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Community and Q&A

ERV Bathroom Ventilation

woodguyatl | Posted in Mechanicals on

I see that Zehnder and some other ERV makers say that they can be used to vent a bathroom in lieu of a dedicated exhaust fan. We intend to install one in our new house and eliminating the exhaust fans would be nice, but there is something I can’t seem to figure out.

Exhaust fans are used to remove moisture from the bathroom. What prevents the ERV from transferring that moisture to the incoming air and simply redistributing it around the house?

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  1. lacroixb | | #1

    From everything I've been reading - ventilation is not a well integrated system yet, so an ERV + humidifier + dehumidifier are all sort of needed together to keep the humidity in the sweet spot and keep fresh air flowing.

    Take that with a grain of salt, as I'm relatively early into my education on this front.


  2. plumb_bob | | #2

    The code I work with requires kitchen and bathroom exhaust fans to exhaust air at a substantial flow rate, something an ERV or HRV will not do.
    Kitchens and bathrooms can produce high loads of moisture and odour, and a dedicated exhaust fan is the best bet for both rooms.

  3. AlexPoi | | #3

    Would bypassing the ERV and dumping the air directly outside saves energy when the humidity is higher inside then outside ? Yes but not as much as you think. Afterall, you're not taking your shower all day long and the humidity get spread all over the house.

    On the other hand, bypassing the ERV unit would require some sophisticated controls which would increase the unit cost. So at the moment, it's just simpler to run a dehumidifier or let the AC handle the small extra load and not bother about it. Plus, in a cold climate, during winter, you want to keep that extra humidity inside the house anyway.

    We'll probably see that sort of controls one day but the industry is not there yet.

    1. woodguyatl | | #5

      Thank you for the excellent input.

  4. tim_in_nc | | #4

    There are a lot of conflicting opinions of this on the forums! I live in a new-built house with just 2 residents and the biggest moisture load in the bathroom is morning showers; the ERV seems to be totally acceptable for venting that and I love how quiet it is.

    But I can totally see from the data we've gathered that it would probably not be sufficient for a larger moisture load like a big family.

    For what it looks like in action -- We have a humidity sensor in our main bathroom that's linked to the ERV; after the bathroom is used the humidity seems to mostly follow an exponential decay curve -- it goes down really quickly and then much more slowly; so my sense is the ERV is enough to clear the moisture but it needs to stay on longer than the 15 minutes that a wall timer might provide. We have ours set to switch to boost mode and stay there until the dewpoint in the bathroom is within a few degrees of the ambient dewpoint in the house, which seems to work pretty well.

    The graph attached shows bathroom dewpoint (lilac, top line) vs. dewpoint on another floor of the house (purple, middle line) vs outdoor dewpoint. You can see that the bathroom humidity does get slightly redistributed to the rest of the house; but not enough to make a significant difference.

    1. woodguyatl | | #6

      This a very helpful answer. Thank you for taking the time and posting the graph.

    2. amorley | | #7

      This sounds like a smart set up! What humidity sensor and controls are you using?

    3. stevedavis | | #8

      I too am very interested in what controls you are using. Are you actually using a controller that can compare two different dewpoints?

      1. tim_in_nc | | #10

        Yeah! I thought about doing a forum post with more detail actually, maybe I will get to that in the next week or two.

        The original motivation for this whole set-up (besides that it's fun to hack on things) was to trigger the ERV into boost mode when CO2 levels in the bedroom went too high, but then since I had Ambient Weather temperature sensors throughout the house anyway, I figured that could also replace the physical booster switch in the bathroom.

        I've got a Broan ERV140TE, and wired up an arduino microcontroller to control that using a digital potentiometer (right now I can put it into boost mode or not, but I'm planning to update the controller to also switch recirc mode on or off soon). The arduino is running ESPHome ( which lets it communicate with a Home Assistant hub I have running, which also pulls in sensor data from an EcoWitt WiFi sensor gateway (, which is connected to the same temperature sensors that my Ambient Weather logging station uses.

        So once all those pieces are set up, I can write some home assistant automations which trigger the ERV on based on the state of other sensors; specifically when the dewpoint in the bathroom is more than a certain degree difference above the dewpoint elsewhere in the house, and then trigger it back to normal mode once they've dropped back close together. That works well in winter when the dewpoint is consistently lower outside than inside. I'm not totally sure what the best logic is gonna be in summertime though, we'll see.

        1. stevedavis | | #12

          Oh that makes me so happy to see a fellow Arduino nerd. Ha! I'm in the planning stage of putting in an ERV for my house and lamenting the total lack of (reasonably priced) CO2 sensors on the market and was similarly thinking about tying it to an Arduino. The dewpoint controller isn't something I had considered for bathrooms but makes total sense.

          On a related note, I have looked endlessly for a "differential dewpoint" controller (or whatever you'd call it) but found no commercial options. In my case, I have an enclosed crawlspace and wanted to add a supply fan that would blast air into the crawlspace during the summertime when the outside dewpoint is lower than the crawlspace dewpoint. This occurs occasionally in my climate during summers and would keep my dehumidifier from running as much. I came to the same conclusion that I'd need to build a custom Arduino to so - and maybe will when I can find the time. Thanks for the links and knowledge that there are other tinkerers out there!

        2. stevedavis | | #13

          And - if its not obviously - please do do a detailed post on your set up!

  5. STEPHEN SHEEHY | | #9

    We don't have a bathroom fan. We use the HRV boost, set for ten minutes, after a shower and it clears the air well.

  6. plumb_bob | | #11

    What flow rate does the boost option have? Seems to eliminate the bath fan would be smart if the hrv can handle the load

  7. scottsil | | #14

    We are planning a build right now and I’ve been researching this topic extensively. Here’s a good thread from years ago on the topic. The comments section provides some good first-hand experiences.

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