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HRV/ERV with Integrated Dehumidifier

blos | Posted in Mechanicals on

I built a passivhaus a year back (0.4 ACH). It’s great and everything but the area of South Japan is very humid. The house was finished late fall so I didn’t notice the humidity problem until spring came and well, it’s quite bad.

The ERV (Stiebel) was great during the wintertime when humidity outside is in the low 40’s and it kept the house nice and comfortable around 50% humidity (I have wireless sensors in every room), but when the outside humidity rose to 70%+ 5 months ago, problems became more apparent. HVAC outlets generate condensation that drops to the floor, some rooms are uncomfortably moist if they don’t have HVAC output but have fresh air output etc…

So obviously, air should be dehumidified a bit, and, well, I don’t want to buy a dehumidifier for every room that has a fresh air outlet. They eat a lot of power like 300-500W just to run and need to be emptied periodically. Also, it seems silly to attack the problem where it manifests and not at the source, and the source is the input air to the Stiebel that then gets distributed all around the house.

So I started researching and it seems like there’s just no product on the market that would be either a) integrated dehumidifier in an ERV/HRV box or b) in-line dehumidifier for the supply air to the ERV/HRV.

There ARE in-line dehumidifiers, but using one at the supply side to the ERV seems wrought with problems. They can’t vary their output to the demand of the ERV and running one at over-or underpressure in regards to what the fans in the ERV pull sounds like a good way to damage the ERV.

Has anybody else run to this kind of problem? Or is there some products on the design board that would come to the rescue later?

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Replies

  1. William Hullsiek | | #1

    Check the Messana ATU, Build Equinox CERVANTES/AprilAire, or Minotair. They all incorporate humidity control.
    HiDew is not available in North America, but also incorporates a dehumidifier.

    1. Charlie Sullivan | | #3

      As I understand it, of these, only the Messana has an HRV or ERV core. The others can only do active heat recovery, not passive. I would not want to give up the advantage of passive heat exchange and only have the active option, so I would put that at the top of the list of those options.

  2. Expert Member
    Akos | | #2

    There are a couple of items to unpack there.

    In a climate with cooling, all rooms of the house should get some conditioned air, your ERV without active cooling will never be able to supply air that is comfortable enough when it is hot and humid outside.

    In a well sealed and insulated houses, most AC systems will still not provide enough dehumidification in the summer time. This means that you do need a dehumidifier. Your AC system should provide enough air changes in all rooms that you only need a single dehumidifier in the house, usually a whole house dehumidifer ducted to the air handler is the most efficient option.

    Once you have humidity under control, your registers should stop sweating. Also since the house air is dryer, the fresh air supplied by the ERV will also be a bit less humid and more comfortable.

  3. Bob Irving | | #4

    we install air source heat pumps in our "net zero" (not quite Passive House) homes. They have a "drying' cycle that works great for removing moisture.

    1. blos | | #7

      Yeah, I have a good AC from Daikin at the house, I was just wondering why isn't it a standard to integrate a dehumidifier into a ERV/HRV, or make an add-on product that would match the input air demand with an in-line dehumidifier.

      Dehumidifyhing when the damp air is already in the house is a bit of a fight at the wrong place, I don't have AC ducts to all rooms, there's the bathroom and powder room that we didn't deem needed AC, just ventilation to get the moist air out. Would be nice if that ventilated air would already be moisture controlled. Or at least lowered a bit.

  4. DCContrarian | | #5

    The CERV by Build Equinox seems to do what you want.

    1. blos | | #6

      Yeah the CERV is the only one that seems to do it :) But man that's a beast.

  5. Josh Durston | | #8

    I agree about ERV's needing dehumidifier (or just a built in AC unit with the evaporator on the fresh air discharge and the condenser in the exhaust). I put a renewaire ERV in this summer, and needed to turn down the volume during high dew point weather to get my indoor humidity under 60%. My house is pretty shaded so I think the sensible to latent load is skewed towards latent.
    My mini split struggled to get the humidity down into the 50% range (in cool or dry mode). It didn't have this issue last year before I had a ERV.
    I've found things are manageable if I reduce the airflow to about 35cfm (would rather be 80-100cfm). It would be nice to handle the humidity at the source, especially with the high dew point whether we've been having September/October this year.

  6. Jon R | | #9

    > have HVAC output but have fresh air output

    There is a fundamental issue that each closed off room's need for fresh air, dehumidification, heating and cooling aren't in the same ratio. So comfort and efficiency wise, any sharing of ducts creates compromises.

    Non zoned systems are also a compromise (because room-to-room ratios of any of the above aren't constant either). The ultimate hvac system controls each one independently for each room.

    > in-line dehumidifiers, but using one at the supply side to the ERV seems wrought with problems

    You can create an air handler that always circulates max(ERV flow, dehumidifier flow). Then the ERV and dehumidifier can be attached using closely spaced Ts. This provides neutral pressure balance to each device. The same design can also accommodate heating/cooling as an additional T'd device.

    1. Josh Durston | | #11

      I think during dehum shoulder seasons the temperature of the air doesn't matter so much, efficient dehumidification of the saturated outdoor air would be very valuable.

      I don't think it's really necessary to try and heat and cool to maintain room setpoints, with the ERV.
      The goal should be to achieve a relatively neutral air temp, (probably 14-27C would be fine given the low flow rate into each room), and a less than 12-13C dewpoint air.

      I don't mind throwing temperature targets out the window during dehum season. There is really no scenario where it is humid outside and I would be concerned with "over" dehumidification of the fresh air. Even with extremely dry fresh air the house humidity would probably still be in the 40's due to internal humidity sources.

      I would love to have the ERV core do the initial heavy lift, and then after that have a simple ac unit cooling the ERV FA discharge that would chill the output air to sub 12C to take core of any excessive remaining humidity. ( I wish I could buy a cheap little 6000 btu chiller and use a chilled water coil). Whether the heat is rejected to the exhaust or back into the house doesn't really matter since the load is so low, I would probably prefer to have it in the basement since it runs cool anyways.

  7. John Clark | | #10

    Answer: Because it's expensive.

  8. KurtGranroth | | #12

    Taking that a step further, I would love an "all-in" house heat pump that took of all aspects that a heat pump excels at. That is, have one incredibly variable heat pump for your entire house that:

    Heated the house
    Cooled the house
    De-humidified the house
    Ventilated the house via ERV/HRV
    Chilled the refrigerator
    Froze the freezer
    Heated the water

    Each of the heating and cooling cycles have "waste" energy on the other end and that can be used in closed loop. That is, the waste heat from air conditioning and the fridge can be used to heat the water and vice versa. Seems like it could be insanely efficient.

    There are surely technical reasons why that wouldn't work (maybe it can't be the dynamic) but it does seem wasteful to me to see so many heat pumps in my house, all doing their own isolated thing...

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