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“Cycling” ductless mini-split

39Chev | Posted in Mechanicals on

I’m working on my wife re: her dislike for the appearance of ductless mini-splits, but I have a question regarding their operation.

My local electric utility has a program where they control the power to heating and cooling in your house. In the winter, they shut off the power to the heating appliance (normally electric furnace or baseboard heaters) for up to 8 hours at a time. They REQUIRE a fossil based back-up system to take over when the power is cut to the electric heat. In exchange, they cut the rate you pay for power sent to the heating appliance (only…dual meters) to 5.4 cents per KWH. I would think this would be perfect for a ductless because they shut off power on the coldest days (from 0 to negative 30 degrees) when the ductless would not be very beneficial anyway.

In the summer, it is different. They cycle the power on and off in 15 minute increments on the hottest days from 4 PM to 11 PM. Would this cycling be an issue for a mini-split?

Thanks…

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    The cycling on and off will reduce the efficiency of the ductless minisplit and, in very hot weather, your house may be warmer than it would otherwise be. Only you can decide whether these drawbacks are worth the tradeoff for low electricity bills. The tradeoff may well be worth it.

  2. Expert Member
    Dana Dorsett | | #2

    The degree to which a 50% duty cycle of 15 on/15 off would affect the efficiency of a mini-split depends quite a bit on just how oversized it is for the cooling load. If it's 1.5x or more oversized for the cooling load it won't be too bad, if it's sized EXACTLY for the load it won't keep up, and will have long cycles running at it's least efficiency max-modulation after the power cycling stops.

    If the mini-split was optimally sized for the heating load in a cold climate odds are pretty good that it's WAY more than 1.5x oversized for the cooling load. It won't hit it's super-high SEER numbers when cycling on/off, but it'll meet or beat them when modulation.

  3. charlie_sullivan | | #3

    The issues are different in summer and winter. It would help to know your climate, so we know the balance of heating vs. cooling, and the importance of dehumidification. Also, what are your plans for the fossil backup?

    But I think you'd be fine. If you have any issues with the summer comfort, you could do a little pre-cooling to get the moisture out of the air so that if the cycling impairs the dehumidification you are still OK. But it sounds like you are in a pretty cold climate if -30 F is common, so as Dana says, it will probably be oversized for summer anyway, at which point it doesn't matter much.

    On the aesthetics, it looks like a mini-split with a duct is now available in the Mitsubishi hyper-heat line. You can do something like hide it in the top of a closet and have very short ducts coming out. I'm not really clear on what is available but it seems like there are more options for mounting in a closet or in the ceiling than there used to be.

  4. 39Chev | | #4

    Sorry...Northern edge of zone 6, right on the border of zone 7 in Minnesota. Cold winters and it can get pretty muggy here in the summer.

    I'll check the Mitsubishi ducted mini-split. I'm not sure what I'll use for back-up yet. No natural gas around here, so choices are limited. I have Geo-thermal in my present home (on this dual-fuel program) and have used propane gas fireplace as my backup and it has worked pretty well, but the Electric Co-op frowns on using only a FP for back-up. I have also considered a small wood stove in the walk-out lower level but I'm not sure about a cement block chimney (and metal ones scare me). Might end up with a regular ducted gas furnace. Propane is cheap right now...but I remember buying propane for my fireplace a couple of years ago and it was up to $6 a gallon for a short time.They blamed it on a bumper crop of ag crops and running out because of all the propane grain dryers...

  5. user-1135248 | | #5

    Every *15 minutes* ?! Don't they realize what that's going to do to
    most air-conditioning systems? That's nuts. Six-hour chunks of time
    controlled at different rates makes sense, maybe, not what they're
    doing. IMHO you'd be better off ignoring such ill-founded meddlesome
    interference and controlling your own systems the way *you* choose
    under flat-rate billing.

    There are some weird time-of-use schemes out there, but that one
    takes the cake. Which power company??

    _H*

  6. 39Chev | | #6

    The power company is Crow Wing Power. In the winter, they shut off for a block of time (4 PM to 10 PM), but in the summer when it's hot, they do the 15 minutes on, 15 minutes off.

    It would be great to ignore the "dual fuel rate", but the difference in price is quite high. I think part of the problem is that this works OK with an old style "normal" AC unit, but I thought it would not be very good with a mini-split.

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