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Sizing a Mitsubishi mini-split heat pump for heating and cooling

ESS13 | Posted in General Questions on

I am planning on installing a Mistubishi ductless mini-split heat pump for heating and cooling.  I need about 27K-30K BTUs for heating.  I’m trying to figure out if the Mitsubishi 18K BTU Hyper Heat system will work for me.  According to the specification document is says that it has a maximum capacity of 30K BTUs at 47 degrees and 27K BTUs at 17 degrees.  But the rated capacity is only 19K BTUs at 47 degrees and 12.8K BTUs at 17 degrees.  Should I use the maximum capacity or the rated capacity to determine whether the unit has enough capacity for my needs.  

Really appreciate any advice or guidance.


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  1. CramerSilkworth | | #1

    From what I understand, the Rated numbers are the performance of the system with the compressor locked at 60 Hz (and therefore unable to modulate above 60 Hz and get higher output) to comply with the testing standard (make it "fair" to compare to non-inverter models I think....but that seems almost counter-intuitive). So Rated numbers aren't actually worth much. Dig into the manufacturer's data tables for that specific unit and find its capacity at *your* local design temperature - that's the number you want to know and compare to your calculated load.

    You'll find the detailed data here: (if you haven't already.)

  2. Expert Member
    Dana Dorsett | | #2

    Cramer has it mostly-right. The "rated" numbers are the output levels at which it was tested for efficiency at AHRI specified temperatures and inverter speed. Mitsubishis units usually have way more max capacity at those temps than the "rated" numbers.

    But even in temperate climates no 18K Mitsubishi isn't going to deliver more than about 22K of heating. You're probably looking at 2 or 2.5 tonner (24K- 30K), assuming a multizone configuration. eg:!/product/59079/7/25000///0

    How did you estimate the heat load? And what is your 99% outside design temperature?

    It needs to be sized for it's max capacity at your local 99% outside design temp. Do NOT oversize it "just to be sure", which will lead to lower efficiency AND lower comfort, especially with multi-zone units (which as a rule have a much narrower modulation range than single-zone units).

    To hit it at least reasonably close, use the (free) Better Built NW load calculation tool, which was designed specifically for sizing heat pumps (to get around rampant rules-of-thumb oversizing problems used by many/most contractors.) :

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