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Community and Q&A

Sizing an HRV

steve02 | Posted in Mechanicals on

I am working on sizing an HRV for my son’s house. Using ASRAE 62.2 I get 77cfm. Using ACHnatural of .35/hour I get 144 cfm. Using Venmar’s tables I get 160/80 cfm for the high/ low. Using 10 cfm/room and 20cfm for master bedroom and basement I get 180 cfm. I wonder if some of the higher numbers from the manufacturers are trying to account for high speed venting of bathrooms. However I wonder even at 180 cfm with ten supply and return grilles if he HRV can pull enough air out of one bathroom to stay ahead of a long hot shower on a cold winter’s night. So am thinking that I should focus on the range of 77 to 144 cfm and install bathroom (and range hood of course) exhaust fans to cover the short term heavy ventilation demands. I am using a friction loss worksheet developed by Carrier that I found on line for trying to get an idea of the static pressure the fans will be operating against in order to size the unit. So I am looking for help on criteria for determining the cfm operating range of the HRV, and a good procedure for predicting the system static pressure. Any thoughts. steve02

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  1. davidmeiland | | #1

    I would size the unit per ASHRAE and use dedicated vent fans in the bathrooms, to be run for short periods during/after showers.

    How many square feet, bedrooms, occupants?

  2. steve02 | | #2

    I figured 5 occupants, 5 bedrooms, 3300 sqft. There are 7 supply and 7 exhaust grilles.

  3. davidmeiland | | #3

    That's a fairly sizable installation. I would be looking for some real-world data from the manufacturers (and your installer?) on how their equipment performs, i.e. how many CFM you can expect to move and how much from each room, assuming many different lengths on the duct runs.

    When you say 7 supply and 7 exhaust, does that mean a pair in each room, or something else. The infrequent installs that I see usually have a couple of supply and more exhaust (although they sometimes seem to me to be in the wrong rooms).

  4. GBA Editor
    Martin Holladay | | #4

    If you are installing an HRV with dedicated ventilation ductwork, I don't think that you also need to install separate bath exhaust fans. Others disagree with me, and there has been a lot of back-and-forth on this issue in several GBA threads.

    If you decide to take David's advice, and you install both HRV exhaust grilles and (redundant) separate exhaust fans in your bathrooms, I would experiment before I used the exhaust fans. You will probably find that you don't need to run the exhaust fan -- that the HRV is perfectly capable of handling your bathroom exhaust requirements.

  5. davidmeiland | | #5

    Martin could certainly be right. It might also turn out that you don't need the HRV if you install bath fans and use them to provide whole-house ventilation. I don't know what climate you're in, what your heating will be, and what your envelope is, but those things matter when deciding to invest in an HRV, And of course, I don't know how tight your house will be. It might not really need a whole lot of mechanical ventilation. My view is jaded by the mediocre tightness of the houses I see built. Some if them have HRVs that will probably just result in uncomfortable over-ventilation and high heating cost.

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