air exchanges

In preparing an HVAC-CALC estimate of my house's heating load, the assumption that I am least comfortable with is about infiltration/air exchanges. I once had a blower-door test result of 7250 CFM at 50 pascals. Is there a way to derive a CFM figure for infiltration (and therefore an air-exchange value) from this number?
Thanks.

 

To convert the CFM derived from the blower door test to air changes/hour, multiply the CFM by 60 to get cubic feet/hr (in your case, 7250 x 60 = 435,000 CFH). Then divide the CFH figrue by the volume of air in your house (for example: 2000 sq. ft. of floor space x ceiling height of 8 feet = 16,000 cubic feet). The resulting figure is air chages per hour.

I'm pretty sure my calculation is correct but just to be sure, check with Don at hvaccomputer. If I remember correctly, you get 3 free "consultations" with the rental of the program. Please let me know if I am in error on either the calculation or free consultations.

 

Grady,
Thanks; your formula looks right for finding air exchanges/hour under the same conditions as the blower door test. But what I'm trying to find out is how the CFM at 50 pascals relates to what should be a lower CFM value without the fan-induced pressure. [Also, I'm realizing that I may be misreading the handwritten number on this old report, that looks like "7250"; for my 20,700 cubic-foot house, that would mean an enormously high value for air exchanges/hour.]
- Old Vic

 

The old standby conversion from CFM50 to natural was to divide by 20. The new lbl number try's to account for wind exposure, zone, and height of house and I'm sure I can locate a chart somewhere on the web. However, your concerns about having the correct reading are valid. In fact, I don't think a standard blower door goes up to 7250, at least I have never run one that high. In any case, that is way too high, unless you have an extremely leaky old house, or a door open.

Yes, just checked and that is beyond the capability of a standard Minneapolis blower door. You might find their manual interesting:
http://www.energyconservatory.com/download/bdmanual.pdf

The one thing you have to realize about all of these numbers, besides your measuring tape, they are all estimates, judgment, guesstimates. Even some of the better energy auditing software will try to adjust their calculations to match what the home owner is actually using, fudge factor.

Bud

 

Bud,

Thanks again. Based on your message, I've re-examined the number, and realized that it just might be 2250, not 7250 (unfortunately, the company that did the audit is out of business now, so I can't check with them). In that case, I'm in good shape, with around 0.32 air exchanges/hour for my 20,700 cubic-foot house.

It seems odd, though, that 7250 isn't achievable, since using the 1/20 factor of with 7250 would come out to exactly 1 air exchange per hour for my house, which I've read is reasonable for a house with average insulation (although I think mine is better than average). I don't understand why a blower test wouldn't be able to get up to the CFM volume for an average-sized house with average insulation (or is 1 exchange/hour far above average?).

 

The blower door has a "can't reach 50" mode that will interpolate to give me a number, or we would have to run two blowers. I have a bunch of 5,000 to 10,000 sq ft home in this area, but as yet none have called. I guess energy costs weren't on their list when they built.

Yes the 2250 sounds more reasonable, but be careful with the answer 0.32. That says your ventilation is probably fine. But it is an average for the whole house, meaning one area may be too tight while another is too loose. And again, these numbers are base upon estimated inputs.

In my experience, a 0.32 ACHn would be an existing home that has been very well sealed. New construction can reach that and better, but retrofit, that would be very good. So any confidence in that number would be based upon what has been done to seal up your house.

Bud

 

I agree that the 0.32 seems too good; but the 1.0 ACH seems too high (doors are weatherstripped; windows are tight; . . .) So, I guess I'll just discard the figure - whatever it is - that's on the old blower report.

And thanks for the reference to that Energy Conservatory manual; it's a real treasure trove of helpful information over and beyond the blower-door technical stuff.

 

The 2250 does sound a lot more reasonable for your house's volume. With an air change rate of 1.0/hr., you would need to do some major sealing up.