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Ductless HVAC and humidity control?

Thomas Roberts | Posted in General Questions on

We live in Pensacola, Florida and area with extremely high humidity. We are building a new house which is about 2800 sq ft. House consists of a main floor at about 1800 sq ft, second floor at about 800 sq ft and a loft at about 200 sq ft. The current plan is to go with an all ductless Mitsubishi HVAC system, well actually 2 systems. 1 controlling the first floor and a 2nd for the 2nd floor and loft. We have a very competent local installer and I have zero doubts about their capabilities and the pricing I feel is fair.

The real concern that I have not addressed with them yet since they are off for a few days is humidity control. We have incredibly high humidity here. I did some reading on whole house dehumidifiers and they seem to be geared towards ducted HVAC systems. That begs the question what is available for dehumidifiers (whole house) for ductless HVAC? Are these systems something that we must install during construction OR can we do the install say a year later? I want to keep the RH at around 50% inside the house year round. The ductless apparently have a mode where they’ll dehumidify to a point but I am just a bit leary of these claims. Perhaps they do but I am here to learn what I can and try to make an informed choice. I have reached out to several references from the HVAC company who have either all ductless homes or have ductless systems on a second floor of the house and they’re very very happy with the company and systems 3 plus years later.

Thanks in advance for any and all thoughts to our questions.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Thomas. Your new house shouldn't need a whole house dehumidifier if your builder is using proper air sealing, ventilation, and insulation techniques.

    For more on why ventilation is important, see https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system for more information on why this is important.

    If your building team hasn't talked to you about air sealing, insulating, or ventilating your new home, that raises some flags.

    Just out of curiosity, how was the cooling strategy selected? Did the contractor suggest the mini splits after reviewing your plan and running a Manual J? Or did you suggest mini splits as your preferred system and the contractor simply worked up a quote?

  2. Thomas Roberts | | #2

    Thanks Steve, we have discussed insulation to a point but still getting bids on the project. As for a choice of systems, a friend designs HVAC systems commercially and when he looked at the actual floor plans, he noted that due to the architectural design of the house that this was a perfect candidate for ductless at least on the 2nd floor and loft with no doubts. Having reviewed the information on a fair amount of sites, I'd have to agree that it makes sense. I will have a read at that link.

  3. Jon R | | #3

    When you have humid but mild weather, you will need a dehumidifier. If your home is fine with ductless cooling, then it will also be fine with a ductless/standalone dehumidifier.

  4. Steve Knapp CZ 3A Georgia | | #4

    Thomas. It's fortunate that you found this site while still in the planning stage. It offers a wealth of information for building an energy efficient and comfortable home. The "pretty good house" posts are a good place to start: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house

  5. Thomas Roberts | | #5

    Steve, thanks very much for the link. I will read it here tomorrow as time permits while at the office. We plan to start to build in Jan or Feb so timing is indeed decent. Glad I found the site which was actually found while reading on ductless and Matt Risinger who I have been in touch with very briefly via email.

  6. GBA Editor
    Martin Holladay | | #6

    Thomas,
    A stand-alone dehumidifier (connected to a drain so you don't have to empty a bucket) may be useful in your climate. It shouldn't cost more than $250 (plus installation). Put it in a location where it has access to the indoor air in your main living area but where the noise won't bother you.

    For more information, see All About Dehumidifiers.

  7. Thomas Roberts | | #7

    Thank you Martin, it is a very viable option. Much appreciated !

  8. Expert Member
    Dana Dorsett | | #8

    With the Daikin Quaternity series ductless mini-splits you can set both temperature and humidity setpoints. Unlike most ductless systems, the Quaternity is capable of providing the dehumidification either with or without sensible cooling. For most floor plans just one Quaternity would be enough to handle the latent loads, the others could be Mitsubishi.

    If you set up the Mitsubishi's to run in "DRY" mode it shifts the balance of latent/sensible cooling for maximum latent cooling, but the sensible cooling load determines whether it runs or not. Unless there's a sensible cooling load it won't dehumidify. For most homes it's still pretty good, if sized correctly for the load, not so much if it's oversized, with a limited ability to modulate down to cover very low loads at a reasonably high duty cycle.

  9. Thomas Roberts | | #9

    Thanks Dana. There are no Daikin dealers even remotely close by, I checked that or I would have also gotten quotes on their equipment. It's either Fujitsu or Mitsu.

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