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Community and Q&A

Ventilation ducting advice

Trevor_Lambert | Posted in Mechanicals on

It looks pretty likely I’ll be running my own ducting, as I have been left with a supply of specialized duct pipe and distribution accessories after my main builder turned into a ghost. I understand the basics, but some of the details I’m not sure about. I have 13 distribution boots, and it’s pretty clear the intent was to have 3-4 exhausts, and 9-10 supplies. Two bathrooms and the kitchen obviously need exhausts. He actually installed half of the boots, and I notice he put two in the large living area, at opposite ends. Should one of those be an exhaust, or should they both be supplies? Secondly, since there are more than double the supplies than exhausts, I think it would be a good idea to twin the exhaust ducts. There are two 3″ ports on the boots, so it would be pretty trivial. I have two 12-port manifolds, so all the ducts will be “home runs”, pretty much a necessity with duct so small. The ID of the ducting is about 2.4″, so six of them would yield a total cross sectional area of about 27.5 square inches, pretty close to the area of the 6″ ducts on the ERV itself. Does that make sense? Would it make sense to twin the longer supply runs as well? Should I be twinning them all?

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  1. GBA Editor
    Martin Holladay | | #1

    I have no idea if your builder knew what he was doing. I suspect not.

    Ventilation air flows tend to be quite low. An entire house often needs just 50 to 90 cfm.

    Typically you'll need to deliver fresh air in each bedroom, and sometimes the living room. That's usually three or four fresh air registers.

    Typically you'll need to pull stale air from the bathrooms, and sometimes the laundry room or kitchen. (Many people skip the kitchen). That's usually two to four exhaust grilles.

    If your air flow is 80 cfm, each register or grille will be handling only about 20 cfm. As I said, these are small air flows.

    Divide the air flow up even more, and you'll have a big balancing and commissioning headache. It's hard to get a duct to accurately deliver 10 cfm.

    Keep it simple.

    For more information, see this article: Installing a Heat-Recovery Ventilator.

  2. lance_p | | #2

    Martin, your link directs to the main Q&A page, not the intended article.

  3. Trevor_Lambert | | #3

    I'm getting a quote/design from Zehnder. I guess I will wait for that and see how it looks compared to what I am guessing the builder intended. I think Zehnder uses a similar type duct. I suspect the large number of supply points has something to do with the exceedingly small duct being used. If that's the case, it would make more sense to twin the ducts and have them go to fewer places as you suggest. Per ASHRE 62.2-2016, my house requires 104CFM. I know there's disagreement over whether the increase in CFM vs the 2010 standard is justified. Considering they put it in there to address tighter houses, and we're expecting our house to be below 0.2ACH, I'm going to go with the higher number for now. It's easier to dial it back than crank it up after the fact.

  4. GBA Editor
    Martin Holladay | | #4

    Sorry about the bad link. I just fixed it. Thanks for letting me know.

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