GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Mitsubishi heating way past setpoint

clusty | Posted in Mechanicals on

Last week I had my Mitsubishi heat pump installed:

External: MXZ-3C24NA 

Internal: MSZ-FH09NA and 2x MFZ-KJ09NA


The 2 floor consoles are installed on the second floor in 2 bedrooms that are fairly small 12×12.

What I notice is that as soon as there is one unit that heats, the 2 floor consoles will also start blowing hot air, heating the room to 77F even though the set point 70. This feels like a huge difference.

I was told by the service guy that this is “normal” since the units cannot close completely.


Was he right ?

Did I just buy myself a useless hunk of junk that I cannot use to heat?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    For multi-splits to really work well the sizing of the coils/cassettes relative to the zone loads have to be fairly consistent. When only one head is calling for refrigerant the excess coming out of the compressor gets shunted to the other zones and causes some heating/cooling of that zone even when the blower isn't running (or forced to run at low speed to protect the compressor from receiving liquid refrigerant on it's intake.)

    Run a room by room, zone by zone load calculation. It may have better balance trading one of the 3/4 ton heads for a 1-1.5 ton , and another or others for a half ton. Even if they'er ALL oversized for the load (likely) the amount of overshoot won't be as severe.

    Unfortunately they don't make half-ton floor consoles, but a 3/4 ton mini-duct splitting it's output between the bedroom zones or a pair of FH06 wall coils and a bigger wall-coil for the third zone to achieve similar oversizing factors may be the right solution.

    The load balance issue is yet another reason why it's important to run the load numbers BEFORE installing the system rather than taking the "head in every room" approach to multi-splits (which also tends to grossly oversize systems for the whole-house load.) Cassettes/coils that can modulate when slaved to a single zone compressor generally don't modulate on a multi-split. Rooms with tiny loads really can't have their own ridiculously oversized private zone heads if the other zones aren't also ridiculously oversized to the same degree.

  2. clusty | | #2

    Thanks for the answer.
    I got the smallest floor consoles for the top floor due to space constraints, that's 9k BTU.
    The third ground floor could have been over sized, but contractor told me the MXZ-3C24NA can take 3x 9k BTU internal heads, and I would not be able to add a 12k or larger on the ground floor.
    The guy is said he was talking to Mitsubishi to make the units stop blowing air when they reach the setpoint, mitigating problem.
    So I am what they call shit out of luck?

    1. Expert Member
      Dana Dorsett | | #3

      Any chance of being able to install a 3/4 ton mini-duct cassette? (Maybe just below the ceiling level in a closet between the two bedrooms, mayhaps?)

      Before swapping out any equipment, run a room by room, zone by zone heat load calculation.

      At 11,000 BTU/hr nominal fixed output the KJ09NA isn't just oversized for a ~150' bedroom, it's LUDCROUSLY oversized.

      Even a corner 150' bedroom with leaky clear glass double panes and only R11 in the walls and R20 in the attic is going to come in under 4000 BTU/hr, it could be 2500 BTU/hr or less than 2000 BTU/hr for your actual house (which is why you need to run the load numbers.)

      A KD09 puts out almost as much as the KJ09, but split between a pair of 2000- 2500 rooms it would the be merely ridiculously oversized. It's possible to include a third duct to send some the output downstairs or the stairwell or something the overheating could probably be tamed.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |