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Parallel duct runs

BryanBL | Posted in Mechanicals on

Installing a Panasonic ERV

The exhaust air must be within insulated ductwork. However, the ducts must pass through a 2×8″ joist cavity to reach the point of exhaust (not movable).

Manual D and common sense give me a 6″ round duct for this. Obviously a 6″ insulated round duct doesn’t fit in a 2×8″ joist.

Ductulator suggests that at the design CFM, 3x 4″ ducts would be equivalent. Those would fit. Obviously I would have to account for the fittings to split from 6″ to 3×4″ and back again.

Is this a no-no? I couldn’t find information online.

Thank you in advance!

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  1. user-626934 | | #1

    You should just switch to rectangular duct, 3x10, for the section that runs between the 2x8’s.

  2. this_page_left_blank | | #2

    I can't comment on the actual sizes, but in principle there's nothing wrong with ducts in parallel. This is how Zehnder and some others design all their systems.

  3. BryanBL | | #3

    Thank you for the replies. John, the rectangular duct would be much simpler. My only concern is about sealing and insulating it well. Should I use closed cell spray foam? Am I overthinking this?

    Thanks again!

  4. GBA Editor
    Martin Holladay | | #4

    As you probably know, either 4 inch round duct or 6 inch round duct is permitted by the Panasonic ERV instructions (see image below).

    Most building supply stores, including Home Depot, sell galvanized duct fittings that transition from round duct to rectangular duct (see second image below).

    Home Depot should also sell duct insulation (see third image below).


  5. Expert Member
    Michael Maines | | #5

    Bryan, for sealing the ductwork, you can either use "pookie," aka reinforced duct mastic, or you can use high-quality foil tape.

    In addition to round and rectangular sections, you can also get oval ductwork, which can be easier to deal with in some situations. Big box stores usually stock a variety of shapes, sizes and fittings, but for more selection you can search out an HVAC supplier near you, or look online at or similar sites.

  6. BryanBL | | #6

    Thank you again for the replies. The background was I had read some concerns (likely on the Energy Vanguard blog) that duct mastic on rectangular ducts still doesn't quite result in a complete seal. That said, the tone of all replies tells me what I really need to know, IE that the sealing just isn't so critical for this context as it is for other types of ducts, like for example cooling ducts in an unconditioned space.

    Michael, thank you for the suggestion of trying HVAC supply stores. I wasn't able to find one in the area that sells ductwork to the public. I did, however, find a decent tutorial on bending 8" round into oval ductwork - the benefit there being I can use 8" round sleeves as insulation and sealing is much easier.

    Thanks again! I will report back on performance once this project is completed.

  7. user-626934 | | #7

    Rectangular sheet metal ductwork can be completely sealaed with mastic duct sealant. There’s no technical barrier to doing it right.

  8. Expert Member
    Dana Dorsett | | #8

    I've seen mastic on rectangular sheet metal ducts that was still very tight after 60 years of service, and tape that failed within 6 months. The diligence/competence of the person sealing the ducts can make all the difference, assuming appropriate materials are being used.

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