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Community and Q&A

Control Options for Whole-House Dehumidifier

user-5946022 | Posted in Green Building Techniques on

For a whole house dehumidifier such as a DV-070, that pulls from the living area into the crawl, dehumidifies it and distributes dehumidified air via its own duct system in the crawl, what are the best controls options?

1. I’ve read some here use a DEH3000. How accurate are it’s sensors? The DEH3000 seems to do alot that I don’t think I need, but I’m open to being corrected.  I have no damper in this system, don’t know why I would run the dehumidifier in fan only mode, and don’t use my dehumidifier for ventilation (separate ERV) and I don’t think the dehumid will run without it’s fan on.

2. Even though the dehumidifier is permanently installed in the crawl space, it is powered via  plug.  So theoretically I could set the dial on the unit to constantly run, and either
a. plug it into something like this
but I’m not sure that is any more reliable than the thumbturn on the unit, and it is still not visible and in the crawl.
b. plug it into a z-wave outlet, have at least one z-wave humidity sensor in the crawl and one in the living area, and program the z-wave plug to go on when either humidity sensor gets to a certain point.  That would allow me to conveniently see the humidity readings in the crawl, and have the dehumidifier run if either the living area or crawl get too high in humidity and keep a log of humidity.  I would control it local via a Hubitat or similar.  However, perhaps there are too many failure points in that system or maybe Z-wave is a bad idea….

3.  Someone posted they use a simple humidistat control.  How does that work – are they remote where the sensor and readings of the humidity in the crawl are accessible?  Are they available with a zwave or zigbee options to log and view?

4. Other solutions? (The dial control on the unit seems inaccurate and is difficult to access, and I’d like more automation than the on /off via the breaker)

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  1. Expert Member
    NICK KEENAN | | #1

    Skimming the manual for the DV-070, it looks like it's controlled by a standard 24V humidistat. You want to locate that in the living space. You can get combination thermostat/humidistat units so you just have one device on the wall. You just have to make sure the control circuits for the HVAC and dehumidifier are in phase so you don't get double voltage across the terminals. (If that sentence doesn't make sense to you, you need to get someone who understands it to help with the wiring.)

    1. user-5946022 | | #2

      Thanks DCContrarian - my HVAC is a totally separate Mitsu mini split system, so not sure it would be a good idea (or even possible) to combine the controls.

      I'm trying to understand why I want to control the dehumidifier based on readings from the living space. I thought controlling humidity in the crawlspace was the most important, and if so, shouldn't the sensor be there?

  2. Expert Member
    NICK KEENAN | | #3

    Two ways to control humidity in a crawl space are to put a self-contained dehumidifier in the crawl space, or to have air exchange with the rest of the conditioned space of the house and use a whole-house dehumidifier. What I thought you were trying to do was the second.

    The air exchange method can be more efficient, the crawlspace contributes to cooling the house in the summer and you don't have the dehumidifier fighting the AC if the AC can provide sufficient dehumidification. You can either have a fan run continuously to provide air exchange, or have a humidistat in the crawl space that triggers the fan. If the AC can provide sufficient dehumidification the dehumidifier doesn't have to run at all, which is why you want the sensor in the living space.

    1. user-5946022 | | #6

      Thank you. This is really helpful. Maybe I should first be asking if my system is properly set up, because I seem to have a hybrid of what you describe.
      My DV-070 pulls air from the living space, via a wall grill & filter, through a duct, to the dehumidifier, which then distributes the air in the sealed crawl via a small ducted system in the crawl connected ONLY to the dehumidifier. So it is a self contained dehumidifier that pulls air from the living space. I have often wondered about this setup.

      The dehumid is effectively pressurizing the crawl with dehumidified air, because there is no dedicated return from the crawl back to the main living space. So the air in the crawl is either escaping to the exterior, or back to the main living space via cracks and crevices.
      a. This setup seems to be pulling presumably already dehumidified air from the main living space (dehumidified via the minisplit), running it through the dehumidifier to make it even drier, and then putting super dry air in the crawl to accelerate it's dehumidification. Is this analysis correct, and if so, is that the right way to do it? I fear that if there are more leaks from the crawl to the exterior than from the crawl to the living space, this is effectively depressurizing the living area...
      b. Are the Mitsu minisplits more efficient at dehumidification or is it the DV-070?
      c. Can the DV-070 be run in fan only mode with no dehumidification, and if so, would this be a more efficient way to dehumidify the crawl?

      1. Expert Member
        NICK KEENAN | | #7

        A lot depends on your climate. Pressurizing the crawl space and de-pressurizing the living space increases infiltration of outside air. If the outside air is more humid than your interior air you are bringing in moisture that needs to be removed. If the outside air is drier, most likely you are in heating season and that outside air needs to be heated. In either case the infiltration is uncontrolled, and when you have a separate ventilation system infiltration works at cross-purposes to it. Having balanced supply and return vents is the way to avoid pressurization.

        To the question of whether mini-splits or a dedicated dehumidifier is more efficient, it depends upon what you need. An AC removes heat and humidity and dumps the removed heat outside the house. A dehumidifier removes humidity, which removes heat, and dumps the removed heat inside the house. For each pint of humidity removed the dehumidifier dumps about a thousand BTU into the house. The dehumidifier typically uses about 400 BTU of electricity to remove that humidity, and the heat from that electricity is also dumped into the house. So a dehumidifier is a heater, and a rather efficient one. If you need heating and dehumidification -- in a cool, damp climate -- a dehumidifier is more efficient. If you need cooling and dehumidification together, an AC is more efficient. Where it gets a little tricky is if you need cooling, but you need more dehumidification than the AC alone can provide. In that case you might need to run both AC and a dehumidifier together to hit both your temperature and your humidity goal.

        Crawl spaces don't typically have temperature goals, so it's not uncommon in humid climates to just run a dehumidifier in the crawl space and not tie it to the rest of rest of the house. Conversely, in less humid climates you might be able to meet your humidity goal by conditioning the crawlspace and letting its air mix with the rest of the house.

        1. user-5946022 | | #8

          Thanks. I'm in a climate that is warm and humid for much of the year.
          House is reasonably tight; probably equally sealed between living and crawl, and crawl and exterior.
          Maybe I should consider a transfer duct between the crawl and living space...

          1. Expert Member
            NICK KEENAN | | #9

            Transfer ducts generally disappoint.

            If you want to transfer either heat or humidity using air, the bigger the delta between the air you're moving and the goal you're trying to hit, the less air you have to move. If the air you're moving is already near the target point you have to move an enormous quantify of it. If you're trying to heat a room to 72F it's much easier using air at 120F than at 72F. Similarly if you're trying to get the dewpoint to 55F it's much easier with air with a dewpoint of 35F than 55F.

            I think you're best off treating your crawlspace as a separate zone, it's going to have a different heating and cooling profile than the rest of the house. Have the dehumidifier just recirculate down there and see how it goes. Unless it gets really hot I think that's your best solution.

  3. walta100 | | #4

    I have found this type of low cost industrial controls coming out of China very handy and for $20 shipped to your door very cheap.

    This one lets you control temp and humidity independently

    1. Expert Member
      NICK KEENAN | | #5

      I could see that coming in handy in a lot of places. Too bad it's not UL-listed, that would probably make it ten times as expensive.

  4. user-5946022 | | #10

    Bumping this to bring back the discussion on the controls part of this dehumidifier.

    The purpose of the dehumidifier is to control the humidity in the sealed crawl space.
    The dehumidifier is not connected to the HVAC; it draws air from the living space as it's "return" pulls it through the dehumidifier, and distributes air in the crawl space via a ducted system of the dehumidified air only in the crawl.
    The contractor installed it with it's only control being the dial on the unit which is sort of unreliable, so the only real control is the breaker.

    Is there any reason I should not control the dehumidifier this via Z Wave or Zigbee controls as follows:
    a. Plug Dehumid into a controlled Zwave/Zigbee outlet rated for the Dehumidif wattage
    b. Install 2 or 3 Zwave/Zigbee humidity sensors throughout the crawl.
    c. Program the hub to turn on the dehumid when any sensor reaches 51%, and turn off dehumid when all sensors reach 48% or below.
    d. Program the hub to alert me if any sensor has a reading absolute value of greater than 2% from any other sensor.

    Is there any concern with controlling this by an outlet that cuts off power when the condition is met? Is that any different than controlling the power to the unit via it's internal low voltage control wiring?

    Is my range of on at 51% and off once it reaches 48% correct? It takes between 20 minutes and an hour to drop 1%. I'm trying to avoid the thing cycling.

  5. Jon_R | | #11

    A couple of points about a dehumidifier with supply and return coming from the crawlspace (the better way to do it):

    a) it works when the main house AC/heat is off. In most climates, no AC/heat happens for long periods, more than enough time to grow mold.

    b) with a well air and vapor sealed crawlspace, the operating cost is so low that efficiency isn't a concern.

    I've controlled a dehumidifier with the controller shown in #4. Seemed to work OK.

    1. user-5946022 | | #12

      My dehumidifier does not have the supply and return coming from the crawl. The supply is ducted throughout the crawl and serves the crawl. The return is pulled from the conditioned air in the living space.
      The humidity in my crawl often increases well above the living space during times the living space is being conditioned. These are the times the dehumidifier needs to run.
      The home is well air sealed with a sealed crawl, but clearly the vapor sealing could have been better or else the crawl would not get the higher humidity.
      It seems to cost about $1.50/day to run the DV-70 dehumidifier if I let it run via the dial; I wish it were more efficient. It is enough that I want to only run it when the crawl humidity raises to enough to need to run it.
      Also, the dial seems to vary - I can get it to go on when humidity is 52%, but then the next time it goes on at 46%. I don't trust it.

  6. walta100 | | #13

    In my mind your plan does not make sense you are taking the cleanest driest air in your home running it thru a dehumidifier and discharging it into the dirtiest wettest spot in the crawlspace forcing the dirty wet air to find a ways to get into the house bring the smell of the crawlspace into your home.

    I would think feeding the dehumidifier the wettest possible air from the crawlspace and return that air to the crawlspace drying and warming the crawlspace would work better.


    1. user-5946022 | | #14

      Thanks for the reply.
      Unfortunately the setup described is not a plan, it is what is installed, but the contractor installed no controls, so I now control it only from the breaker, and I am looking for a better controls solution and input on that.
      In regards to your concerns, I would not describe the crawlspace as having "dirty" air, as it is sealed, or "wet" air. It is just a bit more humid than the house - usually 3-5% points. In my situation the crawlspace air does not smell (or at least not yet). I understand your point, but that is not how it is set up. Perhaps I'll consider looking into revising this at some point, but that point is not now, and either way I still need to be able to control the thing, so back to the controls question.

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