Spot Ventilation Fan Flow Measured With the Wrong Reference
A quality control inspector is verifying a bathroom exhaust fan installed in a 90 sq ft bathroom with an 8 ft ceiling in a home enrolled in a program that requires 50 CFM intermittent or 20 CFM continuous operation. The installer's report claims 62 CFM measured with a flow hood at the grille. The inspector's own measurement with a calibrated flow hood at the same grille reads 41 CFM. The duct run is 12 ft of 4 inch flex with two 90 degree elbows and terminates at a roof cap. The fan nameplate is not visible because it is above a finished soffit. The installer says the hood reading varies with how the hood is seated and that 41 CFM is 'close enough.'
1. How should the inspector resolve the discrepancy between 62 CFM and 41 CFM, and what is the compliance conclusion?
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The inspector should not average the two readings or defer to the installer's number. First, confirm the measurement method: the flow hood must be properly seated and sealed against the grille, the correct hood range or correction factor applied, and the reading taken after flow stabilizes. A loose seat leaks air around the hood and typically reads low, while an oversized hood range or a mis-set correction can read high. The inspector's calibrated instrument and documented technique should govern unless a defect in that technique is demonstrated.
With a 41 CFM reading and a 50 CFM requirement, the installation fails as measured. 'Close enough' is not a standard; the program threshold is a hard line. The inspector should record both readings, the instrument used, the calibration date, and the seating method. Because the fan nameplate is hidden, the inspector should determine the model from the submittal or the fan curve and check whether the measured flow is consistent with the installed duct static pressure. The conclusion is noncompliant pending correction or a validated retest by an independent party.
2. The inspector suspects the duct design is the cause. Explain how the duct run affects delivered flow and what should be verified before blaming the fan.
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Delivered flow depends on the fan curve and the system static pressure. The 12 ft of 4 inch flex with two 90 degree elbows and a roof cap adds friction and fitting losses. Flex duct that is compressed, sagging, or sharply bent at the elbows raises resistance substantially, and 4 inch duct is already restrictive at bathroom fan flows. As static pressure rises, the fan moves less air along its curve, so a fan rated at 62 CFM free air can deliver well below 50 CFM at the grille.
Before blaming the fan, the inspector should verify the duct diameter against the fan outlet, that the flex is fully extended and supported, that elbows are swept rather than crushed, that the roof cap is the specified type and not blocked by a damper stuck closed or debris, and that the duct is not disconnected or leaking in the attic. The inspector should also confirm the fan is the specified model and that it is receiving full voltage. If the duct is the cause, the correction is duct repair or redesign, not a larger fan alone. The report should state the measured flow, the observed duct defects, and the required correction.
3. What documentation should the inspector produce so the finding survives a program audit?
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The record should be contemporaneous and reproducible. It should include the room dimensions and the required ventilation rate with the program citation, the fan model and rated flow if obtainable, the instrument make and model with calibration date, the exact measurement location and seating method, and the measured value. It should record the installer's claimed value and the reason it was not accepted, without personal criticism.
Photographs should show the grille, the hood in place, the duct run including elbows and roof cap, and any defect such as crushed flex or a stuck damper. A sketch of the duct route with lengths and fitting counts supports the static pressure reasoning. The report should state the finding (noncompliant, 41 CFM versus 50 CFM required), the required corrective action, and the retest requirement. It should be dated and signed and stored with the project file. If the installer disputes the finding, the inspector should note the dispute and the basis for the inspector's measurement rather than quietly changing the number.