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Balanced ventilation questions

calum_wilde | Posted in Mechanicals on

I don’t have access to the proper flow measurement equipment to test if my HRV is balanced properly, but I do have a slack tube manometer. I set it up and saw that the pressure differential was about 0.03 inches of watch column with the inside being slightly pressurized.

So, my questions: Is that good? Should I try to adjust it down to dead even? Is this even a reliable means of testing?

Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Are you comparing the indoor air pressure with the outdoor air pressure? If so, wind effects and the stack effect can overwhelm the effect of a slightly unbalanced HRV.

    Your measurement of 0.03 i.w.c. corresponds to 7.5 pascals -- not much. The stack effect in a two-story house can amount to 8 pascals during cold weather.

  2. calum_wilde | | #2


    I should have given more information on my measurement technique. I measured at the top of my front door which is on a landing of my split entry. My hope was to reference the midway point in the house vertically to get the most neutral point in the stack effect. It was a calm day with very little wind and watching the reading showed no variation due to wind. The house is pretty air tight for a retrofit project on a 123year old house, 1.0 ach50.

    Given your description of how little that pressure differential is, I'm guessing that's not worth worrying about.

    (If you'll remember my last question about sealing my dryer duct after the ventless dryer is delivered this weekend, I'm anxious to see if that makes a difference in the differential pressure as that's likely a significant amount of the present air leakage.)

  3. user-626934 | | #3

    If you want to see how your HRV is affecting building pressure with respect to outside (a rough proxy for HRV balance), you would need to test the pressure differential with the HRV off first as a baseline, and then again immediately with the HRV on. A calm, mild day is essential.

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