Carbon Monoxide and Flue Installations for Residential Gas Appliances

What it is

Carbon monoxide, abbreviated CO, is the colourless and odourless gas produced when natural gas, LPG, wood or any hydrocarbon fuel burns without enough air. In a gas appliance the CO concentration in the combustion stream stays low when the appliance is tuned and the flue draws cleanly. If the flue spills or reverses, CO can build up in the living space and reach lethal concentrations within minutes.

For a builder, CO is a risk concentrated around flued gas heaters, gas storage water heaters located inside, and gas cooktops in poorly ventilated rooms. The rules that govern flue installation and post-install testing changed materially after two Victorian fatalities in 2010 and a third in 2017.

The 2010 Victorian fatalities

In May 2010 brothers Chase and Tyler Robinson died of carbon monoxide poisoning from a faulty open flued gas space heater in their home in Shepparton, Victoria. The state coroner investigated and the Victorian government and the Victorian Building Authority introduced a series of reforms aimed at open flued heaters and flue spillage testing.

The reforms tightened gasfitter servicing requirements, required testing for negative pressure and CO spillage on every service of an open flued appliance, and led to the supply restriction on new open flued gas space heaters announced by Energy Safe Victoria. A further fatality in 2017 reinforced the program and led to the current Carbon Monoxide Safety Program run by the VBA.

AS 5601.1 flueing rules

AS/NZS 5601.1 Section 6 covers flueing of gas appliances. It sets the minimum flue height, the termination clearances from windows and air intakes, the materials for single wall and double wall flue pipe and the support requirements through ceilings and roofs.

The standard distinguishes between Type A flueless appliances such as a cooktop, Type A flued appliances such as a flued space heater, and Type B appliances such as commercial water heaters. Each type has its own rule set. For residential work the focus is on Type A flued appliances and the natural draught flues most often fitted to ducted heaters, in-room heaters and storage water heaters.

Open flued vs room sealed

An open flued appliance draws combustion air from the room and vents flue gas through a flue terminating outside. A room sealed appliance draws combustion air from outside through a dedicated inlet and vents to outside through a dedicated flue, with no air exchange with the room. Room sealed appliances carry a much lower CO risk because flue spillage cannot reach the living space.

Modern continuous flow water heaters and many ducted heaters are room sealed. Older in-room gas heaters are commonly open flued. The shift in the Australian market since 2010 has been toward room sealed designs for any appliance installed inside the building envelope.

Negative pressure and spillage

Negative pressure inside a dwelling can pull flue gas back into the room. Range hoods, bathroom exhaust fans, clothes dryers and clothes dryers ducted to outside all draw air out of the building. A tight building envelope amplifies the effect. When the negative pressure exceeds the flue draught the gases reverse.

The post-2010 reforms require a gasfitter to test for negative pressure and CO spillage on every service of an open flued appliance. The test uses a smoke pencil at the draught diverter to detect spillage and a CO meter near the appliance for concentration. The result is recorded on the service record. The VBA and Energy Safe Victoria publish the test method in their guidance sheets.

Builder obligations

A builder fitting a new gas appliance must use a licensed gasfitter and a compliance certificate is issued at the end of the work. The compliance certificate covers pipework, appliance connection, flue installation and combustion air provision. The certificate is lodged with the gas regulator in the state of work, for example the VBA in Victoria or NSW Fair Trading in NSW.

Flue termination clearances are the single most common defect. AS/NZS 5601.1 Table 6.9.4 lists the minimum clearances from openings, mechanical air intakes and adjoining structures. Boundary additions, deck extensions and air conditioning condensers added after the gas appliance was installed can pull the clearances below the minimum and create a fresh CO risk.

CO alarms in homes

There is no national requirement to fit a CO alarm in an Australian dwelling. State guidance recommends a CO alarm in any room with an open flued gas heater. AS 3786 covers smoke alarms but a separate Australian standard for residential CO alarms is not adopted by reference in the National Construction Code. International alarms conforming to UL 2034 or EN 50291 are commonly available through Australian retailers.