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How to size HRVs and how to determine how much they need to run?

jculbertson | Posted in Energy Efficiency and Durability on

We are building a super tight house in a very cold part of Wyoming. We currently have three Fantech variable speed HRV’s planned with a total CFM capacity of 640. We have about 6,500 square feet of conditioned space with some pretty high ceilings.

Two questions. Is this the right amount of total CFM capacity? Are there guidelines for how many air changes per hour we should try to achieve? I have heard that Building Science Corp. may have come out with some guidelines but I have not been able to find anything on this.

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  1. GBA Editor
    Martin Holladay | | #1

    ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 3 cfm for every 100 square feet of occupiable floor area.

    If your house has 2 occupants, your ventilation rate would be (2 * 7.5) + (3 * 65) = 210 cfm.
    If your house has 3 occupants, your ventilation rate would be 217.5 cfm.
    If your house has 4 occupants, your ventilation rate would be 225 cfm.

    The bottom line: instead of 3 HRVs, you only need one HRV.

    For more information, see Designing a Good Ventilation System.

  2. STEPHEN SHEEHY | | #2

    When Zehnder designed my HRV layout, I learned that it just multiplies floor area by ceiling height, but limits the height to 8.2 feet(2.5 meters), for purposes of figuring total volume. I was told that is how Passivhaus calculates ventilation volume.

    Applying that to your house, the planned 640 cfm is well above the recommendation that I got from Zehnder. 640x60=38400cfh. Divided by total volume 53300 (6500x8.2) gets you .72ach, which is way above the .3 which my architect identified as the standard. He also pointed out that many people think that is too high. Ventilation needs (as opposed to codes) may be influenced more by the number of people and their activities, than by volume of air. Building Science Corp. has a lot to say about ventilation.

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