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What happens if one has mini-splits with ratings that exceed the heat pump?

willib | Posted in General Questions on

One configuration we are considering is a
– Mitsubishi MXZ-4C36NAHZ2 – 4 port ,36K BTU  with 

– SVZ-KP18NA  18K air handler for the 1st floor master bedroom, bathroom, office
– 3 MSZ-EF09  for 3 upstairs rooms

Total for units is 45K.

– Does anything bad happen if all units demand maximum heat other than it isn’t available? 
– Is having 45K for the units on a 36K HP a problem?  The expected heating and cooling load of the upstairs rooms is less than 9K for each room, but there are aren’t smaller EF style units.  6K would be plenty for at least one of them and maybe 2.
–   If one of the EF cycles regularly is that ok?  I don’t expect that the HP itself will short cycle due to all the other rooms on the system.

Part of the thinking is that in extreme cold, the upstairs rooms can  be turned down redirecting more BTU to the bedroom and office which sticks out of the main house on its own.  The upstairs rooms will benefit from heat rising from the rooms below which are on a separate HP.  In the summer, the reverse would happen, but being in MA, cooling isn’t really an issue with any system.


  1. Expert Member
    Akos | | #1

    Multi splits are very hard to get right.

    First question, is your house heat loss (actual, real loss, not some rule of thumb) close to the 45000BTU the 4c36 can deliver? If it is much less, the system will be pretty inefficient.

    Generally it is a bad idea to stick oversized wall mounts into small rooms, even worse when those are on a big multi split. You'll get bad humidity removal and overcooling in the summer and cycling in the winter time. Cycling on these is pretty annoying when heating as the heat exchanger will expand and contract which makes random popping noises. Last thing you want in a bedroom.

    A better option is to use a properly sized, probably 9k or 12k, slim ducted unit for the 3 bedrooms.

    There is no issues with installing a bit more indoor head capacity as long as the total is kept bellow 130% of the outdoor unit capacity. What happens when everything is running at max is the capacity of each zone is slightly reduced to match the overall output of the outdoor unit.

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