I post the content of a message to a customer. The exercise in math and logic may clarify my problem with any blower door testing, as prerequisite and director, for maintenance. I hope it will raise Diligence Honored, as a replacement for almost all Home Performance testing.
I monitored the conversation with your condo-complex General Contractor, about the 110 CFM Panasonic fan in your master bathroom. This fan is presumed 12” by 16” through ceiling. In fact the cut is 14” by 18”. The gap is hidden by the fan cover, but is unobstructed as a house infiltration path.
The involved flow area is 14*18 - 12*16 = 60 sq in.
CFM50 = 7.5 * (Path Area, sq in) = 450 cfm. CFM = CFM50 / 20 = 22.5 cfm
The annual cost of this leakage at fair $2 per therm, is 0.074 * CFM50 = $33 per year.
The 22 5 cfm constant flow is equal to fan flow running 22.5/110 * 24 = five hours per day.
Would you intentionally ventilate this much, or is this objectionable and involuntary?
Energy Trust would rebate half of the cost of fixing the gap, say $50 times half = $25. The rebate would require blower door tests before and after, surely in separate-day unhappy visits for you and the tester, at cost of several hundred dollars. The tests and rebate are of course absurd.
The general contractor did not give good advice.
A lesson for all of us, is that diligence must happen, at fair least cost, without a fuss, and without bad feelings because someone would pull the public string. All “performance” conditions for rebates, and all rebates should end. They are just a disadvantage to this good person, who will not abuse the public trust.
My customer liked this advice, and I will fix his fan installation. No testing needed. The math is far more accurate than testing, for any one repair.
The repair is not insignificant, and that is why an electrician or handyman walks away from his error. I will fully remove the fan, and rebuild the opening with real plaster, total job time about an hour. I will help anyone to be comfortable using plaster (not mud!), here Structolite. Weatherization sponsors should step in to require repair of installer error. Repairs should not be left to the rare capable weatherization person, and must not be rebateable.
I belatedly admit a simplification, and over-estimate of this problem. I well know that Panasonic fan housings are bottom-flanged, to align on and seal against the ceiling. That flange blocks the flow path, probably by about two thirds. This detail does not change resolve to do the repair. It does nothing to justify attendant testing.