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Infiltration calculation

Zman Member Posts: 7,607
Does anyone know of a way to calculate air infiltration which is any better than a SWAG using the blower door test and calculating based on 50 pascals or just estimating ACH based on perceived tightness?

I find that most methods being used significantly overstate infiltration rates. This becomes apparent when you compare actual usage data to calculated. I think this is due to the non lineal nature of infiltration. The convective infiltration will vary greatly based on the height of the building, elevation of the penetrations in the structure (building with gaps high and low will flow) and the outdoor temp.

Is this just the nature of the beast or is there a better way?
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein


    HVACNUT Member Posts: 5,890
    As far as I know, the blower door at 50 pascals is the only way based on total volume of the condition space. One of the major areas for infiltration is where the top wall sheetrock meets the top plate. Unsealed, the air will pull through, go down the interior wall and out past the base molding. The more conditioned space, i.e. a hot roof or floor to ceiling insulation at the basement foundation adds to the total volume. Anything over 3 air changes per hour needs some buttoning up. Under 2 air changes should have an Enery Star bath exhaust fan with a timer to force the air change, an ERV, or a fresh air intake with damper (approximately 100 CFM to the return duct. If the HVAC system puts out 1000 CFM, it only pulls in 900 to keep positive pressure and force the air change.
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,389
    Hello, This could be another good place to ask the question: https://homeenergypros.org/

    Yours, Larry
    EBEBRATT-Ed Member Posts: 15,840
    It's always not much more than an educated guess. Blower door test is probably the best.
  • kenjohnson
    kenjohnson Member Posts: 87
    Hi Zman, I did a lot of research on this, and there seems to be no good answers. I believe that if you do a reasonable job of air-sealing (probably 3 ACH or less at 50 Pa) then you can probably ignore real-world infiltration when there is not a blizzard outside.

    Having said that, I did stumble across a website at one time that gave a multiplier for the ACH50 value to get a "normal" day value. It had some adjustments for height of house (1, 2 or 3 story), location (open clearing, sheltered), etc. Pretty much no matter what you entered, you multiplied the ACH50 value by something like 0.1 or 0.15.

    So, in summary, tighten up reasonably well and then ignore infiltration for real-world heat loss calcs.

    I did this on my renovation and I'm not wondering why my detailed calculated result diverged so much from my real-world performance.