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Minisplits in the humid southeast?

CMOD | Posted in Mechanicals on

I’m currently working on design for a house in the southeast, on the border between climate zones 3 and 4 (officially 4). Tight site, 700sf footprint, 3 floors over a walk out, unfinished but minimally conditioned basement.

Wall composition isn’t settled yet, though the minimum is likely 2×6 studs with blown cellulose or open cell foam and 2″ exterior xps. Basement is possibly thermomass if the numbers shake out OK. Given the small footprint and envelope, I’m hopeful that we can opt for decent walls, roof, and windows.

I’m thinking of proposing a mini split or multi split system in combination with an ERV for HVAC. Assuming a pretty tight house, I think this’ll be 2-4 indoor units (max of 1 per floor). I am struggling to find relevant data and cases where people have taken this strategy outside of the Midwest and northeastern US. While I don’t have the usual concern about cold bedrooms in winter, I am wondering about warm summer nights in our mixed-humid location – that a couple humans in a closed bedroom will generate too much heat and humidity, which the unit won’t be able to take care of with the door closed.

Daikin quaternity units look ideal due to the humidistat control, but it appears they don’t offer a concealed ducted unit that would allow short duct runs to each bedroom (bedrooms are on the upper 2 floors).

I can think of a few ways to move air into the bedrooms – fan transfer grills or possibly a whole house dehumidifier connected in. But either of those seems to invite penalties from a cost, efficiency, or noise standpoint.

Any suggestions or examples to look at? Are any of the mini split manufacturers in the process of integrating humidistat control on their ducted units?

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  1. kloopster | | #1

    My 2 cents. I build in the same zone. Things I might look at if I were you:
    Thermomass is expensive, in our area CMU with exterior foam (I use EPS) is much cheaper and more flexible.
    All of the major Splits now offer a "de humidifying" setting which equates to lower fan speeds and longer rum times.
    I would skip the ERV - there are cheaper options.
    The thing to worry about with two people in a closed bedroom is fresh air first, then comfort second, Jump ducts (flex between rooms) will help with return and balance.
    You can always put the head from the split unit in a closet and run a separate fan unit from the closet to the other rooms (supply) and have a small fan on a timer to bring in fresh air( you could also park a humidifier (plug in / energy star) in the same closet if yuo find you can't get where you need to be with the split unit.
    Good luck!

  2. GBA Editor
    Martin Holladay | | #2

    You've asked several questions, and I'm going to leave it to GBA readers in the Southeast to answer most of them.

    You may want to read this article by Carl Seville of Atlanta, Georgia: Loving My Minisplits.

    When it comes to achieving summertime comfort in a bedroom with a closed door, the main issue is whether there is a (large) west-facing window. Afternoon heat gain is your enemy. If there are no west windows, or the west windows are small, or protected by a wide porch roof, or if the windows have glazing with a very low SHGC, the bedroom will be much easier to keep comfortable than if the bedroom has a large west-facing window.

  3. kloopster | | #3

    @ Martin, you are so correct on the West Facing window sizing and here in the Southeast it is routinely ignored.

    @ Chris the orientation of glazing advice from Martin should extend to the entire house design (all floors) not just the bedroom, unless of course there is a great view out the Western side, then you could be pretty energy screwed. Better windows and overhangs can help but are costlier and can be more complex, respectfully.

  4. Expert Member
    CARL SEVILLE | | #4

    Chris - You would do fine with any manner of mini split equipment currently available. You could use ductless units - 1 per floor would be more than adequate, although you may run into some minor comfort issues with closed doors. As everyone else mentioned, glazing is a big issue - you need to avoid overheating in the summer.

    The single head per condenser units are the most efficient. Mitsubishi has one that is rated at 30.5 SEER. Mutli head and ducted units are less efficient, but not so much to avoid considering them. You could use a ducted unit where there are doors that will be closed and wall mounted heads on lower floors where (i assume) there won't be as many distinct rooms. Bathrooms and closets should be fine without any direct heating or cooling. Good luck

  5. Expert Member
    Dana Dorsett | | #5

    With ductless systems sizing it correctly for the load correctly is more important for as-used efficeincy & comfort than how well it performed on a SEER test. A single head 3/4 ton unit serving a peak cooling load of only 3500 BTU/hr will not meet it's efficiency numbers, and will not modulate except on the hottest hours of the hottest days.

    In tight higher-R houses with very low room loads, if you keep the doors closed much of the time it can be better to go with a mini-ducted system and split the output appropriately between a few adjacent or nearly adjacent rooms so that the max-output of the mini-duct cassette is at least no more than 1.5x the peak load of the rooms served. The SEER rating may only be 15, but the comfort and power use may be better than an SEER 30 ductless that's 3x oversized for the load.

    SFAIK the only vendor that includes dehumidistat control is Daikin. Most others have a "dehumidify" mode that cools at low speed while ignoring the temperature setpoint. Those do not sense the actual humidity OR temperature in dehumdify mode, and will continue to run continuously until the mode is changed back to heating or cooling mode under a temperature control.

    Many homes in the Caribbean are cooled (and sometimes heated) with mini-splits successfully- the latent cooling loads of the sticky southeastern US aren't any higher than that.

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