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Community and Q&A

Accuracy of blower door test at low fan speed

A7nzCAez4q | Posted in PassivHaus on

I am trying to get my new built cottage down to .6ACH at 50 pascals. I had my third blower door test done today and initially received good news – we had achieved .6ACH. Then the tester ran the test again at ACH 150 and ran the results through some interpolation software. This test indicated an ACH 2.4 at 50 pascals. Thinking that there might be an error in the interpolation, he then used the duct blaster attached to a window and came up with an ACH 2.8 at 50 pascals.

So is the blower door test less accurate at low fan speeds/fan pressure? As I understand the tester’s explanation, a blower door test becomes less accurate at low fan pressures. Are duct blasters typically used to measure air infiltrations for tighter buildings? I am new to this and am trying to figure out next steps.

Thanks for any insight.

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  1. user-788447 | | #1

    Your tester will have to obtain the small fan size for the blower door to test a house approaching .6ACH.
    You will not get accurate results with a standard size fan. If I remember correctly it is referred to as the 'C' ring.

  2. user-626934 | | #2

    For REALLY tight houses, using the duct fan (as pictured above with Gary Nelson) is an even better way to go. Energy Conservatory sells the red nylon blower door frame cover with a smaller opening for the duct fan. Why carry around a 30lb fan when you can carry a 5lb one?

    Down the road, there will be a day when the big blower door fans will be just about least for testing new houses.

  3. A7nzCAez4q | | #3

    Thanks Gary and John for your response.

    Prior to the test discussed above, I had another test where the technician put a ring around the fan to shrink the circumference of the fan opening. Does that serve the same purpose as using a smaller fan? And is it commonly done? I am trying to figure out the methodology and how much confidence I should have in the folks doing the testing. (I have had two different companies in to do the tests).

    Thanks again

  4. user-788447 | | #4

    I would call the folks at the Energy Conservatory (located in Minnesota). They would know in detail if there is anything that a typical blower door contractor may need to learn in order to obtain accurate results for highly air tight envelopes.

  5. davidmeiland | | #5

    Seems to me that you can easily use the Minneapolis "C" ring to accurately test houses at .6ACH50. Let's say you have a smallish house at 10,000 cubic feet. To accurately run .6ACH50 you will be right at 100CFM50. If I am reading correctly, the C ring will be at less than 1% error at that level. If you want to go lower, get the D or even the E ring. Flow range info is at the very end of the user manual at

    I'm guessing your tester came out there with the A and B rings, and got caught with his panties down. I bought the C ring after testing a fairly tight house and having to stop the test at -30pa with the B ring. Since then I've done 1-2 tests entirely with the C.

  6. user-723121 | | #6


    .6 ach 50 is the Passive House standard for infiltration, are you looking for PH certification? Will your cabin have a maximum annual heat load of 4,750 Btu per square foot.?

  7. A7nzCAez4q | | #7

    Thanks again for all of your responses. I just looked at the Minnesota blower door manual (online) and see that the "C" flow ring is good for 80-300 cfm. The flow rate in my cottage is definitely less than 300 cfm. I think David that you are right - that the tester(s) only brought the A and B rings. It will be interesting to see if they even have the C, D and E rings.

    The interior volume of my cottage is 5475 cubic feet.

    Doug, I am not going for passive house certification but my contract specifies 4726 but/ft2 heat load and .6ACH 50.

  8. davidmeiland | | #8

    You say 5475 cubic feet of "interior volume"... which I take to mean inside the drywall. Depending on where the air barrier is (sheathing?) you might have slightly more volume. Anyway, let's say 5475... .6ACH50 is just about 55CFM50, so even with the C ring you'd have to run the test at -25pa and extrapolate from that. Better tell 'em to dig out the E ring if they want to actually test your job competently. You could do it with a duct blaster fan, too.

  9. A7nzCAez4q | | #9

    Thanks David - Yes it is the interior volume. Currently, the cavities are filled with foam so we are effectively at "inside the drywall".

    Interesting that the test would have to go all the way to the E ring. The last tester did use the duct blaster in a second test but we had a hard time keeping the everything sealed. We used a jerry-rigged window insert taped to the inside of the window and the tape kept pulling out even though I was trying to hold everything in place... That's why I would prefer to use a blower door - better fit and standard equipment.

  10. Skylar Swinford | | #10


    You stated that "my contract specifies 4726 but/ft2 heat load and .6ACH 50." Could you clarify what you mean by this? Is your contract with the designer? What software did he use to calculate the "heat load"? By "heat load" are you referring to "Annual Heat Demand" or "Peak Heating Load"? I am assuming your designer modeled the energy usage of the home in a software such as PHPP to help reach this target, is this correct?

  11. A7nzCAez4q | | #11

    Skylar - Yes, the architect used PHPP to come up with the 4726 number and refers to the annual heating demand. The contract mentioned is with my general contractor. We are not going for passiv haus certification but wanted to use the ideas behind passiv haus to inform the design and construction of the cottage/ADU.

    I am less concerned about the precise 4726 heating demand number although I think the building will perform quite well - but would really like to hit the ACH50 of .6. So before the drywall and plaster application, I want to get as low as possible - there will be little opportunity to correct once the plaster (with integrated color) is up.

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