Bath Fan That Won't Clear the Mirror
A homeowner in a tight, well-sealed 1,800 ft² house complains that the bathroom mirror fogs for 20 minutes after showers and that the bathroom smells musty. The existing exhaust fan is rated 80 CFM at 0.1 in. w.c. and is ducted through 25 ft of flexible 4-inch duct with four 90-degree elbows to a roof cap. A flow hood measurement at the grille reads 28 CFM. The house has a heat recovery ventilator (HRV) but it serves only the bedrooms and living areas, not the bathroom. The homeowner has sealed the house tightly and reports the bathroom door has a 1-inch undercut. The blower door reads 1.9 ACH50.
1. Diagnose why the fan is delivering only 28 CFM and propose a corrected design.
Read the worked answer
The fan is rated 80 CFM at 0.1 in. w.c., but 25 ft of flexible 4-inch duct with four elbows adds enormous static pressure - flexible duct is rough and each 90-degree elbow adds equivalent length. The fan is operating far off its curve, so delivered flow collapses to 28 CFM. The fix: replace the flexible duct with smooth rigid or semi-rigid duct sized to the fan (6-inch for 80–110 CFM), minimize elbows, and use a short, direct run to a proper termination. Then select a fan rated to deliver the target CFM at the actual system static pressure, not at 0.1 in. w.c. Target at least 50 CFM intermittent for a bathroom per ASHRAE 62.2, or 20 CFM continuous. Verify with a flow hood after installation. The musty smell is consistent with chronic under-ventilation and elevated humidity.
2. The house is at 1.9 ACH50 with an HRV serving only living areas. Explain the interaction between tightness, the bathroom exhaust, and makeup air, and whether the HRV should be extended to the bathroom.
Read the worked answer
At 1.9 ACH50 the envelope is tight, so a bathroom exhaust fan must have a makeup air path or it will depressurize the house and starve itself. The 1-inch door undercut helps but may be insufficient for a high-CFM fan; a transfer grille or jump duct sized for the flow is better. Depressurization can also backdraft combustion appliances and pull soil gas or garage air indoors. Extending the HRV to the bathroom is a strong solution: continuous balanced ventilation removes moisture without depressurizing the house and provides makeup air. If the HRV cannot be extended, provide a dedicated makeup air path and verify the fan's delivered flow. The combination of tight envelope plus unbalanced exhaust is the root cause; balanced ventilation plus a properly sized, short, rigid duct run solves both the moisture and the pressure problem.