When exhaust fans and tight construction pull more air out of a home than can get back in, a naturally vented gas appliance can backdraft its exhaust into the house.
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Quick answer
Backdraft happens when negative pressure inside a home overpowers the natural draft of an atmospheric-vent gas appliance, pulling combustion exhaust, including carbon monoxide, back down the chimney or vent into the living space instead of outside. It is more common in tightly sealed or renovated homes where exhaust fans, dryers, and fireplaces compete for a limited supply of makeup air. A licensed gas fitter checks draft under worst-case conditions, not just at idle, to confirm an atmospheric appliance is venting safely.
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Every exhaust device in a house, a bathroom fan, a range hood, a clothes dryer, a central vacuum, even a fireplace drawing combustion air from the room, removes air from the building. In a leaky older home, replacement air seeps in easily through gaps around windows, doors, and the building envelope, keeping pressure roughly balanced. In a tightly sealed or recently renovated home, especially after upgrades like new windows, added insulation, or air sealing during a renovation, there may not be enough natural leakage to replace that exhausted air quickly.
When several exhaust devices run at once, such as a range hood and a dryer while a bathroom fan is also on, the resulting negative pressure can become strong enough to overpower the gentle natural draft that an atmospheric-vent boiler or water heater relies on to push exhaust up its chimney.
Atmospheric boilers and water heaters vent by natural draft alone, meaning warm exhaust gas rises up the chimney because it is lighter than the surrounding air, with no fan assisting it. This draft is a relatively weak force compared to a powerful exhaust fan. If negative pressure in the home exceeds the chimney's natural draft, the exhaust can reverse direction and spill into the mechanical room or living space instead of exiting the roof.
Sealed-combustion, direct-vent condensing appliances are far less vulnerable to this because they draw combustion air directly from outdoors through a dedicated intake and use a fan to push exhaust out, independent of the home's indoor pressure. This is one reason Metro Vancouver homes undergoing tightening renovations are often good candidates for converting older atmospheric equipment to sealed combustion.
None of these signs should be treated as minor. If a CO alarm sounds or spillage is suspected, everyone should leave the house and call for help from outside rather than trying to diagnose the cause while still inside.
A proper backdraft assessment goes beyond checking draft with everything else in the house turned off. A technician should run a worst-case depressurization test, turning on exhaust fans, the dryer, and any other competing devices simultaneously, then checking whether the atmospheric appliance still drafts correctly under that load. This kind of testing reflects how the home actually behaves day to day, not just in ideal conditions.
| Check | What It Reveals |
|---|---|
| Worst-case depressurization test | Whether draft holds up with all exhaust devices running |
| Spillage test at the draft hood | Whether exhaust is escaping into the room instead of venting |
| Combustion air supply inspection | Whether the mechanical room has enough makeup air |
| Chimney or vent condition | Whether blockage or damage is contributing to poor draft |
Solutions depend on the cause. Sometimes adding a dedicated combustion air duct to the mechanical room restores enough makeup air for the appliance to draft properly. In other cases, converting an atmospheric boiler or water heater to a sealed-combustion, direct-vent model removes the dependence on room pressure altogether. Homeowners planning to tighten their building envelope with new windows or insulation should raise this with their contractor beforehand so combustion air needs are addressed as part of the same project, not discovered afterward.
Safety note
If you smell gas, hear hissing near a gas line, or your carbon monoxide alarm sounds, leave the home immediately with everyone inside, do not operate switches or light anything, and call FortisBC at 1-800-663-9911 from outside the house. Gas and combustion work must be carried out by a Technical Safety BC licensed gas fitter.
It's when exhaust from a gas appliance's chimney or vent reverses direction and comes back into the house instead of going outside, usually because something else in the home is pulling air out faster than it can be replaced.
Yes, a powerful range hood can create enough negative pressure to overwhelm an atmospheric water heater's natural draft, especially if the home is tightly sealed. This should be checked when installing a strong hood.
It greatly reduces it, since these units draw air from outdoors and use a fan to push exhaust out, independent of indoor pressure. Atmospheric appliances remain the ones most at risk.
Signs include soot around the draft hood, a musty exhaust smell near the appliance, or a carbon monoxide alarm sounding, particularly when other exhaust devices are running.
You can watch for the visible warning signs, but a proper depressurization test that safely evaluates draft under worst-case conditions should be done by a licensed technician.
It can, if the renovation tightens the building envelope without addressing combustion air for existing atmospheric-vent gas appliances. It's worth having this checked as part of any major renovation.
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