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Mitsubishi mini split HEAT issue

PeteW12 | Posted in General Questions on


 I have 4 MSZ-FH heads 06,09,12,12 connected to MXZ-4C36NAHZ-U1 in a 1500 sq ft raised bungalow. The two 12’s are in basement (on ground level) and kitchen (upstairs), the 6 and 9 are in bedrooms. In cooling mode I have no issues but because heat rises I’ve had multiple issues in HEAT mode. I am not able to keep my house warm on cold days in AUTO fan mode because the fan does not push enough air away from the intake. Apparently the thermistor in the head tells the brain the temp has been reached when in reality the temp of the room is about 5 degrees colder than what the head is reading. I am able to keep the fan on SUPER HIGH in the basement with vanes constantly moving and AUTO in the other rooms for a steady ish temp, but I am concerned that’s not how this unit should be running? From what I’ve read you can install a wireless thermostat so it can read the temp from a different part of the room?  I just spent $15k on these units so for me to spend anther $600 ish to install a wireless thermostat seems absolutely absurd. 

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  1. kyle_r | | #1

    Sorry if this is obvious, but have you tried increasing the setpoint until you are comfortable?

    1. PeteW12 | | #2

      Yes, the problem with that if I set the fan to AUTO and crank the heat, the fan speed doesn't seem to change from the lowest speed which then causes the hot air to rise quickly around the head and tells the brain its reached its set temp (I believe this is called short cycling?) without having the fan set to SUPER HIGH the temp is so inconsistent in that room. For example if I want my room at 20 degrees Celsius on AUTO I would have set the remote to 23-24 the room temp would fluctuate between 18-23, when I set fan to SUPER HIGH and the remote to 20 degrees the room stays at a steady 20 degrees but every 20-30 min it drops to low speed and seems to be doing a defrost?. Do you think cranking the heat to 23 degrees on remote and fan on AUTO would be more energy efficient or fan on SUPER HIGH and remote set to 20 degrees?

      1. kyle_r | | #4

        Given what you describe, I think the remote thermostat is the best option. If you haven’t already done so, I would recommend air sealing your attic as well. This will help with the temperature difference between floor and ceiling.

  2. walta100 | | #3

    I think a remote temp sensor would fix your problem.

    I think PAR-33MAA is the part number for your heads. This is a wired unit so you would need to fish a wire. This is a full remote control there maybe a sensor only option.


  3. ohioandy | | #5

    Pete, what remote are you using right now? Can't you change the settings to "sense room temp at remote?"

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