Recirculating-Range-Hoods-Do-Not-Remove-Moisture-or-CO (Home Systems)

Recirculating Range Hoods Do Not Remove Moisture or CO

idea

Claim: a ductless (recirculating) range hood passes kitchen air through a charcoal filter to absorb odour and some grease vapour, then returns the air to the kitchen. It does not remove steam, moisture, or combustion gases (including CO). For homes with gas stoves, this is a meaningful limitation.

Mechanism

A ducted-to-exterior range hood physically removes a volume of kitchen air — including all its contents (grease, steam, CO, odours) — and exhausts it outside. Replacement air enters through leaks or a makeup-air system.

A recirculating hood draws kitchen air into the hood, passes it through:

1. A metal mesh or baffle filter (captures some grease particles)
2. A charcoal/activated-carbon filter (adsorbs odour molecules and additional grease vapour)

…and then returns the filtered air to the kitchen from vents on the front or top of the hood.

What the charcoal filter removes: odour molecules, some grease vapour.

What the charcoal filter does not remove:

• Steam and moisture — water vapour is not captured by activated carbon. All the steam from boiling, sautéing, or pressure cooking stays in the kitchen and eventually condenses on surfaces and inside walls, promoting mould.
• Carbon monoxide — CO from gas combustion is not captured by activated charcoal. A gas stove’s burners produce CO. Without exterior venting, this CO remains in the kitchen.
• Other combustion gases (nitrogen dioxide, particulates) — also not removed.
• The home is not depressurized — the same air volume is returned. This means recirculating hoods do not carry the backdraft risk that high-CFM ducted hoods do.¹

Scope

• The CO limitation is more significant with gas stoves (which produce combustion gases at the burners) than with electric or induction cooktops (which produce no combustion gases at the surface).
• The moisture limitation matters more in humid climates and tight buildings (Metro Vancouver’s mild-damp climate; newer high-insulation strata units) where moisture has nowhere to go.
• Recirculating hoods are sometimes used in strata units where exterior ductwork cannot be installed (no exterior-wall penetration approved, or common-property duct termination issues). Understanding the limitation is especially important in this context.
• BC Building Code requires exterior ducting for kitchen exhaust in new construction (recirculating does not satisfy the BC Building Code’s requirement for kitchen ventilation in most jurisdictions).² An existing recirculating hood in a pre-code-update unit may be grandfathered.

The decision rule

If you have a recirculating hood and a gas stove → CO from the burners is not being removed from your kitchen during cooking. Run the hood for odour and grease; open a window during cooking for air exchange; ensure you have a functioning CO detector in the kitchen area. This is not a crisis — residential gas stove CO output is generally low — but it is a meaningful limitation to be aware of.

If you are replacing a recirculating hood → evaluate whether ducting to exterior is feasible first. A ducted replacement requires a duct penetration through the exterior wall (strata approval + possible permit), but permanently solves moisture, CO, and odour removal. A like-for-like recirculating replacement is simpler but carries the limitations forward.

Idea Compass

North: Where this comes from

• range-hood (Home Systems) — the parent component note; this is the ducted vs ductless trade-off
• gas-appliance-venting (Home Systems) — CO from combustion appliances is the same safety topic at larger scale

East: Tensions / failure

• High-CFM-Ducted-Hoods-Can-Backdraft-Natural-Draft-Gas-Appliances-via-Depressurization (Home Systems) — the risk that ducted hoods carry that recirculating hoods do NOT; trade-off, not simply better vs worse
• A-Grease-Laden-Range-Hood-Filter-Is-a-Fire-Path-Not-Just-a-Dirty-Filter (Home Systems) — both ductless and ducted hoods accumulate grease; this failure mode applies to both

South: Where this leads

• smoke-co-detectors (Home Systems) — the detection layer that matters more in recirculating-hood kitchens with gas stoves
• range-hood (Home Systems) — the Bottom line one-time-setup note on confirming hood type

West: What’s similar

• HVAC air recirculation — recirculating hoods behave like an air cleaner (odour removal only), not like ventilation (air exchange)

Sources

Footnotes

1. GreenBuildingAdvisor — recirculating hoods explained: charcoal filter removes odour but not moisture or combustion gases; does not depressurize the home; comparison with ducted systems — https://www.greenbuildingadvisor.com/article/makeup-air-for-range-hoods ↩

2. BC Building Code, Section 9.32.3 (kitchen exhaust requirements) — an exhaust fan must discharge to outdoors; a recirculating hood fan does not satisfy the BC Building Code’s kitchen ventilation requirements in new construction — https://free.bcpublications.ca/civix/document/id/public/bcbc2006/building_b_p9_9.32 ↩