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Community and Q&A

ERV Duct Size

connorkatz | Posted in Mechanicals on

Getting ready to install an independently ducted ERV as part of our remodel (3,700sqft), nothing super fancy like a Zehnder, probably this one from Broan

My question is in regards to the duct sizing. I know the trunk lines should be 6in but we will have a couple branch lines that split off to different bedrooms/living room for supply and bathrooms/mudroom for return.

It seems like the short (mostly straight) branch lines could step down to 4in, 6in seems like overkill. The Zehnders for instance seem to use a flexible home run system that looks like they are maybe 2-3in

Should I just not worry about it and use all 6in or does it make sense to step down at the branch lines.



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  1. norm_farwell | | #1

    Can be a bit complicated—your desired duct diameter depends on length and transitions (elbows, eyes etc) and flow rate and what static pressure the unit wants. Higher flows and longer lengths will bump up the diameter requirements.

    Allison Bailes has good technical advice on duct design at his energy vanguard website. For ex:

    Zehnder systems typically deliver around 12 cfm to a register with one three inch tube and 24 with two. At 4” you’ll be in the 24 cfm range when you compare cross sectional area. That’s plenty of supply for a bedroom or exhaust from a bathroom. A kitchen without a range hood to the outside might want a bit more than that.

    You might want to go larger with the trunk, depending on distance.

    Also you can buy zehnder components a la cart so you could buy manifolds, tubes and registers and pair them with a less expensive erv and maybe save yourself trouble and probably some bandaids if this your first time with sheet metal.

  2. Expert Member
    Deleted | | #2


  3. Expert Member
    Akos | | #3

    You do want to minimize duct size to limit sound transmission between rooms. This is more of an issue with hard pipe.

    6" is a bit on the small side for a trunk on that unit on boost especially if your runs are longer. I would step it up to 7". Low loss ducts also reduce the fan power which can add up even on an ECM unit.

    ERVs are pretty quiet but not silent. If you have mostly hard pipe, make sure to install a length of flex between the unit and supply ducting or a duct silencer. Without this you can hear the unit go on boost in a bedroom which is annoying.

    As a ballpark, you are looking at 4" for a bedroom and 5" for a kitchen pickup. I would use a 5" for the pickup for the most used bathroom, the rest can be 4". Rooms that will rarely be used can even go down to 3" pipe as these only need 10cfm.

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