GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Mold on ERV intake side of core

user-7538414 | Posted in General Questions on

Help! I love my zero energy house that has functioned superbly for 3 years, but I am now plagued by mold growing on my ERV core and can find no information about this. My family is suffering from severe allergies, we need help! I don’t know if this is related in any way to how the unit is installed or the paths of the ducts. It is very expensive to replace the core and I hate to have this happen again.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Akos | | #1

    Which climate zone are you in? What is your interior relative humidity and temperature in the summer and winter?

    If I had to take a stab at it, either your interior is too humid or the ERV is significantly out of balance.

    With a very tight house, an ERV is not needed as you generally need to remove the humidity generated by the occupants, swapping the core out to the HRV version might be better (for most manufacturers the cores are interchangeable between the same series HRV/ERV unit). The HRV core is metal and won't support mold growth.

  2. user-7538414 | | #2

    Thanks so much for the reply! We live in Northeast Ohio. We can have prolonged very cold snaps in the winter, heavy rains in the spring. The intake opening outside is close to snow on the ground in the winter. Humidity is not that big of an issue here. The mold is on the side directly coming from outdoors. We have 2 mini split units we run on “dry” in the summers for cooling, eco mode for heating in the winter.

    1. Expert Member
      Akos | | #3

      If a good part of your spring/fall is mild and humid with lots of rain especially if you are near forested area, there is a good chance the intake side of your ERV spends a lot of time at the temperature/humidity where mold likes to grow.

      Combine this with a tight house with not much cooling load, thus higher humidity during the same time, you get very little drying from the air exhausting from the house.

      So your options are:
      -dehumidify your house more in the spring/fall
      -run your ERV in exhaust only mode during this time
      -swap out the core for an HRV

  3. user-7538414 | | #4

    Thank you! My indoor humidity was 67% this afternoon, dropped to 55% when I opened all windows and turned off ERV. I will look into switching out core for HRV and dehumidifying. I have no idea how to run my ERV in exhaust only mode. I very much appreciated your help!

    1. Trevor_Lambert | | #6

      Since there is no moisture transfer between incoming and outgoing airstreams with an HRV, indoor humidity won't affect the problem at hand. So try one or the other first.

  4. Jon_R | | #5

    When you take moderately humid warm air from outdoors and cool it, you get very humid air - which is what mold likes. With the ERV, some of this moisture is removed. With an HRV, it isn't - so the very humid conditions will extend all the way through your fresh air supply ducts - more places for mold to grow! The best way to stop mold - do something that causes it to dry out once per day. Perhaps ERVs should do this automatically (as some ACs do).

    In warm weather with AC, exhaust only ventilation is a bad idea for wall moisture/mold - if you are going to do unbalanced, you want intake only and the slight pressurization it causes.

    1. mikeolder | | #7

      Exhaust only in any home is a bad idea, pulling sewer gasses radon and other contaminants into the home. And this EVR/HRV controls choice is left up to the home owner to decide, instead of a smart controller?

      User-7538414 declaration of love for his house is over shadowed by his plea for help and unsettling for those of us headed down the same path. Because we've been told tight homes need EVR's to prevent sick homes.


      1. JC72 | | #10

        Radon is not an issue and neither are "sewer gases" with exhaust only ventilation. If sewer gases are getting into a home there's a plumbing issue.


        1. mikeolder | | #16

          No radon in Ohio? And no way during a negative pressure condition for a pee trap to dry out and let sewer gasses back into the home?

          1. MattJF | | #17

            Off topic at this point, but there was a least one study that showed exhaust only radiation reduced radon levels in the main home. More dilution air came from outside than from below slab essentially. Now that was comparing exhaust only vs no ventilation from what I recall.

            Traps drying out would be an issue with any or no ventilation. Stack effect forces in a drain system will likely overwhelm ANY ventilation system.

            The OP has balanced ventilation, which they should be able to get working with the right setup. There are wetter, warmer places than Ohio.

        2. mikeolder | | #18

          How is this off topic? Were talking about a family suffering from severe allergies that could have originated anywhere, but presumably from the leaky zero energy house which dilutes whatever radon is pulled in by a exhaust fan, so that's OK somehow. I cant count how many easy trouble calls Ive fixed by simply pouring water down a dry floor drain. The stack effect may be enough to prevent poisonous sewer gasses from entering a home, but not if there is a fan pulling them. Been there done that.

          1. MattJF | | #19

            I said off topic because the ventilation choice doesn't influence drying of drain traps and a dry trap will present issues even with supply only ventilation.

            Ventilation choice also does not have a significant impact on radon levels, it reduces radon in all cases vs no ventilation.

            OP has an ERV, which is the BEST ventilation choice for humid exterior conditions. They should stick with it and just change how it is operating. The only time an HRV is superior is if you have excessive indoor humidity in winter conditions.

  5. Expert Member
    Akos | | #8

    My suggestions were that any of those steps should prevent mold growth, you don't have to do all of them.

    Usually going from HRV->ERV is pretty straight forward. Going the other way is harder if the existing ERV does not have drains or installed in the wrong orientation.

    Before going too far down any path, the OP needs to get their interior RH under control. A house with above 60% RH is asking for interior mold issues never mind the ERV. A stand alone humidifier might be needed.

  6. MattJF | | #9

    Are you running the ERV 24/7 or cycling it? It would be much less likely to develop mold running continuously if it is not. You will need to rebalance the unit if it is not running continuously to get the correct ventilation.

    I don’t think this is an issue that is going to be solved by switching to an HRV. ERV’s generally have less condensate issues. An HRV probably won’t fix your humidity issues, and will make summer worse. As said before get a stand-alone dehumidifier. Get the humidity at the top floor ceiling down to around 50%.

    What is you summer cooling set point?

  7. JC72 | | #11


    Have you cleaned the cabinet and core as prescribed by the manufacturer?

  8. user-7538414 | | #12

    Yes, I’ve been religiously changing filters, vacuuming the core and cleaning inside the case. We usually run the ERV between 20-50%. Im getting dehumidifiers, air purifiers to bring the indoor humidity down to appropriate levels and take care of any mold etc. I’m having difficulty sourcing a replacement core. We are also having the ducts cleaned and hope to get a fresh start with all of these measures. I will look into making sure the system is properly balanced. Thank you all for your help. Thank goodness it’s a great time of year to just open the windows until we get all of this sorted out.

    1. MattJF | | #13

      Have the ERV setup to run continuously. I think this is the primary issue you have. The indoor RH should be fixed, but is not the dominate problem. Condensation is likely forming every time the unit cycles off. There will be a slug of still warm humid air against a cool core and condensation can form. With continuous airflow, condensation is unlikely form. Do you have dampers that close on the exterior supply exhausts? If you don't, that is another likely source for moist air and condensate. You won't need them in continuous operation.

      I think the mold develops from summer time use. During winter the core should be a bit too cold to encourage mold growth in most homes. An ERV has a lower summer latent load contribution than an HRV, so it "helps" dry the house in the summer. It doesn't really dry the house, but it contributes less moisture. HRV's can have winter time condensation issues and need a drain installed. Many ERV's do not need a drain. I would stick with the ERV.

      Drier indoor air from using a dehumidifier during the summer will help dry the core a bit, but the moisture source is humid outside air.

      You can likely clean the core you have, immersing it in a mild bleach solution. See if the manufacture has any guidance.

      If your exterior supply and exhaust ducts are insulated, they probably don't have any mold in those. Duct cleaning is more snake oil than anything in most situations.

      How is the unit ducted? Is it independently ducted or tied into the heating cooling ducts? I am thinking about if there is any unintended movement of air through the ERV. Edit - I see you have minisplits, so unless ducted, I the ERV is independently ducted.

    2. MattJF | | #14

      Use the formula here to determine the CFM to balance the unit to:

    3. JC72 | | #15

      What about cleaning the core itself (i.e. rinsing)?

      Second Question: How are mold spores getting past the MERV filters?

      1. Build_once | | #24

        And HRV filter is usually just merv 8 or less.
        Its a partical filter, it won't stop smoke, pollen, mold spores and other micron particals

    4. jdhbuilder | | #21

      I have been having the same problem with my ERV, and have shut my system off. Did you end up finding out what the cause was, and how to fix the problem? It is a new home, and I ran the system continuously, and suspect that the issue was most likely caused by running the system when exterior humidity was high - the intake is 10' off the ground.

  9. larkomundo | | #20

    I realize this is an old post. But after reading the replies, I have to offer my thoughts. I've been worried about this risk since a friend, who is one of the best mold hounds in the U.S. told me she's never tested an ERV core that didn't have mold. I think it's inherent with the design. We just cannot expect two opposing temperatures clashing in a highly conductive cell without one side or the other seeing dew point condensation. Think about it. HRV's have a drain for some reason. I don't see this problem going away until we rethink the strategy.

    1. Build_once | | #23

      The only issue is if you aren't controlling this issue in the ERV then this same issue happens in your walls everytime a bath fan is turned on.. two airstream clashing near condensing surfaces.

  10. Build_once | | #22

    Replace the core. And in the future you need to clean the filters every few months and then Clean then core annually.
    Once mold grows its roots will always grow again if "killed". It has to be removed and the core can't be cleaned like that.

    On your future core, You may want to maybe install a filter box that has a higher merv. Especially if you have a 2 motor ERV.

    Also run your erv 24/7 and obviously use dehumidifiers in your house.
    If you are only using an ERV as a bath fan or to control humidity for short times and its a humid day then the core will be wet. But if the ERV is on continuously It will dry out after your shower even on a humid day.

    If mold is growing fast on things in your house the ERV may have alerted you to a problem. You may have a mold source that is emitting spores somewhere in your walls or under your house.

    A uv Light in the intake shining directly on the Core, especially the white core should prevent growth.. put the light on a timer a few hours a day.

    1. Expert Member
      Michael Maines | | #25

      Mold spores are everywhere; there is no hiding from them. If conditions are right for their growth, they'll grow. If conditions are not right, they won't grow.

      It sounds like you might be talking about dormant mold, which can cause reactions, and will quickly re-grow if conditions are conducive.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |