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Thread: Outdoor mini-split fan continues running even when doing nothing, is this a problem?

  1. #1
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    Outdoor mini-split fan continues running even when doing nothing, is this a problem?

    I have a Mitsubishi SUZ-KA15NA outdoor unit that is paired with a ducted SEZ-KD15NA4 indoor blower and I'm extremely happy with its performance for the last couple of years I've had it. The system is controlled with a wall mounted Mitsubishi MRCH1 thermostat.

    At night I have a schedule on the thermostat which drops the temperature to 74º 9:30 PM and then to 70º at 10:45 PM. In the morning at 6:15 AM the thermostat schedule is set to cool at 78º. Shortly after 6:15 AM the indoor temperature will be in the high 60’s or right at 70 and the outdoor unit fan is blowing at a much faster speed than it does when it’s actually cooling. The outdoor fan continues running until I use the thermostat to turn the system off. I can set it back to cooling after a minute or so and the outdoor unit fan remains off until later in the afternoon when it actually starts cooling again. If I do nothing, the outdoor unit fan will run at the faster speed all day until generally around 3 or 4pm when the indoor temperature causes the cooling cycle to begin. The system still cools perfectly fine but it seems like the outdoor unit fan should stop running if it's not actually doing any cooling.

    I had a service tech out to check it and he cleaned a tiny amount of debris off of the rear of the outdoor unit thinking that may have something to do with it but the problem, if it is one, has continued. I've replaced the air filter about a month ago. The outdoor unit doesn't get any direct sunlight on it until after noon.

    Since the tech was out last I've noticed that the outdoor unit fan running or not may be related to the overnight low temperature. If the low is around 63º or higher then the fan will stop running when the schedule increases the cooling set point at 6:15 AM but if it's lower than that the fan will keep running at a higher speed all day until it actually starts doing cooling again.

  2. #2
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    Quote Originally Posted by AJ08 View Post
    I have a Mitsubishi SUZ-KA15NA outdoor unit that is paired with a ducted SEZ-KD15NA4 indoor blower and I'm extremely happy with its performance for the last couple of years I've had it. The system is controlled with a wall mounted Mitsubishi MRCH1 thermostat.


    Since the tech was out last I've noticed that the outdoor unit fan running or not may be related to the overnight low temperature. If the low is around 63º or higher then the fan will stop running when the schedule increases the cooling set point at 6:15 AM but if it's lower than that the fan will keep running at a higher speed all day until it actually starts doing cooling again.
    This is absolutely normal Mitsubishi behavior if the unit is in cooling mode (it doesn't have to be calling for cooling, just in cool mode) and the outdoor temp is below 63°. It'll run the outdoor fan slowly to move air across the outdoor temperature sensor to get an accurate reading.

  3. Likes BALloyd, R600a, jacob-k liked this post.
  4. #3
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    Thread Starter
    Thank you for this. I guess I'll continue turning the system off and back on after the schedule sets the temperature back to 78º since that keeps the fan from running all day.

  5. #4
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    I'd call Mitsubishi.
    I do a triple evac with nitro to remove non condensables.

    I'm feelin' a little peculiar.

  6. #5
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    R600a is offline Professional Member*/Membership Committee
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    Quote Originally Posted by AJ08 View Post
    Thank you for this. I guess I'll continue turning the system off and back on after the schedule sets the temperature back to 78º since that keeps the fan from running all day.
    I think you're over thinking this. That fan uses very little power. If Mitsubishi designed it that way I'm sure it's fine because they're pretty darn efficient units. @FlyersFan knows what he's talking about.
    "Is this before or after you fired the parts cannon at it?" - senior tech
    I'm tired of these mediocre "semi flammable" refrigerants. If we're going to do it let's do it right.
    Unless we change direction we are likely to end up where we are going.
    "It's not new, it's better than new!" Maru.

  7. #6
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    This is straight out of the engineering manual:

    4. Low outside temperature operation:
    If the outside temperature falls to 64°F (18°C) or less during operation in COOL
    mode, the unit will switch to the low outside temperature operation mode.
    <Operation>
    (1) Outdoor fan control
    The outdoor fan rotational speed slows down to maintain sufficient cooling capacity.
    NOTE: Even when the unit is in the "thermostat-off" status under the low outside temperature operation mode, the outdoor
    fan rotation does not stop.


    No need to changed the way you're operating the system, it's completely normal operation.

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