CSLB C-20 HVAC ContractorVentilation and Air DistributionHard
A home has a 400 CFM range hood, a 200 CFM clothes dryer, and two 50 CFM bathroom exhaust fans all operating simultaneously, totaling 700 CFM of exhaust. The home's envelope and passive makeup air pathways can only supply 500 CFM of replacement air. Which hazardous condition is MOST likely to result?
- ABackdrafting of a naturally vented gas water heater due to negative pressure
- BIncreased humidity levels causing microbial growth in supply ducts
- CExcessive positive pressure causing duct blowout
- DOverheating of the range hood motor from airflow restriction
Show answer & explanationAnswer & explanation
Correct answer: A. Backdrafting of a naturally vented gas water heater due to negative pressure
When total exhaust (700 CFM) significantly exceeds available makeup air (500 CFM), the home becomes negatively pressurized. This depressurization can overpower the draft of naturally vented (atmospheric) combustion appliances like water heaters, pulling combustion byproducts including carbon monoxide back into the living space.
Why the other options are wrong
- B. Humidity/microbial growth is unrelated to this specific pressure imbalance scenario.
- C. Depressurization, not positive pressure, is the result of exhaust exceeding makeup air.
- D. The hood motor is unaffected by whole-house pressure imbalance in this way.
Depressurization & Backdrafting
When exhaust airflow exceeds available makeup air, the home depressurizes, which can reverse the natural draft of atmospheric combustion appliances and pull combustion gases indoors.
- Exhaust > makeup air = negative house pressure
- Naturally vented water heaters/furnaces are most vulnerable
- Backdrafting can introduce carbon monoxide into living space
Memory trick: Suck out more than you let in, and the water heater's draft caves in.