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will a mini split work here ?

ddubx6 | Posted in General Questions on

looking for input on a mini split set up. currently have a large wood burning furnace in my basement, i am in northern NH, so it gets cold,  also have  an old oil furnace for back up, the oil furnace is ducted into the house but the wood furnace only blows into the basement. we keep the furnace cranked up so the unfinished 2000sq ft. basement stays at 80 degrees, this keeps the floors warm and keeps the 1st floor at about 65 degrees. we find this much more comfortable than the oil heat which even at 68-70 degrees air temp feels colder because the floors get cold. i want to get away from burning stuff, so i was thinking of putting one large unit or two smaller ones to heat the basement and a ducted one hooked to existing ductwork for as needed use. how do i size these, a manual j doesn’t seem logical because i am overheating one space to heat another, and the back up unit will only be needed on the rare 20-30 below days to supplement a partially heated space. any recommendations, suggestions, input? thanks

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  1. walta100 | | #1

    If you dislike the oil furnace that is likely putting out 130° air my guess is you will hate a heat pump that going to be blowing out 90° air.

    I think heat pumps are great for tight well insulated buildings but that does not sound like what you have.


  2. ddubx6 | | #2

    hello, my house is a mix of good and bad, the bad- approx. 1000 sq. ft. of r13 fiberglass cavity, 2" exterior polyiso, single pane glass, poor air sealing, the good- 2000 sq ft. of modern additions, rockwool cavities,lots of tapes, gaskets, caulking on every seam, 4" exterior polyiso, taped zip, membrane barrier, air tight drywall, low e double pane windows, 20" cellulose attic, etc. renovating the old is hopefully a near future project. so walta do you think mini splits wont work, or will they just have to work harder, my goal is still to heat just the basement and let it radiate up to first floor, so we really wont be "feeling" the air. the only air we would feel is the back up system on the rare really cold days, which i could leave on oil if needed. my gut tells me to do a manual j for the basement and first floor areas and size mini split(s) to the total of both heat loads. cost of electricity is a factor but i hope to offset it with solar in near future. any and allideas , suggestions are welcome. thanks

  3. Jon_R | | #3

    Say the floor is R3. Then heat supplied to the first floor by the basement will be 2000 x (80-65) * 1/3 = 10,000 btu/hr. This comes to 5 btu/hr/ft², which is more useful for adjusting individual first floor room loads downward.

    So do a normal Manual J/S/T/D but account for this load shift by adding 10K btu/hr to the basement heat pump capacity and reducing the first floor one by 10K. Not exactly right, but close.

    In my opinion, this "cheap radiant floor" approach has a lot of merit with a crawlspace (where the space isn't usable even if it were kept at a more reasonable temp).

  4. Andrew_C | | #4

    Since you asked for thoughts,
    1. More air sealing is always good, especially if you're going away from burning stuff and ventilation isn't an issue.
    2. It's my impression that a properly sized mini split unit with a variable speed blower will be quieter and less "blowy" than a standard forced air furnace. You won't notice the air flow as much. This will depend somewhat on placement of the unit, but if it's not blowing down onto your reading chair, should be good.
    3. When you run the Manual J, run the basement with and without insulated walls. If you normally keep the basement at 80 degrees, I would think that you'd insulate the walls even if you don't plan to finish it.


  5. walta100 | | #5

    I do not want to be negative but is long as you keep the wood furnace that is more or less a big hole in the roof that suck warm air out of the house 24/7 you cannot air seal the house well without risk of back drafting the wood and oil furnaces. Since they seem to be working I am guessing the house is at least a little drafty. So I am afraid to tell you to air seal the place too much.

    As for you plan to heat the basement to 80° with a heat pump, you need to understand that heat pumps work by moving heat from one place to another the greater the difference is between the two places is the harder the system must work and the less efficient it becomes.

    In my opinion heat pumps are great in tight well insulated buildings as the primary heat source but when combined with other heat sources I do not see how the occupants can become accustomed to the lower supply temps if they are still being exposed to warmer air supplies from time to time but maybe others have more willpower than I can imagine.


  6. ddubx6 | | #6

    thanks for the input, i should clarify the basement, it isn't finished with drywall and such yet, but it is used, there is storage, an area for my dogs when its too cold outside, a pool table, a full exercise/weight room with surround sound and tv. i just recently finished a basement wall renovation, all the additions have ICF walls so i dug out the original crawlspace, by hand, quarrying the granite boulders to moveable sizes with wedges and feathers, and put a full ICF foundation there too. 80% of the walls are fully buried, one side is walk out with a couple of low e double pane windows and a new french door, the outside of the walls where coated with a liquid membrane from footing to sill and then covered with a peel and stick membrane, belt and suspenders, it is well above the water table and well drained but i wanted it dry, under slab is a sandy gravel mix with xps layer and 2 layers of 6mil plastic. after this was done i noticed a draft issue with wood stove so outside air ducts were added to stove and furnace. an hrv is in future plans. the wood stove goes out the side wall to an exterior prefab metal chimney, so no hole in roof, and the stove is pretty air tight, no drafts but it will get cool to touch if not used at freezing temps, but i could cap wall outlet to prevent this if i don't use it. we are not "keep the house hot people", we are fuzzy socks, flannel jammies and a sweatshirt or sweater if needed people so a little chilly is preferred. Jon R , thanks but you lost me in the math, what am i doing with the 5btu/hr/sqft ? i realize that heatpumps have limits in cold climates, i am trying to lower the risk as much as possible by sizing things correctly, my belief is that they work more efficient if they don't have to turn on and off but stay running longer, this is why i was thinking that 2 units might be better with one doing most of the work and the second for when needed. is this correct? so is it me or is being green a complicated pain in the *** ? my neighbor has fiberglass bat 2x4 with basic oil heat and is sitting on his deck enjoying a frosty cold one while i am still building window bucks, taping seams and hanging foam!!!

  7. Expert Member
    BILL WICHERS | | #7

    >"my neighbor has fiberglass bat 2x4 with basic oil heat and is sitting on his deck enjoying a frosty cold one while i am still building window bucks, taping seams and hanging foam!!!"

    You'll have a lot better thermal performance, so savings on enery, so you'll end up with lots of extra money for frosty cold ones than your neighbor will :-)

    Minisplits can modulate, so they're better about cycling than a conventional unit, but ideal is still to always be running and not cycling on/off too frequently. Using two units may help to balance the temperature in the space (assuming you have them at opposite ends of the room and not right next to each other), and they give you some redundancy in case of a failure in one of the units. Just make sure to size things approprately -- you don't want to have a "way too big" unit.

    You need to make sure the units don't fight each other if you have more than one in the same space too.


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