Heating and cooling an extension: extend the ducted system or add a split?

When an extension goes on, the existing heating and cooling either stretches to cover it or something new goes in. The answer depends on two things: whether the existing system has spare capacity, and whether ducts or pipes can reach the new rooms sensibly.

Get both answered before the extension is designed in detail. A bulkhead for a duct, a platform for a second unit or a pipe route to an outdoor unit is a pencil line at the drawing stage and a variation once the frame is up.

A ducted reverse-cycle system is sized for the area it was designed to cover. As a rough check, the Victorian Government suggests 1.1 to 1.4 kW of heating output for every 10 m² covered, depending on the home. Compare the unit's rated heating capacity with the existing conditioned area plus the extension. If there's little margin, extra outlets spread the same output over more rooms and everything gets slower.

Zoning helps. If the extension becomes its own zone, the system only conditions it when you need it, and a well-zoned system rarely runs every room at once. But zoning doesn't add capacity: on a still July morning with every zone open, the unit still has to cope.

Age matters too. The Victorian Government says a reverse-cycle system more than 15 years old will typically use more than three times the energy of a modern one. If the existing unit is old, the extension can be the moment to replace it with one sized for the whole house, reusing sound ducts where they suit.

Even with spare capacity, the duct has to get there. A long run from the unit to a rear extension loses air along the way, and a flat or skillion-roofed extension with little ceiling space may have nowhere for a duct to go. New ducts should be insulated to at least R1.5 with sealed joints.

The return air path matters as much. If the extension is a closed room at the far end of the house, air needs a way back to the return grille, or its outlets won't deliver their share. A second return or a transfer grille can solve it, but it needs planning.

Adding outlets also changes where the air goes. The existing rooms get a little less, so the whole system should be checked and rebalanced, not just the new run.

Older gas ducted systems often use floor outlets and were sized for the original house. Extending one means checking the heater's capacity and the duct route, and any change to the heater or the gas supply is work for a licensed gasfitter. Energy Safe Victoria says it's illegal for unlicensed people to do any gasfitting work.

There's also a cooling question. Gas ducted only heats, so an extension that needs cooling still needs a split or reverse-cycle system. Owners of existing homes aren't required to switch gas space heating to electric, but many use an extension as the point to add reverse cycle that covers both.

If the renovation seals the house up, with new windows, insulation or draughtproofing, and an open-flued gas heater stays, Energy Safe Victoria warns that the ventilation the heater needs to clear carbon monoxide may change. Raise it with your gasfitter before the work is finished.

A wall split in the extension runs independently of the rest of the house. That suits a new family room, a parents' retreat or a home office used at different times from everything else. The federal energy guide notes that wall units are more efficient than ducted systems, and a single head on its own outdoor unit is the simplest job.

A multi-head system runs several indoor units from one outdoor unit. It suits an extension with two or three rooms, such as a bedroom suite and a living area, where you want each room controlled separately without several outdoor units on the wall. Each head can be a wall, floor or bulkhead unit to suit the room.

Size the new unit for the extension itself. Extensions often have more glass than the original house, such as wide sliders to the garden, and some face west. A unit sized on floor area alone can struggle on a hot northerly afternoon.

Whichever way you go, the rough-in happens during the build. For a split or multi-head, that's the refrigerant pipes, drain, control cable and power between the indoor head and the outdoor unit, run inside the frame. For ducted, it's the duct route, outlets and any extra return, plus a bulkhead if the ceiling space is too tight.

Mark the outdoor unit on the plan, clear of bedroom windows and the side path, and check the switchboard has room for a new circuit. The Wiring Rules require additional safety switch protection when an existing installation is altered, so the extension's circuits are a good time to bring the board up to date.

Installing air conditioning is licensed on more than one front. In Victoria, refrigerated air-conditioning work is a class of plumbing work regulated by the Building and Plumbing Commission, installing a split needs a refrigerant handling licence, and the electrician provides a Certificate of Electrical Safety for the connection.