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Minisplit data & sizing

mhirschland | Posted in General Questions on


We’re about to install ductless mini splits. We want to use as little electricity as possible and so are interested in single-head units. I’d greatly appreciate your review of our thinking:

House basics: The first floor of our two-story home (1940’s or 1960’s) has 1008 sq. ft. of semi-open space with a limestone exterior. The second floor, also semi-open, is a converted attic with vinyl siding. Our white metal roof covered with 15 solar panels. Although the upstairs is 775 sq. ft., because of the eaves and low ceiling, the cubic footage is just 62% of the downstairs – the equivalent of 665 sq. ft. We’re in climate zone 5 with a heating design temperature of 5 degrees. Getting an accurate manual J done in our small midwestern city by anyone other than a contractor turns out not to be possible.  The house is not very well-insulated and there’s not a lot we can do about it.

Heat load calculation: For each of five relatively cold months, I found the number of heating degree days and our gas usage. I calculated the BTUs / heating degree hour and the design heat load and then made three adjustments:

Old inefficient furnace: I assumed that, after 30 years, our nominally 80% efficient furnace is no more than 75% efficient so I multiplied the BTUs/heating degree hour by .75.

No thermostat setbacks going forward: We’ve been setting our thermostat back to 55 degrees 62% of the time – nine hours nightly and eight hours weekdays. To adjust our BTUs per degree hour to reflect that we won’t use setbacks with the mini splits, I calculated what our therms would have been for the month had we not set our thermostat back. This is how I did this:

(38% x therms for the month) +(62% x (therms for the month x ((65 degrees – avg. exterior temperature for the month) / (avg. temperature that the house reaches during setback – avg. exterior temperature for the month))

Heat load calculation by floor: Because the HVAC ducting in the converted attic is inadequate and there is little insulation in the roof, when the first floor is comfortable in winter, the upstairs is typically very cold – perhaps 10 degrees colder than downstairs. Therefore, to get the heat load for the downstairs, I assumed that just 20% of the household heat is going to the upstairs and multiplied the whole house heat load by 80%. Then, to estimate the heat load for upstairs were the upstairs to be heated to the same comfort level as downstairs, I multiplied the downstairs heat load by 62% – the ratio of cubic footage of upstairs to downstairs. I know that we should be looking at other factors but we have no way to do that. I’m most uncertain about the validity of these calculations.

The calculations for four of the five months yielded similar heat loads for the downstairs: between 22,400 and 23,300 BTUs. In turn, this yielded a heat load of 13,400 to 13,900 BTUs for the upstairs. We therefore are leaning towards one 12K and one 9K Mitsubishi FH for downstairs – max heat load at 5 degrees of 24,500 – and one 12K for upstairs – max heat load at 5 degrees of 13,600.

I’d greatly appreciate thoughts on the calculations, results and tentative decisions on the unit sizing.

Air handling: To see whether our space could be heated evenly enough, we set up three small electric resistance heaters on the first floor where the air handler would go, left the 50-degree space for 8 hours, and then checked the temperature in each room. The range was from 68 to 61 – a differential that we could live with. We then did this on the second floor; the differential was just 2 degrees. Does this make sense?

Thank you.


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  1. maheatpumpguy81 | | #1

    Hi Madi,

    I'll chime in and give your post a bump.

    You can perform a Manual J yourself if you're so inclined. A quick Google search should point you to the tools and knowledge to do so.

    I'd also poke and prod your assumptions some as a sort of sensitivity analysis on your heat load to get a range of heat loads by varying these assumptions. Then you could better understand how your proposed heat pumps would handle these various heat loads. Just a quick math check as well, shouldn't you be multiplying by 9/24, not 62% for your heat setbacks? That setback calc adjustment was a little hard to follow. I assume tho that it bumped up your therms a bit after you did it... maybe by 10-15%?

    Of course, you really should consult a couple insulation contractors for possible solutions to improve your attic/roof insulation. What do you have there now? Is there anyway perhaps to blow-in some insulation between the rafters perhaps (if there's space) by opening some small holes in each rafter bay thru your 2nd fl ceiling? I have my attic rafters sprayfoamed w/ closed cell to R-38 (climate zone 5) and I barely need to use any heat in the winter for my 2nd fl. I set my 1st fl heat pumps to 69F and the 2nd fl holds at 66-67F without using any 2nd fl heads. Of course, I have a nice open stairway leading up to the 2nd fl (colonial house).

    When you did your temp variation test w/ the resistance heaters, did they have fans? I bet you will get better distribution with actual minisplit heads or floor units due to their good CFM fans.

    Lastly, you could always wait to see how the house performs with just the 1st fl mini splits and then decide later what you need for the 2nd fl.

  2. Expert Member
    Peter Engle | | #2


    +1 to the comments above about trying to improve your insulation. It sounds like you've got kneewall spaces, and sometimes those are huge energy hogs that can be significantly improved without being super expensive. That could reduce your second floor heating load by quite a bit.

    You will still need a minisplit upstairs for cooling, though and the 9k unit will work fine for that. Overall, your equipment sizes seem to be in the right range for a poorly insulated house in your climate, but it's still worth doing a more diligent Manual J before you invest in new equipment. You can certainly use one of the free online calculators with the understanding that they almost always oversize equipment. There are also online services that will perform a manual J for you. You send them your floorplans, types of windows and doors, insulation levels, etc. and they do the manual J for you.

  3. mhirschland | | #3

    Dear MAHeatPumpGuy and Pete,

    I can't thank you enough for taking the time and sharing your expertise - so helpful.

    As you suggested, I did some sensitivity analysis and decided to up the size of one of the downstairs heat pumps so that they're both 12K's. Although I love the idea of having a unit that is 31 SEER the sensitivity analysis made me see that there's a chance that we'll need more capacity when it gets to 5 degrees and a look at the weather history here surprised me - that happens more frequently than I'd realized. Getting your sense that these sizes and assumptions seem reasonable is extraordinarily helpful.

    Sorry not to have been clearer about the 62% - we have the heat off at night seven days a week and during the workday five days a week for a total of 103 of 168 hours weekly. Yes, you're on the nose about the setback adjustments: they resulted in precisely a 12% to 15% increase in projected heat load.

    The electric resistance heaters have fans but presumably not as strong as those in the mini-splits. With two downstairs and stronger fans, perhaps the temperature differential will be smaller than expected.

    In the summer, when the downstairs is comfortable (with ceiling fans, we let it get up to 78 or 80), the upstairs gets really hot so we think a mini-split up there will be a real comfort even if it's set to 80 or so.

    Your points about ceiling and kneewall insulation are very well-taken. On your suggestion, we plan to decide on one or two upstairs insulation projects - after we restore heat to our home. You've greatly hastened that much-anticipated moment with your feedback.

    Do you have a recommendation about where to order the units from?

    Thank you so much,


  4. maheatpumpguy81 | | #4

    Hi Madi,

    I'm looking at the NEEP data for the Mitsubishi MUZ-FH09NAH and MUZ-FH12NAH and the 9k appears quite a bit more efficient. I wonder if you could stick with those and make up the shortfall with a few small resistance heaters around the house.

    Here's the link for getting the specs on the heat pumps:

    Ideally, you'll want to compare a Manual J to your fuel-based heat load, vary your assumptions some as you did, and choose somewhere around the median for all the estimates. Then you could cover the gap in heating during extreme periods when the heat pumps can't keep up with resistance heaters or wood stoves. The research I've read supports this approach (see:

    Best of luck.

  5. mhirschland | | #5

    Thank you so much for this suggestion and for passing along the presentations. I'm afraid that we know too little about the insulation and windows to do a manual J. I was fascinated by the Powerpoints and, with their conclusions in mind, very much like your idea of just sticking with a 9K and a 12K downstairs knowing that we'll use electric resistance heaters as backup if needed.

    As we'll be using two mini-splits downstairs to heat the same space (though starting from different rooms), I wonder whether we can turn one off in milder winter / shoulder season weather. That may also help with efficiency.

    Gratefully yours,


    1. willymo | | #6


      You CAN do an approximation of a manual J-; in fact, your already have with your fuel based calc. You can make assumptions about the walls, ceilings, and windows, and then see if the results comport with your earlier calc. That's what I did, and the results came in pretty close, so I had more confidence in them.

      You need heat now, and you may do insulation/weatherization improvements later. I would be very conservative, then, in sizing your units, knowing that in the short term you can always add the electric heaters when it's really cold, and improve the heat-loss in the long run. The last thing you want is to over size now, because that will create a big hit on your efficiecy.


  6. mhirschland | | #7


    Thanks so much for your response. After much help from two members of this listserv who responded earlier and lots of reading, we did do what you're suggesting: conservatively estimated what we need knowing that we can use electric heaters if it gets too cold (and also that we can try to add insulation.)

    In response to what Dana Dorsett often notes - that the systems do not work as efficiently when the temperatures outside are more moderate - I'm wondering whether it makes sense to use just one of the downstairs units when the temperature is above say 40.

    Thanks for your suggestions!


    1. STEPHEN SHEEHY | | #8

      If you haven't already installed them, maybe just install one downstairs and see how it works. A thousand square feet isn't a big space. If it isn't enough, you can always add the second one.

  7. mhirschland | | #9

    Thanks, Steven. Much appreciated.

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