The Depressurized House and the Backdrafting Water Heater
A 1,500 ft² house has a naturally drafted gas water heater in a closet with a louvered door, a range hood rated 600 CFM, and two bath fans at 80 CFM each. During a blower door test with the house at 50 Pa, the technician runs the range hood and both bath fans. A spillage test at the water heater shows flue gases spilling into the closet. The house measures 2,100 CFM50. The water heater closet is in the middle of the house and has no dedicated combustion air duct. The homeowner has never noticed a problem but occasionally smells exhaust near the closet.
1. Explain what the spillage test shows and why the combination of fans and a naturally drafted appliance is dangerous.
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Spillage means the water heater's flue is not drafting - combustion gases are entering the closet and can enter the living space. A naturally drafted appliance relies on a pressure difference between the house and the outdoors to pull combustion air in and push flue gases up the vent. When the 600 CFM range hood and two 80 CFM bath fans run, they exhaust up to 760 CFM and depressurize the house. The water heater closet, with only a louvered door and no dedicated combustion air, cannot supply makeup air fast enough, so the house pressure drops and the flue reverses. This is a carbon monoxide hazard. The occasional exhaust smell is a warning sign. The fix is to eliminate the depressurization or isolate the appliance: provide dedicated combustion air, replace the natural-draft water heater with a sealed-combustion or power-vented unit, or interlock the fans with a pressure switch. Verify with a combustion analyzer and spillage test after correction.
2. The homeowner wants to keep the 600 CFM range hood. Describe how you would make the system safe and verify it.
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Keeping the hood is acceptable only if the house can supply makeup air and the combustion appliance is protected. Options: (1) install a makeup air system interlocked with the hood, sized to supply most of the hood's exhaust, with a motorized damper and tempering if needed; (2) replace the natural-draft water heater with a sealed-combustion or power-vented unit so it is isolated from house pressure; (3) add a pressure switch that disables the hood if house depressurization exceeds a set threshold (typically around -5 Pa in the combustion zone). Often the best package is a sealed-combustion water heater plus a makeup air duct and a pressure monitor. Verification: run the hood and both bath fans, measure house pressure relative to outdoors, perform a spillage test at the water heater, and use a combustion analyzer to confirm acceptable CO and draft. Document results and re-test seasonally.