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Community and Q&A

Advice on heating a basement with a ductless mini-split

simpsoap993 | Posted in General Questions on

I’m looking to add some heat to our basement so we can enjoy it more in the winter months when we can’t get outside with the kiddo. Our current basement has 10′ ceilings, poured concrete walls with 2″ foam attached to them and then studs. We plan to either drywall or put some sort of high-end paneling up on the walls to “semi-finish” the space, even though I’m going to leave the ceiling open. We only have electric heat in our neighborhood, so I’m wondering since the basement is below grade, can we get away with a single head from Mitsubishi to heat the space? We would essentially only be using it in the winter and would likely only turn it on when we plan to use the space. The basement is roughly 1400 sq ft – looking for advice? Perhaps a 9k, 12k or 15k head would do the trick to keep it comfortable around 67-70 degrees?

Thanks in advance!


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  1. GBA Editor
    Martin Holladay | | #1

    The usual advice applies: When designing a heating system, the first step is to perform a heat load calculation. Here are links to relevant articles:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    Who Can Perform My Load Calculations?

    We would need to know lots more information to help you. For starters, we would need to know your geographical location or climate zone. We would also need information on any windows and doors (sizes, glazing specs, etc.).

  2. Expert Member
    Dana Dorsett | | #2

    In the lower 48 states of the US almost any insulated 1400' basement that is mostly below grade will be within the capacity range of a Mitsubishi 9K ductless head at +5F. But if your basement is in Fairbanks AK it won't cut it, since the mid winter outdoor temperatures are too low for the compressor to operate. What's your 99% outside design temperature?

    In some climates it's going to be worth putting some fiberglass or rock wool in the 2x4 wall, tight to the foam-board. The U-factor of the 2" foam + foundation + drywall is going to be about U0.1 BTU/hr per square foot per degree-F. Insulating the studwall will cut that in half.

    Assuming you have 160' of perimeter with 2' of exposure above grade (320 square feet) and an outside design temp of 0F, inside design temp of 70F, the main driver of the load is the above grade wall, which has a heat loss of:

    U0.1 x 320' x (70F- 0F) = 2240BTU/hr.

    There will, be some losses from the windows and through the, below grade walls & slab, plus some air infiltration, but all of that won't add up to twice that number (unless you have permafrost under an uninsulated slab), so you're probably looking at a 0F load of not much more than 3 x 2240 BTU/hr= 6720 BTU/hr. Round up, call it 7000 BTU/hr to make the next part easier:

    That's on the order of 10 BTU/hr per degree below 70F, so at 47F the load is probably less than (70F - 47F) x 10 BTU/hr = 2300 BTU/hr, and that's probably on the high side of reality.

    Not all 9K Mitsubishi's are create equal, and depending on where your load numbers come out that may be important. The one that can modulate LOWEST with the best modulation range is the FH09 at ~1700 BTU/hr @ +47F, when married to a compressor that can also modulate that low. (It can't go that low with a multi-head compressor if it's the only head running, but can if the net load from all heads add up to the minimum output of the compressor, or higher.) A mini-split that can modulate down to 1700 BTU/hr would still operate in an efficient and modulating mode with that much load, but one that only modulates down to 3000 BTU/hr @ 47F would be cycling on/off pretty much any time it's above 40F out (or even colder) , taking a toll on both efficiency and comfort.

    The minimum output of the GL09NA and GE09NA @ +47F is 4500 BTU/hr, which means it would be cycling rather than modulating almost all of the time, even in a US climate zone 5 climate.

    As a general rule, a mini-split will deliver higher efficiency if it's allowed to run all the time during the heating season, not just when you plan to use the space. If the floor above is heated with resistance electricity, and not with a heat pump, keeping the basement at 70F with the mini-split will lower total amount of power used for heating the whole house.

  3. simpsoap993 | | #3

    Dana/Martin -

    Thank you both for your replies. To provide a little more information, I'm located in Ohio, so climate zone 5, as I'm in central Ohio. I have two block windows in our basement, on the west wall. 2' of block is above ground, the rest is below. The rim joists have spray foam sprayed around the perimeter, and on the east wall, above grade, is where our attached garage is located. The only door is the entrance door from the first floor.

    Last year we had a new HVAC system installed, and had a multi-head Mitsubishi system installed on our first and second floor, it's one of the Hyper-Heat units. After the kinks were worked out, we have been very pleased with the system, which is why we thought about the idea of getting a single head for the basement. I won't use the previous contractor again due to some issues that I'd rather not get into on here.

    I can further insulate the area between the studs and foam board if that is recommended, I don't have a problem with that. It sounds like from what Dana is saying, is that it is best to leave the unit running, rather than cycle it on/off when we are going to use it. If that is the case, it is recommended to just leave it at 70F, or if we aren't going to be in the space, would you suggest turning the temp down to 60-64F?

    Thanks again!


  4. Expert Member
    Dana Dorsett | | #4

    If adding a head to the existing system either an FH06 or FH09 will probably fill the bill and would make the most sense. If it's a new and separate mini-split with it's own compressor, they both will modulate down to 1700 BTU/hr @ 47F, and it's less than a $200 upcharge for the FH09 over the FH06, which should have plenty of margin even for Polar Vortex events.

    IRC code min for zone 5 foundations is R15 continuous insulation, so with only 2" of foam you're a bit shy of the mark. Adding R13-R15 (unfaced or kraft faced) to the studwall will knock off 1000 BTU/hr or more from the total load, and even if the 2" foam is EPS (R8-ish) it will have more than adequate dew point control on the above grade section for mitigating against wintertime condensation without using interior vapor retarders (other than standard interior latex paint.) R5 would even be enough in your climate.

    Using daily/nightly setbacks with modulating ductless systems almost always results in higher energy use. Only when you're going away for the weekend, or only use the basement on weekends, or any time the set back period is measured in days rather than hours, would turning the temperature down on save power use.

  5. simpsoap993 | | #5

    Dana -

    this would be a new install, as the branch box is currently installed in our attic, and I don't want an additional line set running on the exterior of the house. I have a local account here in town, so the up charge for the FH09 isn't a big deal. It sounds like from what you are saying, a 9k head would be sufficient and doesn't make sense to spend the additional funds on a 12k or 15k head. Based upon this link, it looks like basement walls need an R13 value in my climate zone.


  6. Expert Member
    Dana Dorsett | | #6

    Under IRC 2009 a continuous R10 or 2x4/R13 met code minimum for zone 5. Under IRC 2015 that's been bumped up to R15 continuous insulation, or 2x6/R19:

    More than just the additional funds, bumping up to an FH12 or FH15 would result in LOWER efficiency & comfort due to much higher minimum-modulation levels, of 3700 BTU/hr and 5150 BTU/hr respectively, (@ 47F), compared to 1600 BTU/hr for the FH06 & FH09:

    There has to be enough load at average winter conditions to keep it in the modulation range to have any kind of efficiency, and you mostly likely don't have enough load to make the FH12 or FH15 modulate very much even in mid-winter. The mean outdoor temp in central OH, say, Columbus is north of 25F even in January. If the ~7000K @ 0F WAG is close to reality, the average load in January will be less than 4500 BTU/hr, and much lower during shoulder seasons:

    If you calculate the load more carefully it's likely that the FH06 might still make it, but might not during rare days when it's rime-icing fog at single-digits outdoor temps, where it would be spending too much time in defrost mode. The FH09 would have a better shot at covering the load even under heavy defrost/low temp conditions with it's marginally higher capacity, and would probably cover the load at normal cold-temperature outdoor humidity conditions to well below 0F, even into negative double digits.

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