How gas ducted heating works

A gas ducted heater burns gas inside a sealed chamber to heat a metal heat exchanger. A fan pulls air from the house, blows it across the outside of the hot heat exchanger and sends it through ducts to an outlet in each room. The burnt gases stay inside the heat exchanger and leave through a flue to outside.

The heater sits in the roof space, under the floor or outside against a wall, depending on the house. Plenty of 1970s and 1980s homes in Melbourne's west have under-floor heaters with floor outlets, while homes built more recently tend to have a roof-space unit with ceiling outlets.

When the room drops below the set temperature, the thermostat or wall controller signals the heater. On most modern heaters, a small fan first clears the combustion chamber, an igniter lights the burner and a flame sensor confirms the flame. If no flame is proved, the gas valve closes and the heater tries again or locks out.

Once the heat exchanger has warmed, the main circulation fan starts, so the first air from the outlets isn't cold. The heater runs until the thermostat is satisfied, the burner goes out, and the fan keeps running briefly to draw out the remaining heat.

Safety devices sit along the way. An overheat limit switch shuts the burner down if the heat exchanger gets too hot, usually because not enough air is moving across it. Other sensors watch the flame and the flue. When a heater keeps cutting out, it's often one of these doing its job.

The heat exchanger is the wall between the flame's exhaust and the air you breathe. After years of heating up and cooling down, metal can crack or corrode, and combustion gases can then mix into the air going to the rooms.

The flue carries the exhaust outside. A blocked, disconnected or damaged flue can spill exhaust back into the roof space or the house. Energy Safe Victoria warns that all gas heaters can spill carbon monoxide, a gas you can't see or smell that can make you seriously unwell or kill you.

That's why a ducted heater service includes a combustion spillage test. Energy Safe Victoria recommends servicing ducted gas heaters every two years, with a carbon monoxide test at the heating vent outlets in every room.

The circulation fan moves a lot of air, and anything in its path affects how well the system heats. A clogged filter, a couch across the return air grille or rugs over floor outlets cut the air crossing the heat exchanger. The heater overheats, trips its limit switch, cools and restarts, which looks like a heater that can't make up its mind.

Ducts carry the warm air to each room. Under-floor ducts on older homes and flexible ducts in roof spaces both age: joints pull apart, sleeves split and insulation compresses. YourHome says ducts should be insulated to at least R1.5 with sealed joints. A leaking duct heats the roof or the subfloor instead of the bedroom.

Air also has to get back to the return grille. Bedrooms with doors shut need a gap under the door or a transfer grille, or they get less heat.

The thermostat measures the temperature where it's mounted, usually in a hallway, and runs the heater until that spot is warm. If it sits in a draughty passage or near an outlet, the heater runs too long or too short for the rooms that matter.

Many systems have zones with motorised dampers, so living areas and bedrooms can be heated separately. The Victorian Government recommends zoning so each area can be heated on its own, which saves heating the whole house when you only use part of it.

Controllers and their cabling, zone motors and the heater's power supply aren't gas work. A dead heater is sometimes nothing more than a tripped circuit, a failed controller or a damaged thermostat cable.

Gas work on the heater is for a licensed gasfitter. Energy Safe Victoria says gas heater servicing and repair should be done by a licensed Type A gasfitter, and that it's illegal, and potentially fatal, for unlicensed people to do any gasfitting work. That covers the burner, gas valve, ignition, heat exchanger, flue and combustion testing.

The power supply, circuit, controllers and cabling are work for a licensed electrician, while ducts, outlets, filters and zone dampers are looked after by an air conditioning or duct technician. On our gas ducted jobs, the gas work is done by a licensed Type A gasfitter, and our team handles the electrical, controls and ductwork so the pieces line up.

A dusty smell for a few minutes on the first run of winter is normal as dust burns off. A smell that lingers, soot near outlets, a change in flame colour, or people feeling headachy, dizzy or sick while it runs are reasons to switch it off and have a gasfitter check it. Energy Safe Victoria advises watching for changes such as flame colour and having a qualified gasfitter inspect the heater if anything changes.

Less urgent signs are still worth booking: cold or lukewarm air, several tries to light, the heater cutting in and out, a new noise from the roof or subfloor, or rooms that no longer warm up. If you're weighing up whether an older heater is worth fixing, our repair-or-replace guide works through that decision.