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When do you need an ERV?

chrisjenx | Posted in Mechanicals on

I’ve been reading allot about ERV’s and ventilation for our new house currently being built – one question I struggle to find an answer for is:

 > At what “tightness” does an ERV make sense?

The house is being built to 2018 IECC Climate Zone 5B, meaning 3 ACH to pass. I saw someone mention that it’s not worth it unless you are building to <0.25 ACH? 
To be honest while our current house is technically less tight – I really feel like we could do with an ERV in this property – not gunna lie but there’s been many a time I’ve had to open the bedroom window in the morning to “air it out” per-ce…

Follow up questions 
– For Colorado’s dry climate does it make sense to put in an ERV or HRV?
– If we “should” put in an ERV should I just tell the builder to omit the “fresh air” system they include?

Thanks in advance 

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  1. erik_brewster | | #1

    Yes you should put in an HRV/ERV. It doesn't matter what your ACH figure is if you are trying to make it tight at all. Your CO2 is going to be high if you don't I live in an older house that I've tried to tighten up, but have never measured it. I have measured CO2 problems and installed an HRV with 90 CFM. It isn't enough. Get the ducts in now for a supply and return from each bedroom. There is no better time to do this than the construction phase. I feel like everyone that says that there is a limit where you don't need to have an HRV/ERV has never measured their CO2.

    I can't speak to ERV/HRV in you region.

    1. chrisjenx | | #3

      Thanks for the response:

      I already requested the builder to install an ERV they declined the custom, thankfully the attic and basement should make all rooms that I would want to modify accessible.

      Their reason was something about the house master plan and city permits that have already been signed off. Last I checked, ducting doesn't change any outside/structural aspects of the building...

  2. Expert Member


    The "fresh air' system your builder includes is mechanical ventilation used to maintain healthy indoor air quality and control humidity. It replaces the unpredictability of trying to do those things through opening windows as we used to.

    Whether you need an HRV/ERV to supply that ventilation depends on the payback for the energy it saves in your climate, and how sensitive you are to issues of comfort.

    1. chrisjenx | | #4

      Hey Malcolm,

      Thanks for the response,

      We have fresh air system in our current house and it is crap to put it politely - might as well not be installed.
      Smells gross when it runs - makes the house cold in the winter and hot in the summer. I run it cause I know I need to pull in fresh air in but its not good - most builders here seem to use them cause they are cheap way to pull in fresh air into the house.

      I guess you helped me answered my own question then, from a comfort pov it's something that I want - esp as we work from home (even before COVID).

      1. Expert Member
        MALCOLM TAYLOR | | #5


        "most builders here seem to use them cause they are cheap way to pull in fresh air into the house."

        I think that's a fair way to put it. You can design a good ventilation system without any heat exchange, but the only real incentive to do that is cost.

  3. aunsafe2015 | | #6

    For one data point, my 10-year old house (I'm not the original owner) was measured at 4.3 ach50, which isn't very good, and my Co2 levels tend to hover between about 800 and 1100. So yeah, I probably need an ERV even at a relatively poor 4.3 ach50.

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