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Cost and sizing for a Mitsubishi City Multi ductless

albertoarriaga33 | Posted in Mechanicals on


I wanted a second opinion on the quote and sizing of a Mitsubishi City multi ductless system. The house is 1570 sq ft, in a hot humid climate. I ran a manual J using CoolCalc and came up with 30k BTU load for the house. I got quoted a 3 ton city multi system for a bit over $10k. 

For reference we have R24 open cell in the attic, R13 mineral wool block walls, tilt and turn windows, and 50 cfm of mechanical ventilation.

Does this seem like reasonable sizing, and do you think the price is fair? This is our first build so having second opinions from this community is important as most locals have no idea about these systems and dont even install insulation most of the time.

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  1. Patrick_OSullivan | | #1

    This is a new build? What climate zone are you in?

    1. albertoarriaga33 | | #2

      This is a new build yes. Our region does not have a climate zone classification but it would be mostly representative of climate zone 1 un the US.

  2. Expert Member
    Akos | | #3

    Coolcalc tends to overestimate cooling load. Unless you have some real uglies like large west or south facing windows with no shading, your cooling load should be much less than that.

    Most of your cooling load will come from roof, windows and air leaks. With new construction it is easy to address all of those which should get you closer to a more realistic cooling load.

    Either way, the Multi is a commercial unit designed for much larger spaces. You should be able to go with a standard slim ducted unit or a multi position air handler for the whole place something like:!/product/29082!/product/29060

    1. albertoarriaga33 | | #5

      The MXZ series mutli zone ductless appears to be only marginally cheaper than the city multi, but I will go into further detail with my mitsubishi dealer, thanks for the info.
      As for the load we dont have many "uglies" as you say, the tilt and turn windows have double gasket, attic is pretty well sealed with 6.5" of OCSP. Do you reckon a 3 ton system is oversized? In any case the MXZ and city multi are variable capacity units so they should be able to ramp down to meet part load conditions.


      1. Jon_R | | #6

        Make sure you review the pros and cons of a VRF system (which reportedly the City is and the MXZ isn't).

        1. albertoarriaga33 | | #7

          From reading the technical docs I understand the m series multizone is variable capacity, my dealer also confirmed it is. Am i missing something?

          1. Expert Member
            Akos | | #8

            The issue with any of the multi zone system (MXZ or PUMY) is the outdoor units have minimum modulation range which is around 9000BTU for both 3 ton units.

            This is fine provided you don't have a bunch of very small zones and if the overall cooling load is well matched to the outdoor unit.

            I doubt this would be the case for a 3 ton unit in most reasonable code min 1600 sqft new builds.

            If the system is oversized, you'll get a lot of cycling of both outdoor unit and indoor heads leading to low efficiency and humidity removal.

            Important to go through the Man J from coolcalc and find where the problems are and why your cooling load is so high. Usually it is rated to leakage rate selected (even tight tends to overestimate leaks) or windows details are not inputted properly.

            You can also hire an hvac engineer to run the numbers for you, but this can also be hit and miss sometimes.

            P.S. In the current construction madness, $10k for a 3 ton Mits multi is a great deal.

  3. Jon_R | | #4

    In a humid climate, look carefully at dehumidification performance, over all operating conditions. This is mostly about poorly documented CFM/ton, with equipment capacity vs load (sizing) playing a very minor role.

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