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Is the low temp capacity of a Multi-Split a function of indoor units, outdoor units or both?

moe_wilensky | Posted in Mechanicals on

Say one were to install a multi-split that allowed for up to three zones and has a total heating capacity of 24 kBtu/hr at rated conditions and 18 at kBtu/hr at -25 C. With only 2 x 9 kBtu/hr heads installed, would the full capacity of each head be available down to -25 C?

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  1. Expert Member
    Akos | | #1

    I would check the engineering data but you should get full capacity for those two 9k wall mounts.

    This is the resource for detailed Mitsubishi data, search for engineering data on your specific unit:

  2. moe_wilensky | | #2

    I had a look around at various manufacturer data out there, but it seems capacity at different conditions are associated with the outdoor units or indoor units paired with appropriately sized outdoor units.

    I guess I'm mainly wondering how any decreased capacity at the outdoor unit translates to the rest of the system. To my layperson brain, it seems like either the refrigerant leaves the compressor at a lower temperature or at a lower flow rate (or both i guess). If the former is correct and flow rates through the indoor units maxes out at whatever rate is required to provide the rated capacity at rated conditions, I'd expect that indoor capacity would be reduced proportionally to the outdoor unit capacity reduction, regardless of whether or not the OU has enough remaining capacity to meet the load. Whereas, if the flow rate is reduced but temperatures are constant, I'd expect that (given an oversized outdoor unit) one could still get full capacity from the indoor units.

    This is not pertaining to any particular HP make or model, I'm mostly just curious.

    1. Expert Member
      Akos | | #3

      Generally most inverter units target a fixed pressure out of the compressor, so I would assume you would get lower flow rate. This does mean you should be able to get rated heat out of your smaller indoor units. I was very surprised on my first hyper heat unit when I was measuring 120F vapor supply temps in -20C weather.

      The data for these multi splits is not great and it is really hard to figure out exactly how they operate.

      Also note that for many manufacturers a 9k and 12k (or 9k and 15k) unit is identical so in your case it is pretty likely you'll get full rated heat out of the 24k unit in warmer weather. The only difference seems to be is cooling capacity is artificially reduced in software.

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