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Does the fresh air have to go directly to bedrooms in an HRV retrofit?

JustHousing | Posted in Mechanicals on

When specifying requirements for the mechanical ventilation system in a new home, we typically ask for exhaust points out of bathrooms and kitchen (and sometimes mechanical room) and we call for fresh air points in living spaces and each bedroom.

I’ve done some retrofit work and typically try to follow these guidelines but it can be tricky getting ductwork to deliver fresh air into all second floor bedrooms in an older home. If the doors are undercut, is it okay to get the fresh air upstairs to a central location and not try to duct into each bedroom? Specifically, I’m dealing now with a pretty typical 1 1/2 story c. 1930 home with 3 bedrooms and one bath upstairs, all off a small central hall. I can exhaust out of the bath. How about fresh air to the hall?

Replies

  1. Expert Member
    Dana Dorsett | | #1

    In Net Zero Energy houses or other super-insulated houses heated with ductless heat pumps it's common to use exhaust-only for the ERV/HRV on all rooms except the space where the ductless head is mounted, which is a supply-only space (unless there's a kitchen in that space, which would require a local exhuast even if it's open to the larger common-space with ductless head). This allows the ventilation system to even-up the temperature differences between the room with the ductless head and the doored off areas. Providing pressure equalizing return paths with door grilles or jump ducts etc. is still necessary to get flow to the doored off rooms though.

    Using partition-walls as jump ducts sometimes works, with a grille near the bottom of the stud cavity on one side of the wall, at the top on the other side, but on a circa 1930 house watch out for partition walls that don't have top-plates and air-seal your improvised "duct" accordingly.

  2. user-626934 | | #2

    Hi Rachel, the research shows (for scenarios similar to the one you described) that the air changes per hour in bedrooms with closed doors will be significantly less than the "open" parts of the house. Whether or not this is "okay" seems to be up in the air. At this point, the ASHRAE 62.2 ventilation standard is "silent" on distribution.

    See for example:

    http://www.buildingscience.com/documents/reports/rr-0001-measurement-of-ventilation-and-interzonal-distribution-in-single-family-homes/at_download/file

    http://www.buildingscience.com/documents/reports/rr-0802-field-test-of-room-to-room-distribution-of-outside-air-with-two-residential-ventilation-systems/at_download/file

  3. JustHousing | | #3

    Thank you John and Dana. (and hi John!) Looks like I can easily get fresh air into one bedroom and I'll do that ... and this gives me reason to work with the mech contractor to come up with ways to get into the others or at least create grilled openings that can be controlled. I was trying to determine if pursuing fresh air to all three was pushing for something not really necessary (and it will be more expensive), but ASHRAE aside, it seems best to get the air to the inhabited space.

  4. GBA Editor
    Martin Holladay | | #4

    Rachel,
    I agree with John Semmelhack: if the homeowners want regular air changes in the bedrooms, then you need to have either an exhaust grille or a supply grille in each bedroom. Otherwise, air changes will be lower than ideal.

    That said, plenty of people are happy with exhaust-only ventilation systems that don't provide many air changes in the bedrooms. Satisfaction levels (and complaint levels) are all over the map -- it all depends on the occupants.

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