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Community and Q&A

HRV duct work set up

Svig | Posted in General Questions on

CZ 7 Northern MN, 2×6 standard frame with 4″ exterior foam. Mini-split with back up electric baseboard in a 30×50 1500 square foot one level with unvented crawl space. 50′ southern exposure with master BR and Live/Dine/Kitchen great room facing south. All mechanicals DIY. I read most of the QA on HRV’s yesterday, and I think I will go with an HRV in the mechanical room (center of house north side) and am wondering how the duct work would look. I am thinking; single supply into the great room above the television, which will be on the wall between MBR and Great room. Exhaust in the master bath and laundry on one exhaust branch, which will draw fresh air into the MBR and the master bath. The other exhaust branch in the second bath, dining area just off kitchen area and spare BR. Just the two of us, but we will have occasional over night guests. Each exhaust branch will be somewhat close in length; one will have two vents, one will have three vents. My question is, can I neck down the duct where it splits to pick up multiple vents. For instance, I will have a tee splitting to two branches; can I neck down to 3″ for each branch. 4′ is 12 square inches, two 3″ is 14 square inches, so always maintaining as much or more square inches of duct area after a split. Any other observations also welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    I recommend that you read this article: Installing a Heat-Recovery Ventilator.

  2. Svig | | #2

    Thanks, Martin, it is more complicated than I thought. Exhaust only sounds a little better all the time!

  3. bunney | | #3

    We are south of you on Brainerd Lakes area. We have a Venmar HRV. We were unprepared for the HRV having to frequently go into defrost mode during the coldest periods of winter. Defrost requires max fan speed. Resulting noise in bedrooms is significant disappointment. Contractor turned down fan speed, which naturally means longer time to exchange air in the house. We don'the use spot ventilation in bathrooms. Instead we use HRV, which now is less effective in exhausting odors.We have ventless dryer and heat pump water heater in all electric home with mini splits

    Congrats on your 4 inches exterior foam. We are inadequate with 1 inch.

  4. Svig | | #4

    Thanks Randy, I was looking forward to the automatic functionality of having the HRV. But a lot of time in defrost mode when you need fresh air the most is troubling. And significant amounts of time during the rest of the year, we will have windows open, which I assume means wasting energy if the HRV is running. I guess I need to give exhaust only some more thought, maybe with passive supply vents.

  5. STEPHEN SHEEHY | | #5

    My Zehnder HRV has been running since installed last summer, without any attention from me whatsoever. I don't even know if it has a defrost cycle, but if it does, it doesn't require an increase in fan speed. It runs without complaint at sub-zero temps, although MN has probably been colder than Maine this winter.

    It has boost switches that I run when showering or cooking anything smelly. I haven't seen any need for any additional ventilation.

    Randy-did you talk to Venmar? Their HRVs have a good reputation. Did you have it commissioned?

    For ducts, we have separate ducts from the HRV unit to each supply and exhaust port.

  6. Svig | | #6

    Thanks Stephen, What size is your duct work. Since the inlet outlet of HRV's are 4", I was surprised to see in the article Martin referenced, 6" duct is common/preferred. That size difference might cause installation complications.

  7. GBA Editor
    Martin Holladay | | #7

    Always follow the manufacturer's instructions. The HRV manufacturer should include information on ducting requirements.

  8. STEPHEN SHEEHY | | #8

    Steve- our ducts are about 4." Since Zehnder is a European company, it may be a metric size, but 4" is pretty close, as I recall. Definitely no bigger than that.

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