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  1. #1
    Join Date
    Oct 2019
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    Wisconsin, USA
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    Confused Retrofit HRV ducting

    Hey I'm thinking about having an HRV installed and I'm looking for some good options for ducting from the basement to the two bathrooms on the main floor. I live in a cold winter/hot-humid summer climate so ducting from the ceiling through the attic would probably defeat the purpose of a heat exchanger. That leaves me with ducting through a 2x4 interior wall to a high register. Zehnder has those really neat flex tubes and wall boxes that accept multiples of tubes depending on airflow required. Unfortunately quotes to have a Zehnder installed is way outside my budget let alone would take a lifetime to pay back in savings.


    Here's what I've come up with so far.

    1. Rigid rectangular wall stack. I *think* there would be enough room to have them install rigid duct through a hole in the basement as to not disturb the plaster. I would want something like a 2 1/2 deep duct to keep some separation between the bedroom wall and the duct for noise control.

    2. Rigid oval duct. This is probably not substantially different than the wall stack in option 1 and it would require a box at the end of the oval duct for the register. From my point of view if option 1 or 2 is the best choice I'd leave it up to the installer as to what they would be comfortable doing.

    3. Flex duct. I'm looking for about 150 CFM of ventilation between 3 bathrooms (when in boost mode). Seems like an uninsulated 3" flex duct in each bathroom should work... would I want a larger duct than this or possibly something insulated to cut down any noise? Obviously there would need to be some squishing involved... not sure if that would be best practice. Are there any fittings that would even accept a 3" flex duct (or oval'ed 4/6" duct) for a wall register or is that going to be a custom built thing?

    I really like the Aldes ZRTs that can auto adjust to keep min/max airflow as well as open and close electronically BUT they don't seem to be narrow enough to fit in a 2x4 wall cavity. In addition, I don't want to buy an Aldes HRV and I'm guessing no one is going to want to mix products from different manufacturers. I noticed manufacturers like lifebreath offer rough in boxes but I haven't seen anything for retrofits.

    I've seen some pictures of installs that add soffits all over the house which is something I do not want to do. Any advice you can give me would really be helpful. Thanks!

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
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    How did you conclude that 150 CFM is required to address Your IAQ concerns?
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Oct 2019
    Location
    Wisconsin, USA
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    Thread Starter
    Dan,

    I suppose the 150 is more for sizing of the HRV such that it A) isn't running at full tilt all the time and B) Can boost up to a reasonable amount to handle ventilating any 2 bathrooms at the same time.

    For IAQ and winter humidity control I would stick with the 2018 IRC calculation of 0.01*SqFt + (7.5 *(Bedrooms+1)). Including a partially finished basement I'll have 2800 SqFt of conditioned space w/ 4 bedrooms so that puts me at 65.5 CFM. I do believe the latest ASHRAE 62.2 rates are excessive for IAQ which is why I would stick with what the IRC is calling for.

    Those Aldes ZRTs are starting to look more attractive. With those CAR dampers it looks like it would help to even out the airflow when there are static pressure differences such as duct length and also when you have a bathroom door closed. I called them today and they said their ZRTs can be made to work with just about any fan or HRV/ERV.

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