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Dan1957 | Posted in General Questions on

My basement/family room. Not finished but being used. Would a mini split set on dehumidifier setting work better than a stand alone dehumidifier? Basement is 850 sq.ft. with 7 ft ceilings separated into laundry,mechanical, full bath,kitchen/family room. Eventually intend to stud and insulate exterior walls. Currently using basement while remodeling upstairs. So stand alone dehumidifier, 2 stand alone dehumidifiers, mini split on dehumidifier setting?  Also a thought came to mind would an exhaust fan help remove humidity say one in bathroom and one in kitchen area both running all the time?  I know in perfect world insulate exterior walls put in the mini split all would be fine but at present time with upstairs gutted we need a place to live.  Any suggestions would be great. We’ve been basically living down in. Basement all winter no problems not rainy season upon us has me thinking. Located in north western Pennsylvania. 

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  1. Expert Member
    Dana Dorsett | | #1

    Most mini-splits do not operate to a temperature setpoint when in dehumidfy/dry mode, and will severely overcool the place with a "set and forget" approach in that mode.

    Even a 3/4 ton is overkill for the cooling and heating loads of an 850' basement, but run the load calculations to verify that. A 3/4 tonner that still operates efficiently at a lower modulated output like the LG LAU/LAN 090HYV1 or Mitsubishi MSZ/MUZ FH09NA would be more efficient and more comfortable than something with a minimum rate of 2500 BTU/hr or more.!/product/25817!/product/25894

    (Note that per the NEEP datasets, while the LG has high efficiency at low output in heating mode, it's pretty lousy at it's lowest cooling output level, whereas the Mitsubishi is still cooling efficiently at it's lowest output.)

  2. Expert Member
    BILL WICHERS | | #2

    It sounds like you’re trying to reduce the humidity in the basement while you’re living down there waiting for the rest of your house to be renovated. That sounds like a temporary situation. I’d use a dehumidifier, not a mini split. All a dehumidifier is is an air conditioner with both coils (evaporator and condenser) in one box, so the heat “extracted” by the air conditioner is returned to the space instead of blown outside. The goal is just to make a cold coil to condense water out of the air. A dehumidifier will both dehumidify the space as well as heat the space slightly (the losses of the machine). This is probably what you want since you want dehumidification but not cooling of your basement.

    If you run a dehumidifier, make sure you have one that can drain down a tube and doesn’t need to have a tank get emptied periodically.

    If you’re looking to put something more permanent in your basement So that you can continue to use the basement as living space after your renovation has been completed, you may want a permanently installed dehumidifier, but I’d try to find out where all the humidity is coming from and see if you can seal it off so that it doesn’t enter the space in the first place.


    1. Expert Member
      Dana Dorsett | | #3

      A mini-split alone won't dehumidify much at the low sensible cooling loads of a basement. But dehumidifiers-only solution would heat up the basement quite a bit in summer in the humid "-A" climate zones, converting the latent load to a sensible load. Since the long term plan is to turn it into fully conditioned living space using a mini-split for cooling & heating prior to the full build-out isn't insane. It's probably worth insulating & studwalling the section of wall where the mini-split head is mounted to avoid having to move it later.

      Exhaust fans would only dehumidify to healthy indoor humidity levels when the outdoor dew points are below ~55F (the dew point of 70F/60% relative humidity air.) For much of the summer in the "-A" climate zones the outdoor dew points will be higher than that. While it's important to use exhaust fans to remove high humidity & other pollutants from kitchens & bathrooms while in use, it would only be effective as a dehumidifying strategy in climates with drier summertime air.

      For the record, what DOE climate zone is this house located in?

  3. Expert Member
    Peter Yost | | #4

    As a but of a check-in, I am attaching a chart of sensible and latent loads for representative locations in the US. Pretty sobering, especially for low load homes where the need to dehumidify without heating or cooling (shoulder seasons) is even more significant.


    1. Expert Member
      Dana Dorsett | | #5

      >" Pretty sobering, especially for low load homes where the need to dehumidify without heating or cooling (shoulder seasons) is even more significant."

      The Daikin Quaternity can dehumidify during low cooling load periods without overcooling when in "Quaternity Drying "mode, where it's using half the coil in the indoor head for re-heat, the other for cooling/drying. How well that works in practice depends on the actual loads, but it's a heluva lot better than the typical DRY or DEHUMIDIFY modes of most mini-splits, since those modes don't cool to a temperature setpoint, whereas the Quaternity Drying still operates to a temperature setpoint while attempting to dry to the independently settable humidity setpoint.

      See the description beginning on pages 14 & 15 (p23 &24 in PDF) of the manual:

      If this house is in a climate where it doesn't dip too far below the -4F/-20C minimum operating temperature very often that might make a 3/4 ton Quaternity the preferable unit for the basement.
      That would be despite it's relatively high minimum modulated output (which means it would cycle rather than modulate most of the time on low basement heating loads, even during the winter):!/product/25236

      The COP 6+ at +47F at minimum modulation is still a very high , so even if the cycling knocks it back 20% on efficiency it's still pretty good even while cycling.

      1. Expert Member
        BILL WICHERS | | #7

        Dana, how much of an efficiency hit (or increase in energy consumption) does the unit take when running in re-heat mode? At work, all our data room air conditioners use electric reheat (15kw each), and we try to avoid running that due to the increase in operating costs for the heater, as well as the reduction in cooling capacity since it takes about 3 tons of sensible cooling just to cancel out the heat load of the reheat part of the unit.


  4. Jon_R | | #6

    +1 on getting a standalone dehumidifier. Most climates need one for good comfort.

  5. tommay | | #8

    You are living in a basement, cooking, sleeping, showering, etc. of course there is moisture building up. Open some windows if you got them. Or an opening to the upstairs. Use a fan. Good information to know if you plan on renting the basement in the future.

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