How a negative pressure and carbon monoxide spillage test runs

Why does this test exist?

Since 2010, three Victorians have died from carbon monoxide poisoning caused by open-flued gas space heaters, and there have been several documented non-fatal poisonings on top of that. That figure comes from the Building and Plumbing Commission (formerly the Victorian Building Authority, or VBA). It's why this test is mandatory rather than recommended.

An amendment to the Gas Safety (Gas Installation) Regulations 2018 made Australian Standard AS 4575 the required standard for servicing Type A gas appliances, from 1 June 2022. You have to pay to read the standard itself. Its checklist is in Appendix E, and Energy Safe Victoria (ESV) publishes a free checklist that stands in for it in GIS 59. That's where the steps below come from.

What are the four stages?

1. Get a baselineHeater off, flue cold, all external doors and windows shut, all extraction fans off. Hold a smoke device at the draught diverter relief openings or the combustion air openings and watch which way the smoke goes. That's your reference.
2. Create the worst caseTurn every extraction fan on and set the internal doors to get the greatest possible negative pressure. You're not testing the house as you found it. You're testing it at its worst, because that's when it kills people.
3. Test for CO spillageRecord the room's background CO first. Then run the appliance flat out, on its highest burner and fan settings and at its nominal test point pressure, with the fans still going. On a ducted heater there must be no spillage 5 minutes after ignition.
4. Assess itCompare the CO reading with the background you recorded. If it's gone up at all, you've got spillage and you need to find the cause.

The cold flue matters more than it sounds. A warm flue is already drawing, so it'll hide exactly the fault you're looking for. If you've just run the heater to check something else, you can't do the test until the flue has cooled down again.

Where do you put the probe?

• Draught diverter relief openings
• Heat exchanger joints
• The warm air outlet
• The flue connection and its joints
• The base of the flue hoods

GIS 59 adds one visual check that's worth as much as any reading. In its words, "Burn or scorch marks on the appliance are an indication of overheating and need to be investigated." You can spot that from the front before you open anything.

How much ventilation does it need?

This is the neatest bit of the whole procedure. You don't calculate it. You measure it, with a window.

Slowly open a window in the room the appliance is in until the smoke behaves the way it did in your baseline. The size of the window's open area is the permanent ventilation you need to overcome the negative pressure. Then install that much fixed ventilation.

On a new installation, that ventilation has to be in before you commission the appliance, not after. And if there's any negative pressure at all, GIS 38 says the installation doesn't meet clause 6.3.1 and it's non-compliant until you fix it.

What about your own exposure?

GIS 44 sets limits for the person holding the meter. It says to always take your own exposure readings while you're testing the appliance.

Do CO alarms count for anything?

They help, but they're no substitute. ESV recommends alarms certified to EN50291 or UL2034, and says plainly that Australia has no specific standard covering CO alarm design or installation. Put them in or near rooms with gas heating, close to where somebody actually breathes, loud enough to hear from the bedrooms, and away from gas cooking and dead air spaces.

The caveat is the part worth repeating to a customer. An alarm can't cut off the gas if something goes wrong, and it's no substitute for a proper install and a service every two years.