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Mitsubishi M-Series Minisplit Fan Speeds

ianrking | Posted in Mechanicals on

Hello! Asking anyone with an Mitsubishi M-series system. I have a 9000-class and a 15000-class unit on the same MXZ system. When using auto-fan mode on the 9000-class unit, all of the possible fan speeds are utilized and when the unit is at or near setpoint the “quiet” fan speed is used. For some reason the 15000-class unit does not operate the same way. When near or at the set-point, the second-from-lowest fan speed is used. If I want the unit to operate in it’s lowest fan speed, I have to manually change from “Auto” to “Quiet” fan. Has anyone else noticed this? This seems to be limiting the 15000-class unit’s ability to modulate because the range of possible air flow rates is limited.

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Replies

  1. ianrking | | #1

    Bumping this looking for assistance.

  2. Norman Farwell | | #2

    Had the same problem recently with a couple of FS18 units I installed, one with an MXZ multihead and one with an MUZ one-to-one outdoor. Both operate at the mid-range fan speed in auto mode when there is no call for heating or cooling. Customer was unhappy. After much back and forth with tech support, it appears that this is considered normal operation for this series.

    There are two options: manual over ride to control fan or there is a jumper that can be cut on the control board which will cause the fan to turn off when there is no call in auto mode. The downsides there are that the blower wheel will tend to load up with dirt and the room air will stratify.

    My sense is that FS units are programmed this way for some reason. This was not true if the earlier FH units—those idle on low fan speed.

    1. ianrking | | #3

      Hi, Norman. Thanks for your reply. And for confirming that this is (unfortunately) not a malfunction! I, too, tried to contact tech support and they were not at all helpful.

      The worst part of this is is that without the unit having the ability to automatically cycle between all available fan speeds, the modulation ability of the unit when set to auto fan mode is severely limited and the unit is caused to duty cycle in low load situations. Shame on you Mitsubishi because had I known this beforehand I would have specified a different unit.

      1. Norman Farwell | | #4

        Interesting. On the FS18s, I didn’t see the second lowest speed, only the middle speed.

        I assumed this fan speed issue was limited to the condition when the thermostat was satisfied, and that the unit modulated normally when there was a call for heating or cooling. But you seem to be saying that in auto mode it never modulates down to the lowest speed? If so that seems like a bigger issue. I hope they get this fixed—it’s going to be hard to sell these units.

        1. ianrking | | #5

          Hey, Norman. Yup. Mits finally confirmed this idiotic behavior today via email.

          “Your unit has Auto, Quiet, Low, Med., High, and Super High. Auto fan goes between low and High. It doesn’t go to Quiet and Super High.”

          Makes no sense.

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