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Community and Q&A

Mini-split use in extreme cold

wisjim | Posted in Mechanicals on

My question is about using our minisplits in extreme cold weather. The specs for our Fujitsu (AOU15RLS3H) indicate that it functions at its rated capacity to 3 degrees F, and at greater than 73% of rated heating capacity down to minus 15 degrees F. When we get some days and nights of cold weather, below 15 or 20 degrees below zero, is there any advantage to turning off the minisplits? I find that the operating manual and other info don’t give recommendations for methods of operation.

Thanks for any info or suggestions.

Jim, in Western Wisconsin, looking at high temps of minus 5 F (-20 C)

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  1. wisjim | | #1

    We have an efficient wood burning force air furnace that we use also, but the minisplits provide heat in spring and fall when a wood fire in the furnace would be just smoldering a lot. Also, the minisplits use some of our surplus energy from our solar electric system.


  2. GBA Editor
    Martin Holladay | | #2

    You don't have to disconnect the power from a Mitsubishi or Fujitsu minisplit in cold weather. (If these units include a heating element to defrost the pan, you definitely want to leave everything plugged in.) Even if a unit is only rated for use down to -13F or -17F, plenty of GBA readers have posted comments on better-than-expected performance, with these units providing heat at outdoor temperatures of -20F or -25F.

  3. Dana1 | | #3

    BTW: The AOU15RLS3H has a fully specified output even at -15F, but keeps on chugging away, putting out an unspecified amount of heat at temps below that. How is it that you figured +3F was the low end?

    The "rated" or "nominal" heating spec for a mini-split is the modulation level at which it's efficiency was tested/rated, not it's maximum capacity. The nominal heating spec for the operation range for the AOU15RLS3H is 18,000 BTU/hr, and that is the level at which it's efficiency is tested. It must be able to deliver at least that much at +17F (which it does, with margin). IIRC the specified operating range in heating mode is from -15F to +75F. It's maximum heaiting capacity at -15F at a +70F indoor temp is about 15,000 BTU/hr (not including defrost cycles.). Most of it is spelled out (if tersely) in the efficiency testing submittal sheet:

  4. Svig | | #4

    I got my Mitsubishi MUZ-FH15NA (outdoor) MSZ-15FNA (indoor) fired up on December 21 and it has been heating my home since. We have had some below zero, but last night was -14F, and tonight is expected to be -22F and tomorrow -24F, CZ7 Northern MN. The indoor unit does a little gurgling once in a while, seems like a little more often the colder it gets. I guess this is normal? I am very impressed so far, and folks who have been stopping over to see our new home find it hard to believe that little thing on the wall is heating our home. Wondering if I should turn the baseboard electric thermostat up a little so it forces them to help the heat pump, or just leave them at 65F and see how it goes? Heat pump is set at 71F.

  5. wisjim | | #5

    Thanks Dana for the data sheet. Our units have been doing fine even in the below zero weather, supplemented by our wood furnace. It's nice though to be able to leave the house without worrying about making sure the wood burning furnace is full of wood. The drawback is that for the first time in many years our electric bill is more than the minimal connection charge of our utility. Our PVs couldn't keep up with our usage due to the exceptionally cloudy December of 2016, and our wind turbine is temporarily out of service for the first time in a decade, needing a bearing replacement.

  6. Expert Member
    Dana Dorsett | | #6

    Steve: I'm not sure what the gurgling is about- it's probably worth a call to the installer or distributor.

    Leave the baseboards at 65F, bump the setpoint up on the FH15 if it's not keeping up. There is usually an offset between the incoming air temp sensed at the head and the average room temp- it's not the same as a wall thermostat, and the offset changes a bit with changes in weather/heat-loss. Only when it's clear that the mini-split isn't keeping up would it make sense to bump up the thermostat on the baseboards, since even at -15F the mini-split is nearly 2x as efficient as the baseboard.

    Jim: I'm not sure how old your PV system is, but it's never been cheaper to add more (though it will be cheaper still on 5 years.) The average installed cost for residential rooftop PV in the US in 2016 averaged less than $3/watt (utility scale PV averaged under $2/watt). If not this year, at some point it will make financial sense to add more PV if the mini-split load is causes your power use to exceed the net metered capacity of the existing PV system.

  7. wisjim | | #7

    Dana, we keep adding PVs (our oldest were bought in 1982) and I still have 8 of our last purchase awaiting construction of addition racks for mounting them. The inverters and all other components are in boxes in the barn. Our last PVs cost under $1 a watt, while our first ones were over $8/watt (in 1982 dollars), but even at those old prices, they have paid for themselves over and over. The reason we bought a Nissan Leaf and added 2 minisplits was originally to use up surplus electrical energy that we produce.

  8. Expert Member
    Dana Dorsett | | #8

    $8 in 1982 dollars is equal to about $20 in 2016 dollars, so your more recent buys at under a buck are at a 95%+ discount from the originals!

    Of course we all know that nuclear power has followed the same learning curve/ price track, right? :-)

    Solar still has plenty of room to fall in price, and is likely to dominate the new generation installed between now and 2030 in terms of total capacity. It's a tough time to be in the business of building large centralized power plants, that need to have market security over at least a few decades to have sufficiently low risk. Big-iron is really cool and all, but I suspect ALL gigawatt nukes currently under construction will become stranded assets well before the lifespan on which their financing was predicated. Behind-the-meter PV and storage it going to eat their lunch, and the federal financing guarantees are likely to be invoked to bail out the financiers.

    It's not just nukes- the Kemper carbon capture & storage coal burner is ridiculously over budget and behind schedule, and will likely be uneconomic even if the coal were free. From when they started building very few (if any) analysts were expecting the installed cost of PV to drop by another 60-80% by this time, but you can bet those looking forward understand the financial risks of large generators a bit better now.

  9. Svig | | #9

    Sorry, not trying to hack the thread. Dana, I read something about a noise related to refrigerant circulating back to the outdoor unit/suction line/something, but I can't find it again, the Mitsubishi web site is fairly convoluted. Maybe I'll try the chat...

  10. bencarsan | | #10

    Yes, the gurgling sounds like defrost mode. In heating mode the hyperheat units occasionally reverse and pump heat into the outdoor unit in order to clear ice build up from the condenser. In extremely cold climates, you may want a drain pan heater as well--an electric resistance heater for the bottom of the ODU that keeps the melt from the defrost from building up inside the housing.

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