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Community and Q&A

Minisplit Sizing for Basement

jekeddy | Posted in General Questions on


Looking for the experts to lend a hand. I’m  installing a minisplit in my basement and trying to determine which Mitsubishi unit will best meet my needs. My location is Halifax, NS, Canada. My main concern is re: square footage, but I’ve included a few other questions as well.

The two options are:

MSZ-GL12NA or 

I attempted to do a manual J calculation and determined my heating BTU needs at 5F are 9200 btu/hr for 585 square feet.

One question I have is the basement consists of a main rec room, bathroom, bedroom, mechanical room, and storage closet. Should I be including the last 4 in the square footage? Only some of them?

The mechanical room and storage closet doors are always shut while the bedroom and bathroom doors are always open so I only included the bedroom and bathroom, but if I should have added in the mechanical room and closet then I think the FH12NA is the only option as the additional square footage will increase the btu’s. I’ve made a rough attempt to draw the layout and the red circles indicate where I’m considering putting the minisplit (only one of the red circles will be used). Also there is a stairwell that leads up to the main floor that I factored into the square footage. There is no door at the bottom of the stairs, but there is at the top.

The max output for the MSZ-GL12NA at 5F is 10200 btu/hr while the MSZ-FH12NA is 14000 btu/hr. 

Even if I’ve used the proper square footage, another consideration is the defrost cycle. In another post I read to estimate a worst case reduction of 5-10% of hourly btu’s to allow for defrost and assuming 10%, the GL12 is almost exactly 9200 btu’s at 5F. Would that be fine?

In the event it matters, the minimum 48F rating of the GL is 2000 btu and the FH is 3700. At 40F (typical daytime temps in the winter) my needs would be 3900 btu. At 48F it would be 2900 btu’s (less than the FH), however, I am less concerned about the warmer temps because I suspect I will turn the minisplit off at that point. I believe the FH would not cycle on/off at 40F given the basement requires more than the minimum of the unit, but please advise if you believe otherwise. There are minimal windows in the basement so there would be next to no solar gain.

Appreciate any help I can get.

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  1. Expert Member
    Akos | | #1

    Why not a MUZ-FH09NAH? Should be just about right:!/product/34469/7/25000///0

    Something doesn't seem right on your basement heat load. 9000 BTU for a 600sqft basement seems on the high side. Maybe with older leaky single pane windows. Even if that is the case, fixing the air leaks first would be a much better for comfort and cost less than a bigger unit.

  2. mgensler | | #2

    Our finished basement is 1200 SQ feet. Design temp is 9 degrees. We have an additional 400 sq feet of unfinished mechanical/ laundry room within the basement envelope. We also have lots of glass and a french door. We insulated rim joists and about 25% of the foundation. The rest is brick + cmu. Hoise was built in 1960. We used a Fujitsu 9k cold climate slim duct. It is way more than enough even on the coldest days. We get large heat pockets at the top of the stairs even though we don't have a door.

  3. jekeddy | | #3


    I'm 100% an amateur so I know I'm making errors somewhere along the way. Something seems really off as now I'm up to 11.5k for heat (windows and infiltration making a substantial part of it). The home was built in 2007 and I feel like btu's should be less, not more, given the two comments I've received. I set the infiltration as "semi-tight" with options being tight, semi-tight, average, semi-loose, and loose.

    I've been using "Coolcalc" and attaching the report in case there is any advice folks can offer as to what I'm doing "wrong". It's entirely focused on my basement and doesn't include anything to do with 1st/2nd floor.

  4. mgensler | | #4

    I checked the manual j I did for our basement using ACCA software. It was 12933 btu/h for heating. I set the air changes per hour to 1.0 for the whole house for manual j purposes. House tested at 6 ACH on the last blower door before we did some air sealing. However, I did a fuel based calculation for the whole house and just used the basement as a % of that number. Basement was 16% of the total heating load. Based on upgrades we were planning, I estimated the load would be 9,973 btu/h. I spec'd the ARU9RLF cold climate Fujitsu for the basement which actually outputs 15,700 btu/h at 5 degrees. Minimum turndown is around 3,000 btu/h. Basement is now the warmest room of the house.

    Manual J has lots of cushion in it. If you heat with gas or oil now you can use that to figure out precisely what the heat load is for the whole house and then use manual j to figure out what each room is as a % of the total load.

  5. Deleted | | #5


  6. jekeddy | | #6

    Thank-you very much for all the help. Although I do heat with oil right now, the challenge is that it's also heating my hot water and is the back-up heat source for the upper two levels so I can't figure out what amount of oil is dedicated to heating the basement.

    Re: the mechanical room I mentioned above that I did not factor it into my calculations because it is a space I do not wish to heat with the minisplit and it is separated from the main space by a wall. The mechanical room walls have the lower half as exposed concrete foundation and the upper half being "finished" with insulation and drywall. Note that the wall separating the utility and rec room is just an uninsulated wall (e.g. a sheet of drywall and a door), should I be adding the mechanical room into my calculations? I don't quite understand whether it meets the criteria of a "partition wall" in CoolCalc. And I definitely will consider insulating the separating wall down the road.

    FYI I did reach out to CoolCalc support, but awaiting their response.

  7. mgensler | | #7

    I double checked the manual j and I did include the unfinished mechanical room in the calculation. The total space was 1200 sq feet with 400 unfinished.

    You can subtract off your summer oil consumption and make a guess at how much the upper floors were used. But really, probably the best thing is to focus on turn down. Anything around 3000 BTU at the low end will be fine.

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