CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A client is installing a kitchen range hood rated at 500 CFM. Per the California Mechanical Code, what additional requirement is triggered by exceeding the 400 CFM threshold?
- AA secondary duct must be run to the attic for pressure relief
- BMakeup air must be provided to compensate for the exhausted air
- CThe hood must be downgraded to comply with energy code
- DA carbon monoxide detector must be installed in the kitchen
Show answer & explanationAnswer & explanation
Correct answer: B. Makeup air must be provided to compensate for the exhausted air
The CMC requires makeup air whenever a kitchen exhaust hood is rated at or above 400 CFM, preventing excessive negative pressure that could cause backdrafting of combustion appliances or infiltration issues.
Why the other options are wrong
- A. Attic venting is not a recognized makeup air solution.
- C. Energy code does not require downgrading hood capacity for this issue.
- D. CO detectors are a separate life-safety requirement, not tied to this CFM threshold.
Range Hood Makeup Air
CMC requirement that kitchen exhaust systems rated 400 CFM or greater must be provided with a makeup air source to prevent house depressurization.
- Threshold is 400 CFM per CMC
- Prevents backdrafting of gas water heaters/furnaces
- Makeup air can be passive or mechanically supplied and interlocked with the hood
Memory trick: Four hundred's the line — bring in makeup air time.