Backdrafting Is the CO Entry Point for Natural-Draft Gas Appliances
Claim: When a gas appliance vents by natural draft (buoyancy), any condition that lowers indoor air pressure below outdoor pressure can reverse exhaust flow and deliver combustion gases — including CO — into the living space. This is the primary mechanism by which gas appliance venting fails silently.
Mechanism
Natural-draft (atmospheric) gas appliances — conventional furnaces, atmospheric water heaters, B-vent gas fireplaces — draw combustion air from the room and rely on the buoyancy of hot exhaust gases to rise up the flue. The system only works if the air pressure in the combustion-appliance zone (the room or mechanical space where the appliance sits) is at or above outdoor pressure.
Backdrafting is when indoor pressure falls below outdoor pressure and exhaust flow reverses — flue gases descend into the room instead of rising out. Spillage is a related condition where exhaust gases escape from the appliance’s dilution-air vent (draft hood) before reaching the flue — caused by blockage or insufficient draft.
Both deliver carbon monoxide into the living space.
Primary depressurisation sources:
- Range hoods at 300–1,200 CFM — the most common culprit; large exhaust fans move enough air to reverse B-vent draft1
- Bathroom exhaust fans running simultaneously
- Clothes dryers (exhaust-only venting)
- HRVs set to exhaust-heavy balance
- Tightly closed interior doors (isolating the mechanical room from supply air)
Other backdraft causes:
- Blockage at the vent termination (bird nest, ice, debris)
- Disconnected vent connector
- Undersized or oversized flue
- Strong wind downdraft at the chimney cap
Scope
- Applies to Category I natural-draft appliances only (≤83% efficiency, non-positive vent pressure — the majority of conventional furnaces and water heaters in pre-2000 BC homes)
- Does not apply to direct-vent sealed-combustion appliances — the sealed combustion chamber prevents room-air interaction
- Does not apply to induced-draft / power-vent appliances in the same way — the fan provides positive pressure, though blockages still create CO risk
- CO detectors detect the result of backdrafting, not the backdraft itself — a single alarm means the event already happened; detectors do not prevent the first exposure
Idea Compass
North: Where this comes from
- gas-appliance-venting (Home Systems) — the parent note covering all vent types and the full safety system
- CSA B149.1 appliance categories — the classification that separates atmospheric from sealed systems
East: Tensions / failure
- smoke-co-detectors (Home Systems) — CO detectors are the backstop after backdrafting begins, not before; detection is not prevention
- Makeup air requirements — the building-science fix (supplying replacement air when exhausting) that prevents depressurisation, but is rarely retrofitted in existing homes
South: Where this leads
- Direct-Vent Sealed Combustion Eliminates Backdraft and Depressurization Risk (Home Systems) — the structural fix when replacing an appliance
- Annual licensed gas fitter service with CO draft test — the operational fix that catches a backdrafting condition before it becomes a CO event
West: What’s similar
- chimney-flue (Home Systems) — a blocked or cracked flue liner creates the same CO-spillage failure for wood-burning systems
- Orphaned Water Heater — Oversized Shared Flue After High-Efficiency Furnace Replacement (Home Systems) — a specialised form of the same mechanism: flue conditions cause CO spillage
Sources
Footnotes
-
GreenBuildingAdvisor, a building-science publication — range hood makeup air requirements; hoods at 300–1,200 CFM create negative pressure sufficient to reverse B-vent draft in natural-draft systems — https://www.greenbuildingadvisor.com/article/makeup-air-for-range-hoods ↩