I get asked variations of this question often enough that it’s worth addressing here. If you’re doing a blower door test, how big a house can you test with one fan? Or, put another way, how many blower doors do you need to test a really large house? That question has no answer because it’s based on a faulty premise.
Does size matter?
Yes, it does. But it’s not the size of the house that matters. It’s the size of the hole in the house. You could have difficulty pressurizing a 3000 square foot house to 50 Pascals (Pa) with a blower door. Alternatively, as HERS rater Eric Griffin learned yesterday in the PHIUS+ Rater Training at the RESNET conference in Orlando, you might be able to get a 40,000 square foot house to 50 Pa using the much smaller Duct Blaster fan.
There are two problems with asking how big a house you can test with one blower door fan:
- The amount of leakage scales with the area of the building enclosure, not the conditioned floor area (and certainly not with the volume).
- Every home has a different amount of leakage per square foot of building enclosure.
If, on the contrary, leakage increased with the amount of conditioned floor area and every home had the same amount of leakage per square foot of floor area, then the question would be valid. They aren’t, however, so it isn’t.
What’s the flow rate?
If a 3000 square foot home has an infiltration rate of 10,000 cfm when the pressure difference across the house is 50 Pa, one blower door fan will not get the house to 50 Pa. The Minneapolis blower door can get up to about 5,300 cfm at 50 Pa (cfm50). The Retrotec Q46 can go up to about 5,600 cfm50, so you’d need about two of either type. (You can still get a cfm50 even without getting a 50 Pa pressure difference, though, because of the cool mathematical property known as extrapolation.)
The 40,000 square foot house that could be tested with a Minneapolis Duct Blaster fan, however, must have had an infiltration rate of only 1,000 cfm50 or so, the limit of what those fans can do.
The cfm50 doesn’t tell you if the home is leaky anymore than my telling you I have a 230 pound friend tells you if he’s fat. 1,000 cfm50 could be really leaky if we’re talking about a really small house, or it could be amazingly tight in the case of the 40,000 square foot house. That’s where air changes per hour (ACH50) and cfm per square foot of building enclosure area come in. By normalizing to the size of the house, you can decide if it’s leaky just as if I tell you that my 230 pound friend is 6’6″ tells you he’s not fat.
Speaking of testing homes with duct tester fans, I did that last year in a net zero home near Memphis, Tennessee.
A really small house might be really leaky and need more than one blower door fan to get a 50 Pa pressure difference. A really big house might be really airtight and need only a tiny fan to get a 50 Pa pressure difference. How big a house you can test with a blower door depends on how much air you need to get the 50 Pa pressure difference.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
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