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Supply and return ducts for an HRV

MStaudaher | Posted in General Questions on

I will have one or two HRV units located in a small mechanical room in my garage.  I don’t plan on having any of the ducts for the HRVs in the attic space, but will run them to and from the various rooms in the crawl space (ICF stem walls).

For a regular HVAC system I would put in a plenum routed down into the crawl from the garage side.  My HVAC will be all ductless minisplits so no other ductwork.

My question is should I use plenums for the supply and return through the ICF stemwall or some other penetration option?

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Replies

  1. Jon_R | | #1

    It's not clear to me why you couldn't put the HRVs in the crawlspace.

  2. GBA Editor
    Martin Holladay | | #2

    M.,
    Putting an HRV in your garage is generally a bad idea, since garage air quality is so low (and duct leaks might allow garage air to enter your house through the ventilation system).

    Your question about plenums is confusing. A forced-air furnace generally has two plenums (a supply air plenum and a return air plenum), but an HRV doesn't require a plenum (as you will discover if you read your HRV's installation manual).

    1. MStaudaher | | #3

      I am aware that HRVs don’t have plenums. I was trying to figure the best way to get the supply and return ports to connect to the crawl space from the mechanical room.

      Would it be better to put both HRV units in the crawl space and run the exterior inlet and outlet ports to an outside wall? If I took that approach, because of the possibility of deep snow in the winter, I would need to somehow get those exterior ports up off the ground level.

  3. Expert Member
    Dana Dorsett | | #4

    >" I was trying to figure the best way to get the supply and return ports to connect to the crawl space from the mechanical room."

    The mechanical room should be completely inside the pressure & insulation boundary of the house.

    Is it? Even in the corner of a garage the walls of the mechanical room should be gas-tight and insulated between the room and the garage, as if they were fully exterior walls. (Think of the garage as a really BIG rain screen assembly.)

    >"Would it be better to put both HRV units in the crawl space and run the exterior inlet and outlet ports to an outside wall?"

    Yes.

    >"If I took that approach, because of the possibility of deep snow in the winter, I would need to somehow get those exterior ports up off the ground level."

    That's right- PVC "snorkel" terminations recommended.

    >"My HVAC will be all ductless minisplits so no other ductwork."

    When there is a conditioned crawlspace available for running ducts, it's usually better for heat distribution and efficiency to use a right-sized ducted mini-split approach rather than all-ductless. The heating and cooling loads of individual rooms such as bedrooms are usually well below the capacity of even a half-ton ductless head, and oversizing them by very much negatively impacts both comfort and efficiency. Only if this house is approaching PassiveHouse levels where a single ductless head per floor wouldn't result in large temperature differences in doored-off rooms would ductless-only be the ideal.

    Most multi-zone compressor ductless systems that get installed are ridiculously oversized for their loads, and not delivering optimal comfort or efficiency. To get it right do the room by room load math carefully, using aggressive rather than conservative assumptions or yours will be ridiculously oversized too.

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