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Community and Q&A

ERV retrofit ductwork old house

rondeaunotrondo | Posted in GBA Pro Help on

I want to try to install the duct work myself on an ERV in our 1915 bungalow 1600sqft not including basement zone 5 outside of Boston house. I’ve been monitoring CO2 levels and they’re up to 2k ppm in bedrooms (door has to stay closed  in our nursery). 

I cannot find any “how tos” or articles on GBA/FHB on actually performing this work, meaning getting the duct work into the walls and passing it up to the second floor from an unfinished (finished in the future) basement. We have no chases or current ductwork and have ductless units. From what I’ve read the options are some sort of flex duct similar to zehnder, oval or rectangular. How can I do this work without completely taking down a whole lot of sheet rock?

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  1. charlie_sullivan | | #1

    The Zehnder tubing can be pushed through a 2x4 wall if there's nothing in the way. But getting up to the second floor is harder. One option is to convert a corner of a closet to a chase. The folks at Zehnder also are willing to make a plan for you if you send them drawings of your house.

  2. rondeaunotrondo | | #2

    I talked to New England zehnder rep and got a quote. I asked him about retrofit. He said “it’s much easier on new construction.” Well, thanks Zehnder, I know it’s easier on new construction….

    I likely will go with the CERV2. I had a previous post on Minotair with a quote of 17k.

    Unfortunately the closets are all underneath the knee wall Attics.

  3. rondeaunotrondo | | #3

    Any other thoughts for finding a DIY method for ducting in interior walls for an ERV in an old house particularly up to the 2nd floor.

  4. brian_wiley | | #4

    I've been doing a little research into this as of late in hopes of installing my own ERV in the coming year, and you're right: diy instructions are hard to come by. I have stumbled on a couple of things that may be helpful though.

    While I haven't worked directly with either, the process seems analogous to retrofitting high-velocity ducting to existing walls as the exterior diameter of the ducts (3" for ERV; 2" plus insulation, so about 3"+ for HV duct) is roughly the same. Theres much more info on that process, which may be helpful. It does seem to involve at least some drywall work though.

    The other thing may be helpful is that there seems to be some discussion on the location of the vents, with some expert members stating that its okay to place the registers (at least supply; I was unclear on returns) on the floor or low on the wall (link below). I mention that as it may be easier in your case to pass them through the closets in the first floor to a floor/low wall location on the second through the knee wall. Some of the downsides to the low location of the register that were discussed were additional dust and the potential of kids dropping things down them.

  5. Expert Member
    Akos | | #5

    High velocity ducting is a definitely a good choice, a 2.5" one would be just about enough for most rooms except for the main bedroom that would need two 2.5" runs to avoid noise while the unit is running on boost.

    You can also look at 3" semi rigid aluminum (think dryer went but smaller) which would also fit through a 2x4 wall. HVAC supply houses sell this as vent pipe for fireplaces, usually comes in either 35' or 50' length. Stale air pickups and fresh air supplies to each room also don't need to be insulated as long as the runs are within the conditioned envelope (you only need to insulate the runs from the ERV to the outside of the house).

    The hardest part for any retrofit is getting though wall top/bottom plates and blocking within walls. You can try to get through these from the top and bottom but opening up the wall bay is much quicker especially if you have drywall.

    Stale air pickups tend to be much larger, this generally means a 4" duct to a bathroom and a 5" to the kitchen area. You can use multiple small runs but I think oval ducting in this case is the simplest.

    In a mostly heating climate, you always want your fresh air supplies high on the wall, blowing along the ceiling. When it gets cold the fresh air feed without a post heater could be 10F to 15F bellow room temperature, it would create cold floors if you have a low supply.

    The stale air pickups can be anywhere, but near the ceiling is also usually the best.

    If you are doing mostly rigid ducting, add in a length of flex duct or an inline duct scilencer at the ERV to reduce noise. Most ECM blowers are pretty quiet but still noticeable especially on boost.

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