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Community and Q&A

How to Heat & Cool 3 Rooms via Mini Split(s)

gustave_stroes | Posted in General Questions on


We are building a house in Paso Robles, CA. Central CA Coast technically, but about 20 miles inland (Zone 3C per the map). It get’s 115 deg F often in the summer and in the 20’s in the winter, though never for long.
The East wing of the home will act as vacation rental, though on a per room basis. What the French call a “Maison D’Hôtes”
So we three essentially identical rooms side-by-side, about 300 sq ft each with 10 ft ceilings. Visitors will want to be able to control their own heating and cooling needs.
The home is well insulated. Unvented attic and crawl space. Code insulation in the walls and below roof deck. Also 1-2″ of polyiso on the outside with an air gap between the foil sided foam and the metal siding/roofing (white/gray).
My first thought was three small mini split systems with wall mounted indoor units. But for aesthetic reasons my wife prefers ceiling cassettes or concealed ducted units. I am attracted to a 3-zone system with a single outdoor unit. But I am concerned that the three indoor units may not respond in the same way as three independent systems. For example, would all three indoor fans come on when one of the rooms called for cooling? I just made that up, but it’s in my head that I have read about issues like that.
Can anyone clarify? Would a three-zone system behave identically to three independent mini split systems?
Thank you – Gustave

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  1. Expert Member
    AKOS TOTH | | #1

    Some multi splits do have better modulation range which can work in your case. I would see if you can find an installer to put one of these in:!/product/26517

    There is also a Daikin unit which is not too bad:!/product/33585

    The Midea unit can be paired with three 9k ceiling cassettes (DLFSCAH09) or slim ducted units (DLFSDAH09). The ducted unit can be mounted vertically, you can put just inside the closet beside the door against the bedroom wall for a pretty simple and clean install.

    The unit will still be oversized, which tends to be bad for moisture removal. Since you are not in an area where summer time humidity is a big issue, with a decent modulation range, it will cool just fine. In the case of only a single room running heat, efficiency won't be great but will work which is what you really need.

    1. gustave_stroes | | #3

      May I ask, are my concerns of interaction between multiple indoor units run from a single outdoor unit unjustified? Will the three indoor units act as independent units?
      Thank you.

      1. Expert Member
        PETER G ENGLE PE | | #4

        You do get some "crosstalk" between the units. When a single unit calls for heat or cooling, the refrigerant flows through all of them with only the one calling for heat energizing the fan. Some people have had issues with the natural convection across the other coils causing some unwanted heating or cooling. In your case, I suspect that this effect would be small.

        1. gustave_stroes | | #6

          Thank you Peter, that is what I was wanting to know.
          Btw, where does one find rigid due transitions to go from a concealed ducted indoor unit to a round flexible duct? I have looked around and found nothing.

          1. jameshowison | | #8

            I think the cross-talk can be pretty substantial, at least it was in our house. If the rooms have large differences in solar exposure that would seem to increase the risk.

            And bear in mind that there is no modeling of this, there's seemingly no way to know in advance. And once they are in then you are stuck with it. And if something breaks then all are down while it is fixed.

            I'd definitely go with three separate units, and then you have added redundancy and if something isn't working at least you can focus on that one room and not the rabbit hole of troubleshooting the cross-talk (which is not something that HVAC contractors are prepared to troubleshoot since there is zero logging that can help them figure it out). The modulation is better with the 1-to-1 as well.

            (I couldn't reply to the Akos reply below for some reason. But wanted to say that the imbalance and minimum size issues contributing to the cross-talk make sense. Man, I wish they would actually model this stuff so that we'd really know, though!)

          2. Expert Member
            AKOS TOTH | | #10

            I think the cross talk becomes problematic when there is a large outdoor unit with a combination of big indoor head and a bunch of small ones.

            Since the big head will tend to run at higher load, a lot more refrigerant will be pushed through the small zones causing the overheat issue. Add in there that usually the smaller heads are bellow the min modulation range of the outdoor unit and it creates the comfort problems we see.

            In the case of three similar bedrooms, I don't think this will happen. Since the overall load is still small, the unit will cycle which is not great for efficiency. With the smaller multi split, the min modulation of the outdoor unit is bellow the capacity of each indoor head so some modulation of each zone is possible. This should keep the units from overshooting setpoints.

            If this was a single family home, this is definitely not the optimal solution. In terms of rental, it will work. Overall, I think it would still be a great improvement over a PTHP setup.

          3. gustave_stroes | | #11

            Thanks James.

  2. gustave_stroes | | #2

    Thank you for that Akos.

  3. user-6623302 | | #5

    Get some of those all-in-one units like a motel.

    1. gustave_stroes | | #7

      Hi Jonathan. Are you referring to a wall unit as shown the picture, For example Friedrich? If so then that would fail the WAF (wife acceptance factor).
      If there was a combo unit that hung in the attic above the room and simply communicated with the outdoors via two ducts, then that would be pretty useful.

      1. user-6623302 | | #14

        Have you considered the additional cost for a WAF system. How many additional rental will you need to have to cover that cost. You may be pushing the economic feasibility of this project.

  4. Expert Member
    AKOS TOTH | | #9

    I've used these as adapters before, make sure to get the right size for your unit:

    In a pinch I've also used a set of straight side takeoffs mounted to a plate as well such as these:

    1. gustave_stroes | | #12

      The first link, I just stumbled on that. Those seem a little restrictive.
      I was hoping to find a smooth plenum, like a register adapter.
      I am pretty handy with sheet metal fabrication. I could probably make something somewhat efficient I guess. I am surprised that the mini split manufactures don't make these available.
      Thank you Akos.

      1. Expert Member
        AKOS TOTH | | #13

        Most mini split manufactures do have a duct adapter as an accessory, just not always available in North America.

        I was also skeptical of the sharp transitions until I used it. If you look at the cross section area and the flow rate of most of these, the face velocity is very small. At such low FPM, there is very little pressure loss from the sharp transition.

        In a system, all your pressure losses be from the air filter and the supply ducting/register. Since your runs will be very short, as long as you select reasonably sized ducting and use a larger filter, you can easily keep total pressure bellow 0.1" wg.

        The condensate pump on these is quiet but not silent. If the unit is in a bedroom, you'll hear it. Best to gravity drain. Also the pump on the Midea units stick out the side a fair bit (~12") and it is not shown on the drawings.

        1. gustave_stroes | | #15

          OK, got it.

  5. jameshowison | | #16

    Just out of interest, does anyone have a ballpark for the cost of doing this via AirZone?

    I'm guessing it's $$$ but it does look cool. How could it not be $$$, someone has to train the installer to toss the screwdriver like that!

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