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ERV system: too much humidity

user-6813968 | Posted in General Questions on

We built a custom 2095 sq ft home in the Raleigh North Carolina area and we moved in about a month ago. It’s an Energy Start rated home with a HERS Index rating of 54 and a blower test criteria 2 leakage rate of 1295 CFM 50. We have Mitsubishi mini split ductless HVAC system and have an ERV system. Our problem is the house is too humid. We are running the ERV system on the lowest fan speed because it is bringing in humidity and we can’t get the humidity to go below 50% except early in the morning. My wife is extremely sensitive to mold and can get very sick if she is exposed so we want to take all precautions to avoid mold in the home. We want to keep the humidity below 50% to have a mold free home. We purchased a stand alone home labs 70 pint capacity dehumidifier and we run it 24/7 and still can’t keep humidity below 50%. Currently it has been running between 48 in the early morning to the high 50’s and sometimes 60 in the afternoons. What can we do to lower the humidity? Can we install a dehumidifier that will work with the ERV system? What do you recommend? Thanks.

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  1. user-626934 | | #1

    1) High 40's to high 50's RH% is not typically mold territory...unless you have some very cold surfaces and your house is being depressurized accidentally by the ERV or other fans.

    2) You moved in at the absolute worst time of year...not your fault, just an FYI. The house is likely loaded with moisture from construction, and the outdoor air has been extremely humid...but the outdoor air temperature has also been mostly mild, so not a lot of air temperature cooling needed, and therefore very little moisture removal from the heat pump/s.

    3) You may need a bigger dehumidifier. Yes - you could have a ducted dehumidifier installed inline with the ERV supply air. Or it could be installed stand-alone...Here's an example -

    4) Make sure your measurements are made with a quality instrument, such as Lower quality sensors will be off by as much as +/-10% points from the factory.

  2. user-6813968 | | #2

    Does the Ultra Aire XT155H have controls so that it will automatically turn on and off at specified adjustable settings or will it just run 24/7?

  3. Jon_R | | #3

    My guess is that anyone who is sensitive to mold is also more likely to be sensitive to indoor pollutants - so I'd keep the ERV rate above recommended CFM.

    Sounds like a leaky house - you should improve it.

    Consider positive building pressure which reduces mold in the walls/air (even when inside humidity is low).

  4. Deleted | | #4


    1. lance_p | | #5

      A 1 ACH house in temperate weather without much wind will not ventilate well on its own.

    2. Jon_R | | #7

      > This number equates to more than 12 ach.

      The formula is (CFM50 X 60) ÷ the volume of the house. So 1295*60/(2095*8) = 4.6 ACH@50, less with high ceilings. A believable number and less than 5ACH@50 where you might consider not using mechanical ventilation (see 2018 IRC R303.4).

  5. lance_p | | #6

    As mentioned by John S. above, my bet is on construction moisture. Concrete takes a long time to dry (slab and basement walls), as does lumber. From what I’ve read I t can take a year or more for moisture to stabilize.

    If the house doesn’t require AC until next summer your only option would be to buy another dehumidifier and keep both running. I’d just buy another small 70-90 pint portable unit instead of the considerable cost of having a ducted whole-home model installed.

    Hint: Energy Star models larger than 70 pints are almost 30% more efficient than models 70 Pints and smaller (2.8L/kWh vs 2.0). That can add up if you’ll be running it steady for a year.

  6. Expert Member
    Dana Dorsett | | #8

    Since it's not yet been mentioned I'll throw it out there: Have you also been running the Mitsubishi in "DRY" mode?

    When in DRY mode it's as-used SEER drops, but the sensible heat ratio SHR shifts pretty dramatically toward latent cooling (moisture removal.) That's still more efficient than throwing ever more dehumidifier at the problem.

    A dehumidifier takes the latent heat of vaporization of the water it's removing from the air and heating up the indoor air with it. Running the mini-split in DRY mode puts that heat outdoors where it belongs during the cooling season.

    Until the construction moisture is gone you'll probably have to continue to use dehumidifiers, but be sure to make the best use of the mini-split's drying capacity too- it's not just another air conditioner.

  7. peterbauer | | #9

    Jon R. Thanks for pointing out my mistake. A senior moment........
    4.6 ACH is one leaky home. You must take steps, to correct the air leaks, to get your mechanicals operating efficiently. In the meantime check your Manual J and see if the HVAC is sized properly.
    An oversized A/C will lower your homes temperature too quickly and turn off the A/C prematurely. The result is not removing humidity to a comfortable level.

  8. user-6813968 | | #10

    I had no idea that our house was so leaky. Our builder paid an Energy Star company to do inspections on construction and guidelines that the builder was supposed to follow. I read the report that said it has an ACH50 rating of 3.8 and I just assumed it was a great tight home. We paid a lot of money for this service plus polyurethane foam flash and R-19 fiberglass bat insulation 2X6 construction in walls and R-25 in the ceiling etc..... Now I am very upset and need to find out why my house is so leaky. This is very disappointing.

    1. T_Barker | | #11

      I wouldn't call it extremely leaky. It's just not "super tight" or super insulated. My custom house was built to a better standard than this 20 years ago. It's not a bad standard in my opinion (very comfortable in the winter, no window temp. issues, etc.), but it's not what I would call energy efficient by today's standard.

      In your climate, with this quality of build, I suspect you may need a "real" central air conditioner to keep your humidity where you want it with the ERV running in the summer. In the meantime, I would turn the ERV off during the summer and maybe even spring/fall, and run the Mitsubishi in "dry mode" as Dana says.

      In the hotter months (June/July/August for me) I turn my HRV off, otherwise it just fights the air conditioner by bringing in too much humidity while the AC is trying to remove it.

    2. peterbauer | | #12

      Air-transported water vapor, from leaky wall and ceiling assemblies, will lead to condensation then mold and dust mites, etc.
      Current IECC requires less than 3 ACH @50Pascal in Zone 3.
      Check out some good information from Building Science Corp.

      1. peterbauer | | #13

        Here is a great read from Allison Bailes of Energy Vanguard.

    3. peterbauer | | #22

      US Department Of Energy Table R402.1.2 in climate zone 3 indicates the Ceiling R-value as R38.
      1. Have a contractor perform a blower door test to locate all the air leaks.
      During depressurization he may use a smoke stick to locate any leaks. Seal the air leaks.
      Don't forget, water vapor travels along with leaky air.
      2. Then increase your ceiling insulation from R25 to R38.
      Note: When the ERV is operating, it will pull in outdoor air and Water Vapor through your leakage points. So locate all and seal them well.

  9. Expert Member
    Dana Dorsett | | #14

    While something tighter than 3.8 ACH/50 is desirable, it's by no means a disaster, and not the proximate cause of the humidity issue.

    Even a 12 ACH/50 house isn't going to represent a huge additional latent load that your ductless AC or dehumidifier wouldn't be able to keep up with. This is not a house with ducts & air handler in the attic and a really crummy unbalanced duct design. There is NO air-handler driven infiltration with a ductless heating/cooling system, and the ventilation is a balanced ERV. Outdoor air leakage is not your problem, unless driven by something else we don't know about.

    I asked this already, but are you in fact running the Mitsubishi in it's "DRY" cooling mode? (It makes a real difference.)

    1. Jon_R | | #16

      Right now, Raleigh is 70F and 92% humidity. I get 40 pints/day from natural air infiltration alone (> 70 pints when windy). About 3x these numbers using the fictitious 12ACH@50. No help from the AC.

      With all the moisture sources together, sometimes needing two dehumidifiers isn't surprising. Will be somewhat better next summer (as the house dries out).

  10. user-6813968 | | #15

    The majority of the article is about crawl space which we don't have. The house is built on a concrete slab so there is no crawl space. As for the rest of the article it is a lot of technical HVAC stuff that I do not understand. My main concern now is why do I have so much air leakage when this was supposed to be a very tight house and has an Energy Star certification. And will I have moisture problems in the walls and/or attic space. I am getting scared now. Looks like I have a bad builder. I switched the mini splits to dry mode but when I do the temperature setting disappears so I don't know what's up with that.

    1. Expert Member
      Dana Dorsett | | #17

      Some minisplits will still control to the setpoint in DRY mode, others not. Some that do still control to the setpoint don't allow the setpoint to be changed in DRY mode. Read the manual. But unless it's getting too cold for comfort continuing to run the mini-split in DRY mode removes quite a bit of moisture without raising the temperature, and removes more moisture per degree of temperature drop than running it in it's normal cooling mode.

      Had you been using the DRY mode more when the sensible cooling loads were higher it would have purged a bigger part of the construction moisture already, and the residual latent loads would be lower.

      Don't get scared or assume you have a bad builder- 3.8 ACH/50 is NOT a very leaky house- while higher than IRC code it's still not a big deal. If something lower than that was in the contract the builder should pick up the tab for the retrofit air sealing to the contract level, or to at least bring it within code. But the difference in additional latent load at 3.8ACH/50 compared to a code-max 3, or an even tighter 2ACH/50 just isn't very much. A lot of newly built homes in the southeast before anybody cared (and even now) are in the 10ACH /50 range. If yours tested a 3.8 and not 10 it means somebody was paying attention to at least the basics, and chasing down some more to nail it shouldn't be that hard.

      As Jon R points out, during the fall shoulder season there can be days with almost no sensible load and still a hefty latent load where a standalone dehumidifier (or two) might be necessary. That's particularly true in the northeastern part of the US when the temperatures are lower than the Southeast, but still subject to the same humid air masses moving up from the Gulf of Mexico.

  11. user-6813968 | | #18

    I appreciate all the feedback from everyone and welcome more responses. Just an FYI the house has 9' ceilings and an 11' entryway and cathedral ceiling in the living room.

    1. Expert Member
      Dana Dorsett | | #21

      Without knowing which Mitsubishi you have it's hard to tell if yours is any different, but the older versions I've seen still control to the setpoint when in DRY mode, but runs at a fixed speed cycling on/off rather than modulating when in that mode. The temperature control isn't in as tight a range as when modulating- it will swing in about a 2F-3F degree range around the setpoint (~1.5F over, ~1.5F under). They may have tweaked the algorithms in the newer versions, and it may be slightly different with single zone mini-splits than with multi-splits.

      When in DRY mode it only displays a water-drop, not the setpoint- to change the setpoint requires reverting to the normal cooling mode first.

      In the interest of getting the most out of the existing equipment, start paying attention to your outdoor dew points and adjust the ERV accordingly:

      50% RH @ 70F corresponds to a dew point of 51F. So any time the outdoor dew point is at or below 51F cranking up the cfm on the ERV will not be adding moisture to your indoor air, but unless your indoor temperature & RH are higher than that it won't be removing any either. When the outdoor dew point is 45F taking the ventilation rate to it's maximum will be actively drying the house toward 41% RH @ 70F.

      According to Wunderground's Chavis Park weather station the dew point right now is 49F. So it it's 75F (not 70F) and the indoor RH is hovering around 55%, speeding up the ERV will help reduce the the indoor humidity. But check a few other local weather stations first for verification- not all weather stations on their network measure dew points accurately.

      The dew point is displayed under ADDITIONAL CONDITIONS on the left side, about a third of the way down the page. The weather station it's referencing is displayed just below the bold typeface RALEIGH, NC. To the right of the weather station listing is a "CHANGE" tab which allows you to select the weather station from a map.

  12. walta100 | | #19

    Who thinks someone needs to run the ERV in a house with 4.6 ACH?

    How many people live in the house?

    How many hours a day are they home?

    I think you should get a CO2 monitor like the one in this link and only run the EVR if necessary according to the monitor.

    I was shocked to find how low the energy star home bar is set. With requirement like up to 5 ACH50 and R13 walls.


    1. T_Barker | | #20

      I learned the hard way that in a "medium tight" house in northern climates you definitely need to run an HRV/ERV - at least in the 6 months of winter. I understand this is a southern climate, but I'll bet similar issues will arise if they don't.

      Never mind CO2 monitoring, just air circulation for simple humidity control.

      I agree with you about CO2 monitoring, but in my opinion the problem with non-integrated CO2 monitoring is most home owners aren't going to manually change the setting or turn the ERV On/Off all the time, even if there is remote control. Maybe at best they'll adjust once a season or until they get their house figured out, but constantly adjusting or turning it on or off is asking a lot.

  13. user-6813968 | | #23

    Yesterday I set the evaporators to the dry mode and the house got so cold during the night. In the morning the house was below 70 degrees so I had to change back to the normal AC mode. They were set at 76 degrees prior to changing to dry mode and the house just kept getting cooler and cooler. So using the dry mode is useless unless you want to freeze and pay for the extra electricity.

    1. Expert Member
      Dana Dorsett | | #24

      Read the manual yet? (I swear they have too many features within features!)

      What was the temperature and humidity when you woke up with the chill?

      Changing back to normal AC mode is fine if it's getting too cool. If you can't figure out how to automatically leave you a with an AM chill running in DRY mode overnight, turn it to normal cooling mode (or off) before you turn in for the night. Then when the temperatures picks up in the morning to where it's going to need some cooling, run it in DRY mode until it reaches the temperature you want.

      But you're not really paying for "...extra electricity...", since it's more efficient per gallon of water removed than than running a standalone dehumidifier, which then heats up the house, adding to the "normal" mode cooling load on the mini-split making it run bit harder. The heat of vaporization of water is the same whether it's being drawn by dehumidifier or the mini-split, but the mini-split moves it out of the house in one step, not two, and with a much more sophisticated & efficient control scheme than a standalone dehumidifier.

  14. user-6813968 | | #25

    Yes I read the manual and not much there about any hidden features although they might be there. Now that the temperature has dropped we are no longer having a humidity problem. All day yesterday it was running at 76 degrees and 54 to 46 % humidity and this morning it was 69 degrees and 45% humidity. Hopefully by next summer the house will be dry enough and the AC is all that is needed to keep the house dry. If next summer the indoor humidity goes above 50 I may have to install a Ultra Aire XT105H in the attic space and duct it into the house. I hope this is not necessary because it's expensive but I will if I have to. When it gets cold I will be heating with a wood stove rather than using the mini split heat pump.

  15. Expert Member
    Dana Dorsett | | #26

    Most people only end up using the DRY function when the dew points are north of 70F outside, once the house dries out, but you may also have to use the dehumidifier now & again.

    Still don't know what model ductless you have. Mitsubishi hides some of the functions under SMART SETTING or some similar name. (I don't have one in my house to refer to, but have several relatives heating/cooling with Mitsubishis, just not next door.)

  16. user-6813968 | | #27

    Condenser model MXZ-4C36NA2
    Evaporator model MSZ-GL15NA
    there are three other evaporators same models just different sizes to match room size. I tried to attach photos of the data plates but looks like they would not post. There may be other settings available but not listed in the user manual.

    1. Expert Member
      Dana Dorsett | | #30

      There's nothing under "SMART SET" that is going to change how it works in DRY mode. Anything that gets set under "SMART SET" gets bypassed as soon as the MODE button is pressed anyway. But keep SMART SET in mind if taking off for a cold week in winter. Under SMART SET it's possible to program the heating set point as low as 50F for freeze protection on the house. Under normal heating mode the GL heads can't be set any lower than 59F.

      When married to an MXZ multi-split compressor the heads don't really modulate, but forcing the blower speed to minimum during normal cooling mode will improve latent load handling while still operating to the setpoint.

      A GL series head in DRY mode does not operate to a setpoint (as you have discovered.)

      During the heating season the outdoor dew points are much lower than during the cooling season, and running the ERV at a higher rate will do ever more dehumidification as outdoor dew points fall. Also, during the heating season running a room dehumidifier to turn the latent heat of vaporization into sensible heat inside the house a GOOD thing (unlike during the cooling season, where you then have to pump the heat back out of the house with the air conditioner.) In most homes it would be possible to keep the indoor RH under 40% by adjusting the ERV speed, and in many it would have to run at a lower speed to keep it above 30% (the minimum recommended by most health care professionals.)

  17. user-6813968 | | #28

    See picture of remote control

  18. motoguy128 | | #29

    It's simple math:

    Sensible load is minimal

    Latent load exceeds sensible load.

    Either add more sensible load, or install a whole house dehumidifier. Most times, a WHD with passive filtered ventilation will be better than a ERV because the lower humidity level allows you to raise the setpoint of the thermostat and remain comfortable, thereby reducing sensible load. Further, you can then properly size (undersize) the AC system, and save further in installation and equipment costs and the system runs even more efficiently and evenly with longer run times.

    Minisplits are frequently oversized. While they are inverter driven, ultimately the limiting factor because coil surface area. More surface area, is more area to wet before you remove condensation.

    What residential needs are small self inverter driven chillers and small air handlers with reheat chilled and hot water coils. The outdoor chillers always make hot and cold water, and you use a outdoor coil to transfer heat or extract heat. Add a small ground loop for a hybrid system.

    Then the indoor unit uses small ducts with variable dampers for a mini VAV system. A medium velocity system. Most rooms probably only need a single 4" duct going to a single slot diffuser. Easy to fit in a wall cavity.

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