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Need to maintain between 35-40% humidity

1869farmhouse | Posted in General Questions on

My home has been designed around energy efficiency, but I’m willing to sacrifice that to maintain between 35-40% humidity for my wife’s health condition/allergies.  Note: I do not have central air/heat.  Mini splits only.

But I’d still like to do that as little as possible.  I’m having a hard type finding information on how to tie together either and ERV and a dehumidifier or supply only ventilation and a dehumidifier.

Slight positive pressure would be ideal.  I know this can be dangerous, but I can’t imagine forcing warm air into wall cavities would be an issue if the air is dry?

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  1. Expert Member
    NICK KEENAN | | #1

    During the heating season, the outside air is drier than the inside and increased ventilation brings down the indoor humidity. Positive pressure will increase air infiltration.

    During the cooling season you want to keep the outside air out because it is wetter than the indoor air. The biggest source of humidity is typically infiltration, followed by occupant activity. The first thing I would look at is whether the minisplits have a "dry" setting that prioritizes humidity removal over air cooling.

    The other thing I would look at is sizing. A cooling system that is oversized removes less humidity because it has shorter run times. You may find that if you lower the air flow or even turn off one head it does more cooling.

    I only use dehumidifiers when the space isn't conditioned.

    1. 1869farmhouse | | #3

      I don’t understand how positive pressure will increase infiltration during heating season? Yes I’d be pushing a small amount of air out of the house, but how is that any different than allowing cold air in?

      I do run my mini splits in “dry” mode through part of the shoulder season. But there comes a point where even dry mode will make it uncomfortably cold. From what I understand dry mode cranks the compressor and lowers the fan?

      The system is slightly oversized for cooling because it needed to be for heat, but I turn off some of the air handlers.

      1. Expert Member
        NICK KEENAN | | #8

        If there is positive pressure someplace there has to be negative pressure elsewhere. The positive pressure is pushing air out of the house and the negative pressure sucks outside air in.

  2. joshdurston | | #2

    What kind of mini splits do you have?
    If you have Mitsubishi equipment, I believe the kumo cloud device has some auxiliary outputs that can be used for humidity control when paired with the wireless humidity sensors.

    Below 40%rh is a tough in the summer, you'll likely have to minimize ventilation rates, and use dedicated de-humidification equipment, and possible keep a higher space temp 75F or higher. (since humidity is relative).

    The commercial spaces I work in, that have low humidity requirements usually have some sort desiccant based de-humidification system.

    1. 1869farmhouse | | #4

      I do have Mitsubishi equipment, I’ll look into that. I have Lunos HRV, but I’ve been considering an ERV with dedicated ductwork. Just unsure how to connect the ERV and the dehumidifier without an air handler for central air. Maybe I need an air handler without a/c? Is there such a thing? There’s weirdly little information on this area.

  3. Jon_R | | #5


    better air sealing
    ERV (reduces latent load)
    LGR dehumidifier (works to lower %RHs)
    Daikin Quaternity (which doesn't suffer from the "too cold" issue)

    > forcing warm air into wall cavities would be an issue if the air is dry?

    Depends on how good the air sealing is and how cold it is outside. At some temperature, 40%, 70F air becomes wet.

    > air handler without a/c

    You can use a duct fan to create a primary loop with the ERV and dehumidifier connected as secondaries. This maintains balance and correct flow rates. Hopefully the same distribution ratios for fresh air and dry air meets your needs.

  4. walta100 | | #6

    My first question is where on earth is this home?

    How tight is this house? Has it been blower door tested?

    Where did the 40% requirement come from? This is a very ambitious goal. Most museums peak about 55%.

    Where I live you would be running dehumidifiers 9 or 10 months of the year.

    It seems to me going positive or negative on pressure if only increase the amount of air exchange with the outdoors. I am skeptical that an ERV is going to supply air anywhere close to 40% humidity when taking in 90% outdoor air.


    1. 1869farmhouse | | #9

      I’m in a rural location not terribly far from Kansas City. I don’t have any issues running the dehumidifier a lot and definitely wasn’t banking on the ERV to dehumidify on it’s own at all.

      The numbers are just estimates. So far it’s seemed that as long as I keep the house below 40%, my wife has fewer symptoms. So far this has been doable by running my air handlers in “dry” mode and using big-box standalone dehumidifiers. But the cooler weather is making that harder and the humidity is as high as ever, so my solution is not sustainable (or anywhere close to energy efficient. But 20 bucks a month to run a “real” dehumidifier would be a small price to pay for what we’re trying to accomplish.

  5. Expert Member
    Peter Engle | | #7

    An ERV won't lower the humidity. It will reduce the effect that outdoor humidity levels have on interior air. If the interior air is drier than outside, then the ERV supply air will also be drier than outside, but not quite as dry as the original indoor air. You will still need dedicated dehumidification. How much dehumidification depends on the occupancy of your house and the behavior of the occupants, infiltration and whether or not the infiltrating air is conditioned, and "other" indoor sources (plants, wet basements, etc.). I would look at tweaking your minisplits as much as possible, or even replacing one or two with Quaternity units. Then see how it's going. If you need additional dehumidification, especially if only for the shoulder seasons, consider a free-standing portable dehumidifier or two. Trying to install a duct system for ERV/dehumidification in an existing house is an invasive and expensive proposition.

  6. _jt | | #10

    If you have more than one mini-split try running one to heat and one to cool. That works pretty well for de-humidification for me especially when COP is high due to moderate temperatures.

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