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Where does an ERV’s return go?

qbrt | Posted in General Questions on

With an HRV, the return is typically placed in “wet” rooms like the bathroom, so a shower’s moisture is immediately removed from the house. However, with the same setup an ERV would return most of the moisture to the house, which seems like it would over-humidify the house.

Am I thinking about this right? Where do people put their ERV returns? If it’s not the bathroom, do you also install a separate bathroom exhaust?

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  1. charliepark | | #1

    ERVs have the same "return from the wet / smelly places" guidance as HRVs.

    Hopefully others can weigh in as well, as I might be misrepresenting numbers here, but I'll do my best. Also, I should state that I'm framing this in a scenario where the outside air is more humid than you'd like your inside air to be, as opposed to a cold climate in wintertime.

    Let's say that inside and outside are the same temperature, and the outside air has a 75% RH and your house has a 50% RH. And let's say that your house is broken up so that 25% of your ERV return air comes from the bathroom we're talking about, and 75% comes from the rest of the house.

    Normally, with no shower happening, your ERV does the exchange and has … I dunno … 80% efficiency. As I understand it, your ERV will send 80% of the delta of the two humidities out with the exhaust air. So if your house is at 50% and outside is 75%, the delta is 25% and 80% of that is 20%. So the humidity coming into your home would be 75 minus 20, equaling 55% RH.

    So now you take a shower, and manage to get the RH up to 100% in that bathroom (for easier math) so now your ERV is getting:
    - 25% of the volume @ 100% RH (your bathroom)
    - 75% of the volume @ 50% RH (the rest of the house)

    That all averages out to 62.5% RH for your whole house. So the delta between the 75% outside and the 62.5% inside is 12.5%, and 80% of that is 10. So the humidity coming into your house (while the bathroom is at 100% RH) is going to be 75 minus 10, or 65% RH.

    So for the ~30 minutes that your bathroom is at ~100% RH your dehumidifier will have to handle ~65% RH air, rather than the normal ~55% RH air. Most people would argue that the complexities that would come from additional penetrations for the dedicated bath exhaust (and the requisite make-up air) are not worth it, and would just lean on the dehumidifier a little bit more.

    Also, don't forget that you can always open a window to flush a bit more of the humidity without sending that through the ERV.

    For a bit more on this, check out Corbett Lunsford talking briefly about ERVs and bathroom humidity:

    1. qbrt | | #2

      Makes sense, thanks!

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