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Exaust fans for the bathrooms vs ERV

orange_cat | Posted in General Questions on

Trying to understand how the bathroom exhaust works with ERVs.

We originally had independent bathrooms fans planned for each bathroom, in addition to normal air supply and exhaust via ERV.

The suggestion is to not add roof openings for example in the upper floor bathrooms, and instead rely on ERV somehow to vent the bathrooms, without installing the fans.

That sounds like a bad idea. Is it?

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  1. lord_of_the_manor | | #1

    To prevent pressure balancing problems I am going to use distal fans to draw the air from the bathrooms rather than push the air with ceiling extractors. The advantage of drawing the air is that there is always a slight negative pressure in the duct preventing moist air from leaking out. Then exhaust through the ERV return. At least that's the theory!

    1. Expert Member
      Deleted | | #10


  2. orange_cat | | #2

    I am sorry, I do not really understand your answer. Can you explain it for someone less well versed in terminology?

    1. Expert Member
      MALCOLM TAYLOR | | #11


      I think he is saying he is using a remote fan to exhaust the bathrooms, then send it to the ERV return. It seems like an unnecessary extra step to me. Why not just have an ERV return in the bathrooms, or exhaust the fans directly?

      1. orange_cat | | #15

        Thank you for interpreting this, I appreciate it.

  3. Expert Member
    Akos | | #3

    ERVs work fine as bath exhaust as long as you have sufficient flow. A normal bath with a shower needs at least 50CFM exhaust flow (close to 100cfm with a steam shower), so most ERVs sized for a typical house can only do about 2 bathrooms.

    The fresh air supply from the ERV should not go into the bathroom, you want this ducted either to/near the return of your HVAC or to the bedrooms. Bedroom ERV feeds need a length of flex (even better an in-line duct silencer) to limit noise.

  4. orange_cat | | #4

    Plans show 3 ERVs. The models installed I believe are Lifebreath Metro 120 ECM

    1. Expert Member
      Akos | | #6

      300CFM of ERVs is a bit overkill for 2500sqft. Nothing wrong with overkill but you have to watch the combined flow rate at low speed. Even with an ERV, overventilation of a house in the winter can dry out the air which can create issues with furniture and flooring. Make sure that these are set up properly, depending on the number of beds combine vent flow rate should be between 60 to 100 cfm.

      Each ERV can easily vent up to 2 bathrooms if needed, nothing wrong with a single bath per ERV either, it will just clear it quickly.

      Cooking tends to be largest source of indoor pollution, at least one largish ERV stale air pickup (~60CFM) should be near the kitchen area about 8' away from your range. Also install a boost switch in the kitchen so you can kick it on when cooking.

      1. orange_cat | | #7

        Kitchen has a vent hood which vents outside through the exterior wall right at the hood.

        (And cooking is the source of pollution mostly for gas stoves. I have induction - and no gas line).

        1. matthew25 | | #9

          Do you have a makeup air system for your vent hood?

          1. orange_cat | | #12

            Nothing beyond the same air supply as in all rooms.

        2. Expert Member
          Akos | | #13

          Gas does add its own combustion products, but it doesn't matter what your fuel source, most of the small particulates come from cooking. Even something simple like making toast will spike Pm2.5.

          No hood has 100% capture and most residential hoods have very low capture efficiency. You'll have a lot of bypass which makes it into your house.

          I have an induction burner with a hood (which I do use) and it takes about 2h after cooking for the ERV to bring the indoor PM2.5 back to baseline.

          A stale air pickup in the kitchen is a bonus but ERV boost control in the kitchen is a must.

          1. orange_cat | | #14

            Thank you!
            I will ask for ERV boost. I do not think we have one.

  5. DennisWood | | #5

    Vent the bathrooms via ERV, and make sure supply is to the living space/sleeping areas. It's a smart idea as it not only reduces penetrations, but simplifies mechanical a bit. If you are going to be running three ERVs anyway, it would zero sense not to use one with bath returns :-)

    Your plans likely already call for a boost switch in the bath which toggles the ERV to high speed for a set time to get higher vent rates as required. When in non-boost mode your bath areas will be constantly vented and likely stay drier etc. as a result.

    The unit you spec'd has options for both wired and wireless boost switches to give you 20/40/60 minutes of high speed (boost) operation.

    I just finished going over a system (which was a bit of cluster) as precisely zero of the five boost switches in the home actually worked...and none of them were wired correctly to the HRV...nor was the main HRV wall control. This was a 15 year old system that actually never worked..although the designers did have the the right idea by making sure the kitchen, bath and laundry areas all had HRV returns. The HVAC contractors evidently had no concept of the simple wiring required though. In any case, the home owner is quite happy now with working HRV returns and boost functions in the bath, laundry and kitchen areas. Oh, and the HRV actually turns on now...ha.

    1. orange_cat | | #8

      Thank you!
      Do you ever come down south? I may need to have it checked out soon.

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