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Minisplit Indoor Heads/Outdoor Unit Settings

tech1234 | Posted in Mechanicals on

On the Mitsubishi unit below, if both 6k indoor units are turned off and only the 12k head is powered on (heat, setpoint 66*f) what would the outdoor unit be doing? (btu’s, watts) Is this unit capable of modulating low enough in this setup? Would it be better to have all heads on and set to low? (assuming current outdoor temps do not need the 2- 6k units on for comfort)


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  1. CramerSilkworth | | #1

    The outdoor unit has a lower capacity of 7,200 Btu/h. So there's a good chance with just the 12 head on it'd be cycling a fair bit unless you're near peak load conditions.

    Putting the other heads on low (fan speed), in theory they would be heating only as needed to maintain temperature, but there's this thing called refrigerant bleed-over where even when a head does not need heating/cooling it's still getting a bit of refrigerant, and this can overheat/cool a space. You can change this with the "thermal off" setting in the function settings of the unit, but then the space temp has to be sensed at a thermostat or wall sensor (not the handheld remotes).

    And double check your load calcs - sizing as tightly as possible (better yet, a tiny bit under) minimizes the problems above.

    1. tech1234 | | #2

      Cramer, thank you for the info. I have read about the refrigerant bleed over although this is the first I've heard about "thermal off" setting while using thermostat upgrade. Good info for the future.

      To clarify: If hypothetically the space was calling for 12k btu and (in a fictional world) the heat would spread evenly with just one 12k head, would I be better off with the 2 6k heads turned off or on with fan speeds set to low? or is it really the same thing, as in the loads would just split across the units and the 2 scenarios effectively use the same energy?

      1. CramerSilkworth | | #3

        It really depends on the zoning and how much the spaces communicate with open air pathways. I suspect the efficiency would be a little better with all heads on and handling partial load than putting it on all one, but I don't know if you'd really notice it in the electric bill.

        1. tech1234 | | #4

          Cramer, I really appreciate the response.

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