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Community and Q&A

How to extend an HRV intake

KeithH | Posted in Mechanicals on

Hi guys,

I need to extend an HRV intake from just above the foundation (and therefore just above grade) to a better height.

I’d like to hear from any of you who have placed HRVs in basement or crawl spaces and ended up extending the intake above grade, whether as part of the original plan or as a repair to remediate incoming ground level odor or snow level problems.

Did you extended it outside the envelope?  Inside?  With round pipe?  What kind?  Or did you make a soffit/enclosure and use rectangular duct?  What product did you use to limit debris/bird/insect intrusion?

More specifically:
– If you used rectangular duct, how did you transition from flex duct inside the envelope?
– If you used round pipe, did you use PVC?  Is that safe for an intake?
– If you used polypro pipe, where did you get a mortgage to afford it ? 😉
– How much did you elevate it?

Thanks for sharing any advice or experiences.

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  1. user-2310254 | | #1


    I will give your post a bump. FWIW I had a fresh air intake that exited near the foundation. The low elevation meant it was more prone to sucking in dust and insects. For the most part, this debris was caught by the ventilator filter. If I had continued to live in that home, I probably would have added some metal pipe to raise the intake at least a few more feet.

  2. Expert Member
    Akos | | #2

    Your best bet is stainless flue pipe used for water heaters. You can also try McMaster Carr, it has a good selection of both aluminum and stainless elbows and pipe. Not the cheapest.

    Plastic (I think it is PVC) is commonly used in Europe for air vents, but it won't have any local certification. The sizes won't exactly match up, but close enough to connect with a rubber coupling.

  3. AlexPoi | | #3

    You can't use PVC for the air intake because of off gasing concerns. In Europe they use medical grade High Density Polypropylene which is a lot more expensive but safe.

  4. exeric | | #4

    It would be useful to know if the HRV is already installed in the basement/crawlspace or if it is just your intent to install it there? That isn't the ideal installation location for reasons you may just be discovering. I installed mine in a large closet with all the 6" flexible ducting going through the ceiling and then buried beneath attic insulation. For the intake and outlet air I just used two closets that adjoined an outside wall. Those two ducts descended into the closet and then exited outside with a fitting that is usually used for bathroom outside vents. They work fine to also connect with generic ducting. The reason for the dipping into closet ceiling is because I have a hip roof and because I do not have a raised heel roof. If I hadn't done that the exit would have been in the roof, which is unacceptable to me. You can put a false ceiling in the closet to cover it up without creating any big visual clues.

    Of course, none of this may feasible if the HRV is already installed beneath the main structure of the house.

  5. KeithH | | #5

    Steve: Yup, those are the problems. Thanks for the bump.

    Akos: Why use double-walled vent? That stuff is expensive.

    Alex: Does it? I see a paper from CHEJ saying pvc flooring is associated with lots of respiratory ailments but that isn't causative merely associative (valuable but limited). I also see an industry white paper saying it doesn't off-gas. If I thought PVC was for sure safe, I'd just use it. I'm very familiar with using it for radon exhaust. But I can't find something definitive. I don't want to turn this into just a thread about PVC though it is a key question as that is an affordable, available, customizable product that could work. Competing white papers:

    It's already in the crawl space. Not sure why that is a problem in my case. The intake is the problem and I'm guessing I'll have a balance problem after I modify the intake. My crawl is air sealed, concrete floored, insulated, etc. It's part of my conditioned air space NOT a nasty dirty afterthought. I had a plumber over for a bid on some plumbing repair before COVID; I quote him: "This crawl space is nicer than my house". Is there a concern I didn't understand or does that cover your concerns. FYI my exhaust has exactly the type of vent guard you mention. Works great for the exhaust.

    I guess I'm leaning towards vertical bumpout to accomodate 3.5" x 14" ducting. Tempting to pan it instead of truly ducting it.

    1. exeric | | #6

      Yeah, it sounds like you have it in hand. My reply was based on limited info of your particular situation.

    2. Expert Member
      Akos | | #7

      Use standard single wall stainless pipe. You can get all the pieces including elbow and end termination. More of a buy instead of make. B-vent might be more readily available though.

    3. AlexPoi | | #8

      Do a quick a search on this website, you are not the first one who wants to use PVC ductwork but it's not recommended especially if the air is heated. If you are willing to risk it it's your choice though.

      1. KeithH | | #9

        Oh yeah for sure Alex. I don't want to risk it which is why I asked. It seems like there is not definitive information.

        Akos: I'll what I can find. Thanks.

        1. AlexPoi | | #10

          I don't think there is definitive proof but I think the concerns are valid. For the record, I'm not usually very worried about off gasing issues and I'm not afraid of using OSB, MDF or any engineered wood but when it comes to mixing PVC and fresh air I'm a bit more nervous. PVC exhaust duct would be fine though.

          1. Expert Member
            Akos | | #11

            Most European vent piping I've seen is PVC. Maybe it is a low off gassing version, feel and smell was same as the stuff here. I personally wouldn't have issues with putting in a couple of feet of white PCV into the intake. There are much bigger sources of VOC in a new construction to worry about.

            You can also leave the section out in the sun for a month or two before installing, a bit of heat and sun would release most nasties.

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