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How much should doors be undercut to allow air movement for the HRV?

user-6323668 | Posted in Energy Efficiency and Durability on

How much should interior doors be undercut to allow air movement for the HRV?

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  1. GBA Editor
    Martin Holladay | | #1

    Ideally, a bathroom exhaust fan or HRV shouldn't create a big pressure difference between the bathroom and the adjacent room or hallway. A typical goal is that this pressure difference should be held at no more than 2.5 pascals when the fan is operating.

    A gap of 1 inch between the finish floor and a 30-inch-wide door can provide 47 cfm of return air at a maximum 2.5 pascal pressure difference. That's adequate for most bathrooms.

  2. user-6323668 | | #2

    The supply air enters in the bedrooms and living room. Should those doors be undercut too?

  3. GBA Editor
    Martin Holladay | | #3

    Most homeowners don't close the door (if one exists) between their living room and any nearby hallway. Do you?

  4. user-6323668 | | #4

    No, just the bedrooms (sometimes). Living room doesn't have doors. It's just the bedroom doors I am concerned about.

  5. GBA Editor
    Martin Holladay | | #5

    The fresh air supply rate to bedrooms is too low for you to have to worry about door undercuts. Relax.

  6. Expert Member
    Dana Dorsett | | #6

    Jump ducts can provide a bit more privacy for bedrooms than door cuts. It's pretty easy to convert a partition wall stud bay into a jump duct, with a small grille near the floor on one side of the wall, and near the ceiling on the other side, for a longer path, transmitting less light & sound.

    One could use 4" x 10" or 4 x 12" HVAC vent grilles. In many cases 4"- 5" inch round vent grilles would do it, depending on the size of the existing door-gap (sharing the flow). Be sure to check for fire blocking or other obstructions in stud bays used as jump ducts BEFORE you cut in the big holes.

  7. GBA Editor
    Martin Holladay | | #7

    Jump ducts are used to address air flow from furnaces or central air conditioning systems -- they usually can handle 100 cfm or more.

    The supply grille of an HRV usually only delivers 10 cfm or 20 cfm to a bedroom -- so you don't need a jump duct.

  8. Jon_R | | #8

    The closed door vent area you need to avoid excessive pressure changes with the cfm, but never goes away. About .6 square inches per cfm for a door cut. And more (ie, less pressure change) is better.

  9. user-626934 | | #9

    Most room pressurization/depressurization recommendations for undercuts and transfer grilles are theoretical and do not factor in airflow around door jambs and head (we've found a typical gap of 1/16" to 1/8" for pre-hung doors) nor other smaller, unintentional airflow pathways (via interior walls). We've done a good bit of real world testing of room pressurization and feel confident room pressure difference will remain under 3Pa with supply airflows to bedrooms as high as 75cfm when the door undercut is at least 1/2". See attached graph showing test results of over 60 bedrooms (3 different builders), with just door undercuts (no transfer grilles, jumper ducts or return ducts). Undercuts varied from 3/8" to 3/4", flooring was about 50/50 carpet vs. hardwood.

  10. GBA Editor
    Martin Holladay | | #10

    An answer based on measurements, not math! That's great -- superior in all respects.

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