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Community and Q&A

Mitsubishi heat pump defrost mode

jdorwartcrane | Posted in Green Products and Materials on

I just had a Mitsubishi MXZ-4C36NAHZ with a 24kbtu and 9 kbtu unit installed in my house in Vermont. When the temp drops below freezing, my indoor units blow cold air (as in really cold. Not just a fan circulating air, but COLD). This is a quote from the brochure: “and because the system fan turns on only after the target temperature is reached, it NEVER BLOWS COLD AIR.”

1. First if that’s true, what’s happening?

2. I asked the contractor and he said it’s defrost mode. When the temp’s in the 20s, (which is all it’s been here in VT so far) the unit does this cycle every 15-25 minutes. I called Mitsubishi tech support and they said it should do the cycle only every 40-150 minutes and that if it does it more something is wrong. The defrost mode is especially annoying because THERE IS NO FROST ON THE UNIT OUTSIDE.

I am a tolerant person, so I have trouble believing that less tolerant people would be ok with this. Is there something wrong?

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  1. GBA Editor
    Martin Holladay | | #1

    Two questions:

    1. How long does this phenomenon (blowing cold air) last?

    2. Does the unit successfully keep your house warm?

  2. brp_nh | | #2

    I don't have the expertise to get into tech details, but as the owner of a Mitsubishi mini-split in NH, this doesn't sound normal.

    Could the settings be on some type of auto mode that is running the AC or dehumidify by mistake? Are you blasting a wood stove in cold weather and the heat pump thinks it needs to go into AC mode?

    Just some troubleshooting thoughts.

  3. jdorwartcrane | | #3

    It lasts about 5 minutes. My house is staying warm, but it has only dipped below 30 for short periods of time so far. I'm nervous for real winter because I don't have anything else to heat with. I'm planning on installing electric baseboard but haven't done it yet.

  4. GBA Editor
    Martin Holladay | | #4

    If the unit was recently installed, it must be on warranty. Call up your contractor (and Mitsubishi if necessary) and insist that the unit be fixed.

  5. dedelstein | | #5


    I hope that, by now, you've solved your problem. Not only do I hope things are better for you, but I'm experiencing a similar problem so I'm hoping I can learn from you.

    I have a Mitsubishi multi-zone heat pump. When the temps are 20 or lower and wind is from the north or northwest at 12 mph or more, the pump blows very cold air into the room and does so for 20 minutes or more before the heat starts again. The in-line 6,000 BTU heater seems ineffective. Of course the temp in the room falls well below set point and cannot recover until the weather eases. The heat pump is situated in the wind shadow of the house. Standing near the pump I cannot feel any wind at all. Under all other conditions the systems works fine.

    I'd greatly appreciate it if you'd share your solution, if you've found one and are comfortable sharing it.
    -- David Edelstein

  6. Expert Member
    Dana Dorsett | | #6

    David: If the cold air seems dependent upon wind direction there's a possibility that the line set conduit to the head isn't air sealed.

    BRIAN P: It's impossible for one head of a multi-split to be in cooling mode while the others are in heating mode. They're either all heating or all cooling.

    What Martin said- if you can't get satisfaction from the installer, take it up with Mitsubishi.

  7. Maris016 | | #7

    We are having this same exact issue with our Mitsubishi mini split. Did you ever get to the bottom of the problem?

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