-phase versions, or where the home also runs EV charging, a workshop or other heavy loads. Under the Wiring Rules, the outdoor unit also needs a lockable isolating switch next to it.