Two Bath Fans, a Tight House, and a Spilling Water Heater
A 1,800-square-foot, three-bedroom ranch built in 1978 has two bathrooms and a recirculating range hood. A blower door test measures 2.8 ACH50. The hall bath fan is labeled 80 CFM and the master bath fan 50 CFM; measured with a flow hood at the grilles with the doors closed, they deliver 32 CFM and 18 CFM. Both fans use 4-inch insulated flex duct. The hall bath duct makes two tight 90-degree turns and is compressed to about 2 inches where it passes a roof truss. The master bath duct ends in the attic about 2 feet above the blown insulation, well short of the roof. A natural-draft gas water heater sits in a louvered closet in the hall. With both bath fans running and the air handler on, a smoke pencil held at the water heater draft hood shows smoke being pulled down into the room. The owner reports condensation on the north-facing bedroom windows, a musty smell in the master closet, and mirrors that stay fogged for hours.
1. Using the ASHRAE 62.2 whole-house ventilation approach, calculate a ventilation target for this house and evaluate the measured fan performance.
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The widely used 62.2 formula is Q = 0.01 x floor area + 7.5 x (bedrooms + 1). Here: 0.01 x 1,800 = 18 CFM, plus 7.5 x 4 = 30 CFM, for about 48 CFM continuous. Later editions of the standard raise the floor-area coefficient to about 0.03, which would give 54 + 30 = 84 CFM, so the answer depends on which edition the local code has adopted - that must be checked, not assumed. Either way, the installed system falls short, and the measured numbers understate even the labels: 32 CFM at the hall grille and 18 CFM in the master bath, against labels of 80 and 50.
The shortfall has physical causes, not just undersized fans: the crushed flex section and two tight elbows cut the hall fan's delivered flow, and the master bath duct does not exhaust outdoors at all - it dumps warm, moist air into the attic. That is both an IAQ failure and a moisture problem for the roof structure. Bathrooms also have per-fixture requirements; common intermittent targets are 50 CFM for a bathroom (20 CFM if continuous), and the master bath meets neither. With no kitchen exhaust to the outdoors and a tight 2.8 ACH50 envelope, there is effectively no designed ventilation.
2. Explain the mechanism behind the smoke pencil result and describe how you would evaluate it.
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The house is tight (2.8 ACH50) and the two exhaust fans, the air handler, and the clothes dryer all remove air without a deliberate makeup-air path. The result is negative pressure in the hall relative to outdoors. The water heater's atmospheric draft depends on a rising column of warm gas in the flue; a few pascals of negative pressure in the closet is enough to overcome that weak draft and reverse it, so the smoke pencil shows air moving down the draft hood into the room. This is spillage or backdrafting, and it puts combustion products, including CO, into the living space.
Evaluation: perform a worst-case depressurization (CAZ) test - close the house up, operate all exhaust fans and the air handler, then measure the pressure difference between the water heater closet and outdoors (commonly a -3 Pa threshold is used as a flag, with the appliance's own listing governing) and re-check spillage at the draft hood. Test for CO in the flue and in the room, verify the closet has adequate combustion and dilution air openings sized for the input rating, and confirm the flue is intact and properly sized. Recommend CO alarms regardless. This is a safety finding and belongs at the top of the report, with the caveat that a full combustion safety evaluation is the contractor's scope.
3. List the corrective actions in priority order and state what you would and would not put in the report.
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Priority order:
- Combustion safety. Defer the water heater spillage to a licensed contractor for a combustion safety evaluation; recommend CO alarms now. Do not advise the owner to keep using the bath fans as they are until makeup air and venting are corrected.
- Master bath duct. It must terminate outdoors with a proper roof or wall cap, backdraft damper, and insulated duct; as installed, it is adding moisture to the attic and should be corrected before anything else in the ventilation scope.
- Restore delivered airflow. Replace the crushed flex and tight elbows in the hall bath run, use smooth-radius fittings, support the duct, and verify delivered CFM at the grille with a flow hood (target 50 CFM intermittent minimum for that bath).
- Whole-house strategy. With a recirculating range hood and no outdoor kitchen exhaust, add a designed ventilation system (exhaust or balanced) sized to the 62.2 target, plus makeup air or transfer paths so exhaust does not depressurize the CAZ.
- Moisture symptoms. Window condensation and the musty closet smell are consistent with the ventilation deficit; recommend monitoring after correction.
In the report, state observations and measurements (fan labels, measured CFM, duct termination in attic, spillage observed at the draft hood with fans running), cite the applicable standard as adopted locally, and defer causes and repairs. Do not declare the water heater defective, and do not offer the 62.2 calculation as a code determination without confirming the adopted edition.