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Community and Q&A

Exhaust only ERV system

arnoldk | Posted in Mechanicals on


I am doing an exhaust ERV system and the HVAC design has 20CFM for the both bathroom. Is that sufficient to handle the moisture after hot showers?

Note that I am aware that the mirror fogging level of moisture will not be removed within 5 minutes. I just want to make sure its sufficient or if I should be increasing the CFM a bit higher.

Thank you,

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  1. arnoldk | | #1

    Looking any feedback from anyone?


  2. Andrew_C | | #2

    Is your ventilation system whole house continuous? I think 20 CFM is the minimum continuous flow allowed. If it's not continuous, depending on the size of the bathroom it's probably 50 CFM min to help clear vapor before it condenses on any cold surfaces. Note those are minimums and you need to take your ducting resistance into account.

    Aside: I'm not an expert. I usually think of Energy Recovery Ventilation systems as having inlet and outlet streams crossing to exchange energy and perhaps some moisture. How does an exhaust only ERV work?

  3. user-2310254 | | #3


    This article might be helpful:

    FWIW, it sounds like 20 CFM meets code (at least at the time the article was written), but user behavior/preferences may mean you need more CFMs. For example, the occupant might not want a steamed up mirror, and 20 CFM may not move enough moisture to keep that from happening.

  4. arnoldk | | #4

    Thanks for the link Steve.

    I was just looking for feedback from those who have direct experience. Currently the HVAC design has the Lifebreath 170ERV with a maximum of 160 CFM but if I want more than 20CFM per bathroom, I would need to get another ERV with more CFM like the VanEE G2400E ECM with 210 CFM.

    With that said, I don't want to spend more money if it's not necessary which is why I am looking for anyone with personal experience.


    1. Expert Member
      MALCOLM TAYLOR | | #6


      When this has come up in the past, opinions seem mixed. Some posters, like Stephen Sheehy, successfully rely on the 20 cfm ERV in their bathrooms, others find their 50 to 80 cfm dedicated exhaust fans barely adequate. I'm not sure you will get any definitive answer.

  5. user-2310254 | | #5

    I don't see a boost mode described in the instructions, but it looks like you can wire a Lifebreath timer switch to bump the motor to high speed.

  6. Expert Member
    Michael Maines | | #7

    As with many aspects of the IRC, their ventilation rates are minimums based on input from various experts, and going above code-minimum usually provides superior results. The 2021 IRC requires bathrooms to be ventilated at a minimum of 20 cfm CONTINUOUS, meaning the fan is always on, or 50 cfm intermittent, used only when needed. ( My experience says that in most bathrooms those would indeed be minimums.

  7. walta100 | | #8

    One of us is very confused.

    Do you mean ERV (Energy Recovery Ventilator) the title of the post says EVR maybe I don’t know what that is?

    I don’t see how you can recover energy with an exhaust only system.

    Even if that was somehow possible to recover energy without intake air, collecting air from bathrooms and kitchens is generally prohibited in the ERV installation instructions as the moisture tends to freeze in the heat exchanger and grease from the kitchen clog the heat exchanger.


    1. arnoldk | | #9

      The ERV has dedicated duct for the exhaust but used the central sir system to introduce the conditioned fresh air into the house.


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