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Oversizing a multi-zone inverter heat pump – does it matter?

rk2 | Posted in General Questions on

I’m trying to figure out a strategy to heat a large one level house with many rooms, and it seems that a multi-zone mini split could give me the overall capacity I need with the greatest comfort. But I’ve also seen many warnings about oversizing and its ultimate effect on COP, which makes me a bit nervous.

Looking at the heat pump specs, though, I’m confused as to why there would be a problem. My heating contractors aren’t necessarily heat pump engineers, so I’d like to run some thoughts by the contributors to see where I might be off base.

Looking at the lineup of Mitsubishi H2i unit submittal sheets, it seems that they all have about the same low end of the capacity range – roughly 7KBtu/h. So whether I pick a 48 or a 20, the unit can modulate down to the same level. I see no downside in sizing for max capacity, or am I missing something?

The indoor unit capacities are a bit more mysterious to me. If I have a 12K indoor unit, can it operate down to the 7K minimum from the outdoor unit? Or is the unit always at its max capacity for a given return air temp? And if two units call for heat, can the 7K be “split” between them?


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  1. rk2 | | #1

    Thanks, Jon. So does the same go for the indoor units?

  2. GBA Editor
    Martin Holladay | | #2

    As far as I know, the control algorithms used by manufacturers of minisplits are proprietary and often opaque. Dana Dorsett may provide a more detailed answer.

    Briefly, oversizing is never good. If you've done a good load calculation, use that calculation when sizing your unit. Don't oversize.

    Here are some links to further discussions of these issues:

    Are Seven Heads Better Than Three?

    Tweaking Plans for a Minisplit System

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

    What is minisplit short cycling? Turndown?

    Minisplit efficiency - multiple single zone vs single multi-zone

    How does efficiency in a mini-split vary over various operating levels?

  3. Jon_R | | #3

    Correct, short run-time/oversizing is about minimum capacity - not maximum.

    I'll speculate that the larger model would have a larger heat exchanger, making it more efficient. And a larger compressor - increasing compressor life for a give load. On the other hand, a larger compressor operating at the same 7KBtu/h may reduce pumping efficiency. For a similar price and minimum output, I'd get the larger outdoor unit.

  4. Expert Member
    Dana Dorsett | | #4

    As general rules:

    Oversizing cassettes & heads is bad, oversizing the compressor is worse.

    The coils in an oversized system are bigger, and potentially higher efficiency if running continuously at low speed. But oversized coils also have marginally higher refrigerant volumes and oversized blowers, adding the start-up ramp energy use, making it LESS efficient when cycling on/off, even at the same minimum-modulation levels as a more appropriately sized unit.

  5. Jon_R | | #5

    Larger coils have been an obvious trend in producing more efficient outdoor units - despite their higher volume.

    See below for "Oversizing VC heat pumps substantially decreases cooling and heating season energy consumption and peak demand. "

  6. Jon_R | | #6

    I don't know, but my guess is that the indoor units can modulate down, but how far varies with the model. Splitting between units sounds logical.

    Over-sizing max capacity of an indoor unit (with the same rated minimum output) should increase sensible efficiency but may slightly reduce latent cooling efficiency.

  7. rk2 | | #7

    Thanks for the answers, gentlemen. From all the reading is that the biggest contributor is still the minimum capacity floor, and that otherwise it's in the balance between startup energy use when cycling and backup heat use on too-cold days. Could be interesting to model that, but it seems it might be relevant only to edge cases.

    I just noticed that in the case of the Mitsubishi MXZ series, there may be a simpler answer: the 36, 42, and 48KBtu/h units have the same compressor and refrigerant charge. So with my heat loss calc over 3 tons, I might as well upsize and lower my need for backup stage heat. Does make me wonder what justifies the cost difference on the units.

  8. rk2 | | #8

    * My takeaway from all the reading, I meant to say.

  9. rk2 | | #9

    One other related question: I was looking at page 29 of which describes the MXZ capacity diagrams.

    Unless I'm reading the graphs wrong, the COP of the unit *drops* as it operates below max capacity. At full power, it has 4420W producing 54KBtu/h for a COP of 3.75. At lowest it runs 0.12 capacity ratio but 0.23 power ratio for a COP of somewhere around 2.

    Does that seem right? Or am I misunderstanding the specs?

    EDIT: something doesn't seem right, because the graph doesn't correspond with the operating currents listed on the operational performance charts like that do show increased COP at lower capacity.

    So I'll chalk this up to me missing something.

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