Kai Mallari's Notes'

High-CFM-Ducted-Hoods-Can-Backdraft-Natural-Draft-Gas-Appliances-via-Depressurization (Home Systems)

5 min read

High-CFM Ducted Hoods Can Backdraft Natural-Draft Gas Appliances via Depressurization

idea

Claim: a powerful ducted range hood can depressurize a sealed home enough to stall or reverse the flue of a naturally venting gas appliance (furnace, water heater, boiler), causing combustion gases — including carbon monoxide — to spill back into the living space. This is the depressurization-backdraft failure mode that BC Building Code Article 9.32.3.8 directly addresses.1

Mechanism

Step 1 — Negative pressure. A ducted hood exhausts air from the kitchen to the outside. An equal volume of air must enter to replace it. In a tightly-sealed home with few air leaks, this replacement air cannot enter fast enough at high fan speeds. The house develops negative air pressure relative to the outside.

Step 2 — Flue stalling. Natural-draft combustion appliances (a furnace, water heater, or boiler without a power-vented or sealed combustion design) exhaust by relying on the buoyancy of hot flue gases rising through a metal flue — the same physics as a chimney. This rising column requires the ambient air pressure at the flue base to be at least equal to the pressure at the flue exit. When the house is depressurized, the pressure differential reverses.

Step 3 — Backdraft. The reversed pressure differential pulls flue gas back down the flue and out the draft hood or barometric damper into the mechanical room — and from there into the living space. This reversed gas contains carbon monoxide, nitrogen dioxide, and particulates from combustion.

Pressure scale: studies cited by GreenBuildingAdvisor indicate a home depressurization of roughly 3 Pascals or more begins to create backdraft risk for naturally venting appliances. A high-CFM range hood can easily exceed this.2

The BC Building Code requirement

BC Building Code Article 9.32.3.8 (carrying forward the NBC 2020 requirement) states: where a dwelling contains a naturally aspirating fuel-fired vented appliance subject to backdrafting, a makeup air system must be provided. The makeup air fan must deliver outdoor air at a rate equal to (but not exceeding) the exhaust device capacity plus 10%, and must activate simultaneously with the exhaust device.1

What this means for a homeowner:

  • If you have a natural-draft furnace, water heater, or boiler AND a range hood over 400 CFM → the code requires makeup air (a powered duct that brings outdoor air into the home when the hood runs).
  • If your home was built or retrofitted before this requirement was enforced, or if a new high-CFM hood was installed without checking compliance → you may not have makeup air in place.
  • Sealed-combustion or power-vented appliances (their exhaust is mechanically pushed out through a dedicated sealed pipe, not relying on buoyancy) are not vulnerable to backdraft from hood depressurization.

Scope

  • Applies only to ducted hoods — recirculating hoods return air to the kitchen and do not depressurize the home.
  • Applies only to homes with naturally venting combustion appliances — sealed-combustion units are not affected.
  • The 400 CFM threshold in the IRC (US) and referenced in Canadian guidance is a conservative rule of thumb — the actual threshold depends on the home’s air-tightness. A leaky older home may tolerate a 500 CFM hood without backdraft; a tight new-construction home may backdraft at 250 CFM.
  • This does not cover CO produced by the range hood itself — CO from cooking (gas stoves) is a separate pathway.

The decision rule

If you have a gas furnace or water heater that is NOT sealed-combustion or power-vented, AND your ducted range hood is ≥400 CFM → have a licensed HVAC technician assess whether makeup air is in place and code-compliant. This is not a DIY assessment — it requires airflow measurement and possibly a combustion safety test (sometimes called a spillage test or CAZ — combustion appliance zone — test).

Idea Compass

North: Where this comes from

East: Tensions / failure

South: Where this leads

West: What’s similar

  • dryer (Home Systems) — high-efficiency dryers also exhaust significant air volumes; same depressurization concern in a very tight home, though at lower CFM than a high-powered range hood
  • smoke-co-detectors (Home Systems) — the detection layer that catches backdraft CO if it occurs

Sources

Footnotes

  1. BC Building Code (Division B Part 9 §9.32), Article 9.32.3.8 — makeup air requirement for homes with naturally aspirating fuel-fired vented appliances; system must be interlocked to exhaust device — https://free.bcpublications.ca/civix/document/id/public/bcbc2006/building_b_p9_9.32 ↩ ↩2

  2. GreenBuildingAdvisor — depressurization/backdraft mechanism explanation: negative pressure pulls combustion gases back down the flue; a 3-Pascal differential is cited as a threshold for concern in naturally venting appliances; makeup air solutions — https://www.greenbuildingadvisor.com/article/makeup-air-for-range-hoods ↩

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