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With a multi-zone mini-split, does the indoor unit or the outdoor unit decide when to turn on?

dsmcn | Posted in Mechanicals on

Background: new home to be built in Zone 4A. Living area about 2200 sq.ft. on main level. On lower level is a 778 sq.ft. shop space, and 450 sq.ft. for future use. My energy rater estimates the cooling load at 18,000 BTUs for everything.

I’m thinking mini-splits. Mitsubishi offers several outdoor heat pump units that can have 2, 3, or even 4 zones, matched with any mix of the interior units (with an appropriate combined total of course). I’d like at least three zones to meet both distribution and the diverse needs of the spaces, but there’s a big question.

The low end of the listed capacity range for the outdoor units is 12,600 BTUs (the high end varies with the model). The interior units’ minimum ranges are 2,800 and 3,800. Does this mean that if I have two or three units operating, with a combined call for 10,000 BTUs, then they will not be able to run because they need to get to a 12,600 BTU demand for the outside unit to turn on?

If the answer is yes, then once running, can it modulate itself down lower than 12,600?

Similarly, all of these offer a “dry mode.” I assume that this mode offers some dehumidification when cooling is not called for. Would the units need to meet a threshold demand for the outdoor unit to turn on in this mode also?

This is of course a huge decision: thanks for any help available!

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  1. jinmtvt | | #1

    Have you considere using 3 seperate units instead of a larger with multiple heads ?
    Usually there are almost no price difference and the efficiency of the smaller single systems are much higher than the multiple heads one.

    Will probably also need less load to start on each seperate outdoor unit.

    1. XenOILphober | | #6

      This is exactly what I did and here were the disadvantages of a multi-cassette to single condenser:
      1. if the condenser fails you lose your whole HVAC system.
      2. if you have a generator you may want to consider the impact load of not being able to stage the most important room in your home. (same comment you made)
      3. one condenser to one cassette gives you a backup option (in my case I have 5 single mini splits all with 25+ SEER ratings)
      4. If parts are back ordered or the tech can't get out, you could go days without any HVAC.
      5. Easy upgrade path as technology improves, going with the highest use rooms, imagine in 4 years when a new company has a 60 SEER system. (Tesla, etc)
      6. Current Tax incentives are limited to $15K per family, see here:

  2. GBA Editor
    Martin Holladay | | #2

    I suggest that you call up the Mitsubishi technical help hotline and ask them your questions. The number is 877-391-5550.

  3. dsmcn | | #3

    Thanks all for your timely responses.

    Martin, the number has recently changed to 800 433–4822.

    Yours was a great suggestion: they were very helpful. I learned that indeed calling for less than the minimum on the outdoor unit would result in an error message. Also, "Dry Mode" is not true dehumidification, as there is no measurement of humidity—the effect is minimal and it must be monitored by the homeowner.

    Jin, separate outdoor units in a 1:1 arrangement would be about 20% more expensive. Even worse, it would require an additional 45 sq.ft. (including ventilation room). Given the amount of demand, the difference between 16 and 21 SEER would be relatively minor.

    However, since I'll need less than 12,600 BTUs through most of the day through most of the year, this will in fact be my only option—unless a centralized ducted system is more cost-effective.

  4. gusfhb | | #4

    I don't think you are going to get an error message. In my experience with single unit Mitsu mini splits, they will first ignore the thermostat for a while, driving the room a bit cooler than asked for, then they will eventually shut off the compressor. looks like the prices have dropped on the multi units, they seem like a decent deal now if you don't need low temperature heating

  5. Expert Member
    Dana Dorsett | | #5

    Be careful about the "any mix of indoor units", since not all heads are compatible with all outdoor units in the (perhaps too extensive) Mitsubishi lineup. Make that " a select mix of indoor units", and check in with Mitsubishi on it before installing them- HVAC contractors have been known to screw that up pretty regularly.

    The Mitsubishi dehumidify mode IS "true dehumidification", but it is not controlled by a humidity setpoint, and will also NOT track neither the sensible cooling setpoint or the humidity levels when in dehumidify mode. SFAIK the only mini-splits with a true dehumidistat control (with independently settable temp & humidity setpoints, in both heating & cooling mode) is the Daikin Quaternity series. That is a VERY nice feature to have in high-R homes with low sensible cooling loads, particularly in areas with high summertime outdoor dew points like zone 4A , where you can't really dehumidify by running the HRV under dehumidistat control during the summer. But so far that feature is only available on single-head Quaternity mini-splits, not multi-splits.

  6. greenright | | #7

    Indoor units decide when to turn on based on in- body air sensing (they run the fan) or wired wall thermostat. A multi-split will turn down to roughly 2.5 of total capacity. A fully vrf will go to 3 : 1 or lower depending on brand and model.

    Calling for less heat than the outdoor can turn down to will not result in error- just the extra heat will bleed off another inactive head (usually the biggest one).

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