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Minisplit Short Cycling and Fan Speed

aaron55 | Posted in General Questions on

I am still getting used to my Mitsubishi mini splits.  I’ve noticed that if I set the fan on high or super high, it spreads the heat around a lot better.  However, the units tend to over-shoot their temperature setting and short cycle if they are on a high fan speed and it warms up outside.

Is there some programming that prevents them from modulating down the compressor if the fan is on a higher setting (to keep the air coming out from being “too cold”)?  If so, it seems like these one-to-one units are not really that great at inverting.  They can run from maybe 2/3-full compressor rating regardless of what heat is being called for.  I tried the auto-fan mode, but it just runs on such a low speed that it doesn’t really heat up the house well and speeds up only after the temp has dropped substantially in the room.  I feel like I need to set up an IFTTT script that controls the fan speed on my units to keep them from short cycling.  It seems like I must be doing something wrong here.

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  1. richmass62 | | #1

    The Mitsubishi mini split I installed in 2018 would short cycle because the thermostat was at the unit. I think you should have the unit installed in a big open area close to the floor, like a hotel AC. Otherwise you will get short cycling.

    The way around this is to get the remote thermostat. I installed an MHK1 or whatever it is called and that solved the short cycling problem. The thermostat is located about 20 feet from the unit, not in the direct path of the heat.

    1. aaron55 | | #3

      yeah, i have also been thinking about getting the remote sensor for sure. i wanted to get the wifi adapter and temp/humidity "tile" but they seem to be very hard to find right now, without paying a huge price. thanks for the suggestion.

  2. mjhil | | #2

    My 2014 24k btu mitsubishi 2 wall mount heads (with a second 6k in a bedroom) would short cycle all the time before I got an external sensor. It's still a struggles because it's oversized for the room, but it still helped a great deal. I've said it elsewhere, but an energy monitor (I have an emporia) really gives you a good idea what's going on. With the fan on low, mine can use anywhere from 500-2000 watts.
    It can sit at 500 something watts for hours, which is great, but it seems to only be able to go in 500 watt increments with a gradual slope between those levels. I would've thought -and it would be great- if it had more variability, but I just haven't seen that mine can do it.

    1. aaron55 | | #4

      i'm excited to get an energy monitor. i was looking at the emporia, but i wanted to monitor circuits in two panels, so i thought i'd try getting the energy monitor that goes with the Aeotec Smart Home Hub. Then, i can monitor my two mini split circuits and also use the hub to monitor temperatures and to monitor for water leaks, etc. as well as other stuff. Anyway, in the meantime I have been running down to the panel and putting on a clamp meter to get an idea of what is going on. I've found that my 18k unit usually runs around 1000 watts and can go up to like 1600 or a bit higher (obviously more during a defrost cycle or coming up from a defrost cycle) if i have it on high or super high fan speed. if I have it on auto fan speed i've noticed it running down to 500 watts as you saw. i assume that you run yours in auto fan mode when you are seeing it go down to 500 watts?

    2. PatHennessy | | #6

      When you say 500 watts, what time frame is that? I have a new system and I used over 1000 kilowatts a month over the previous year.

      1. maine_tyler | | #8

        watts is power, watt-hours is energy.

  3. aaron55 | | #5

    Someone on HVAC-talk answered my question as far as why it won't ramp down on higher fan speeds:

    Basically, it wants to maintain a minimum air temperature coming out of the unit, so the only way for it to ramp down is if you turn down the fan speed. Auto fan mode will do that, or you have to just set the fan speed manually, or automate it somehow.

    1. greenright | | #7

      Auto fan is glorified "fan speed depends on set temp and room temp delta - if =>2 run on high, if between 2 and 1 run at mid speed and if =>1 run at low speed.

      Also most minisplits will not allow you to run the fan on higher speed than mentioned above- ie if room temp is 1 degree less than set temp you will not be able to run high or med despite pressing the correct button- only low speed will run... etc. However, low speed is always allowed regardless of delta t

  4. maine_tyler | | #9

    This is an interesting question, and I think a common issue with mini-splits, i.e. distribution. I'm no where near a mini-split expert, btw, but I read your other thread on HVAC and it brings up quite a few relevant points.

    In that thread you say:
    "I feel like if auto fan mode is not adequate, then i would either need more capacity or i would need to set up some kind of custom automated control that allows the unit to cycle through all of its fan speeds depending on where the temp is compared with the set point."

    I don't believe more capacity would change anything, as it would only short cycle even more. This seems like an issue of distribution. I'm beginning to understand that many comfort issues with heat pumps comes down to distribution, not total capacity.
    And isn't auto fan mode pretty much what you describe in that quote? Something that "allows the unit to cycle through all of its fan speeds depending on where the temp is compared with the set point."

    lastly, you mention you have little to no insulation! And that you are used to wood stove heat. That's your battle right there.

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