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Mitsubishi multi-zone Hyper Heat models: true minimum output?

anonymous_khan | Posted in Mechanicals on

Take MXZ-8c48nahz as an example – the submittal indicates that the mitsubishi heating capacity goes from 7200 Btu/h to 54000 Btu/h for DB temperature over 5F. The NEEP spreadsheet with far more detailed spec indicates that the minimum is 27000 Btu/h instead.

Am I looking at two completely irrelevant number? If not, which one of this is the true minimum output capacity?

Thank you.

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  1. anonymous_khan | | #1
  2. Expert Member
    Dana Dorsett | | #2

    I'm not sure what the real discrepancy is. It may be the difference between the minimum compressor modulated output, vs. the sum of the minimum modulated output of 8 individual heads/cassettes.

    The 8 zone Mitsubishi with ALL zones operating at minimum modulation simultaneously may be as high as 27,000BTU/hr @ +47F, (that's only 3375 BTU/hr per zone) but the compressor itself can dial back to 7200 BTU/hr. That happens to be the maximum output of a single half-ton GE06 head.

    When only ONE zone is calling for heat it will run the zone head at highest blower speed to match the minimum compressor output under those conditions rather than modulating the head down and short-cycling the compressor, which would take a severe toll on efficiency.

  3. anonymous_khan | | #3

    Thank you Dana - I have been reading a lot on heat pumps and found your info really helpful! I appreciate the effort you spent on researching this matter and educating the others interested on these systems.

    Follow up question - for FH09NA paired with the single zone outdoor unit, the maximum output capacity @ 47F is given as 18000 Btu/h. As temperature decreases the maximum output of the outdoor unit drops accordingly down to 10900 Btu/h @ 5F which is the rated capacity.

    What about using MSZ-FH09NA with (for example) a 8c48NAHZ? Since 8C48NAHZ has significantly more output capacity than MUZ-FH09NA at low temperature, does the software/firmware in MSZ-FH09NA allows 18000 Btu/h even at low temperature if it's the only head calling for heat?

    Thank you.

  4. Expert Member
    Dana Dorsett | | #4

    You won't get 18KBTU/hr out of FH09 @ +5F even when married to a bigger compressor. Taking look at the output tables of the 3-ton 4-zone multisplit (MXZ-4C36NA) with only one or two 9K heads you'll see that you still only get 10,900 BTU/hr per head. See the tables starting on page 3.:

    Without pulling the submittals I would assume it's the same for the bigger multi-splits too.

    Khan- anonymous kyu? Aapka naam Aamir hain, shayad? ;-)

  5. anonymous_khan | | #5

    Haha no, I don't speak Hindi. I just don't want anyone searching for me be able to easily find everything I said and do.

    I think the table only gives rated capacity - not necessarily the max capacity an indoor unit can achieve given sufficient outdoor unit capacity.

    I guess the only way to tell would me for me to do some measurements on the inlet vs outlet air temperature and use the CFM rating from mitsubishi to calculate! This might work if setting target temp to some max temp allowed would force the indoor unit to run at max capacity. I should have the system installed by the end of the month.

    If I am really in the mood I should probably also grab my clamp meter and measure one of the live wire current to obtain the COP of the system and just be amazed, ha!

    Thanks Dana.

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