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Community and Q&A

Air exchanger prior to air sealing

Sofiane | Posted in Mechanicals on


I live in a late 80’s home that was at ACH 3.12 when we bought it two years ago. We already did some additional air sealing when changing our rear windows. I just realized our current air exchanger is rather useless as it’s one big flex duct going to the attic and pulling stale air from the hallway with no ducting to provide fresh air. 

I am looking at doing a clean install of an ERV with its own ducting system before we do any further air sealing. I live in climate zone 6. Our house is approximately 2800 sqf on two floors without basement and we are 3 in the house. 

As to the questions:

1) Should the ERV be ducted with rigid ducts? Or is there a more appropriate ducting system for ERVs?

Also, we have a bathroom renovation – secondary bathroom – and need to install an exhaust fan. I was thinking of having it connected to the air exchanger, but our contractor suggested against it. He seemed to genuinely believe it was an inferior solution. Do you suggest I have the bathroom fitted with an exhaust fan or to connect it with the air exchanger?

Lastly, I also need to replace our bathroom fan in main bathroom, do you think it best to replace it with an exhaust fan or to connect it to the air exchanger as well?

I’ve met 5 and none recommended what is promoted here: a balanced system with its own set of ducts, which makes me hesitant to trust what they tell me. There aren’t many companies left to meet which leads me to believe I’ll need to know everything there is to make things are done well…

Thank you for your help and insights!


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  1. Expert Member
    Akos | | #1

    Rigid ducting is always the best. Go for dedicated ducting if you walls are open. Shared HVAC/ERV ducting is cheaper to install but cost more power to run. Also fresh air supply to the bedrooms is not as well controlled, which is something you want from a fresh air system.

    If you have complicated runs, I've used semi rigid aluminum ducting (think dryer vent but comes in longer length). It is slightly more restrictive than rigid but way better than a sloppy flex duct install.

    For a house that size, depending on the number of bedrooms, you are looking at around 150CFM ERV, on the exhaust side you need 65 CFM for the kitchen, the leaves you 85CFM. That would do a decent job of exhausting one bath and so-so for two baths.

    I would say the best setup would be to connect the ERV to only one bath and have a dedicated exhaust fan for the rest of them. On the bath with the ERV pickup, have them install a boost switch.

    1. Sofiane | | #2

      Thank you for your input it really helps.

      I have a few follow ups if you do not mind answering:

      Is the Kitchen exhaust recommended even if I have a hood vent with direct exhaust outside my home?

      We have 3 bedrooms and 1 room which I use as an office 2 to 3 days a week. I was thinking of also getting the office connected to the system for fresh air since I spend a considerable amount of time in there. Does it make sense?

      My walls are not open, but part of the route will be opened up during upcoming renovations and even if it sounds a tad crazy, I would rather pay to open up the walls and get it done the optimal way.

      Additionally, according to my guestimate, I would pay in extra electricity over the course of 10-15 years what I would save on installation. I estimated an extra cost of 300$ of electricity per year with air exchanger to heating ducts and an extra cost of 3000-4500$. Does that seem like a fair assumption?

      1. Expert Member
        Akos | | #3

        You always want a kitchen exhaust pickup. A fume hood is great when cooking but you need something that can run for a couple of hours afterwards to clear cooking smells.

        You need to run a fresh air feed to all rooms, office is definitely one of them.

        The electricity cost for connecting with the HVAC ducting depends on what your furnace blower's power consumption is. If it is an ECM blower with decent low speed, it might only use 25W to 50W of power, so the energy consumption might not be that much. If its a PSC blower, than there is definitely a payback for dedicated ducting. Most of this work is install specific, it is hard to put even a ballpark number on it. Not having a basement will add to the costs, make sure they keep the ducting out of the attic though.

        This is Canadian, but a good read on sizing and install details:

        1. Sofiane | | #4

          I quickly glanced over it and I will take the time to read it in detail, but it seems like it's exactly what I needed. I live in Canada, so do not mind it's provenance at all :-)

          Anyway, thank you so much for your help!! I will see if I still need some clarifications or pointers after reading this.

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