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Community and Q&A

How are basement losses and infiltration accounted for with minisplits?

kjmass1 | Posted in General Questions on

I had MassSave finish blowing in insulation on our second floor walls this past week so it’s nice to finally have the whole house minus basement insulated. It was a multi-year process but we’ve made progress after buying a home with no insulation in the plaster walls or attic (1940s era, 2000sf). I’d guess we were close to 1800 btu’s/degree hour a couple years ago and running the numbers now we’re getting down to 500 btu’s/degree hour+infiltration. Much improved but a ways to go. 

We have huge basement loses that we plan to address with CCF prior to next winter. My question is how the basement losses and infiltration are accounted for if we used our minisplits for heating as it stands right now. If we kept the basement cold/boiler off and only wanted to heat the 1st and second floors, how much of the basement losses and infiltration gets added to the heatloss load? It’s pretty leaky down there so I’m assuming it has to get accounted for somehow. I’m completely guessing on infiltration as 3 floors total volume which I read somewhere as a rough ballpark without doing a blower door. 

Thanks again for this site- it is invaluable to homeowners. 


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  1. Expert Member
    Dana Dorsett | | #1

    Your scratch pad shows 21,600 BTU/hr of infiltration loss- how was that derived? Simply from the WAG on the total infiltration? Working backward you're assuming ~320 cfm of natural infiltration? (Seems high if the upper floors have been tightened up with blown cellulose.) Only count the infiltration losses for the floors actively heated- ignore the basement infiltration.

    Unless the windows are really extra-leaky, the industry standard window leakage at some standard wind speed 0.30cfm per square foot of window on each floor. (Tight replacement windows and best-in-class storm windows can be lower than that.) Rules-of-thumb wise on a house like yours use ~5 cfm per window WAG based on the following rationale: A typical early 20th century double hung is about 12 square feet- round up to 4 cfm of leakage per window, than add another 1cfm per window to account for other random leaks and it'll at least be in the ball park. Your random leakage through walls is effectively nil after blowing all the cavities with cellulose, but it's hard to say how much leakage there is at exhaust fans, plumbing chases, partition wall cavity framing penetrations, etc.

    The calculation crib sheet also shows 44 BTU/degree-hour of slab losses, but if it's ever +9F under the slab I suspect you'd have frozen pipes and a frost-heaved buckled slab and ahouse that's falling down. The slab losses are more of a constant based on the seasonal low SOIL temp, not the outdoor temp. I suspect the dirt under the slab isn't dropping much below 50F anywhere. If you spot-check the slab temp in a handful of places to get a reasonable average and compare that to the temperature of the exposed subfloor in the basement ceiling ( assuming it isn't insulated between the joists) and the average basement temp we could make a better estimate of your actual slab losses.

    When the standby losses of the boiler go away the basement temp will be cooler, pulling at least some heat from the first floor, but if/when the basement drops below 50F it will also be pulling some heat from the soil below, so unless the air leakage in the basement is truly HUGE the basement temps probably won't drop below 40F. If one assumes the subfloor, flooring floor coverings and air films add up to about ~R3 as a "whole-floor R" and a basement temp of 45F, with a 65F floor above (70F room temp at the table tops) that's a 20F delta-T on a U-factor of 0.33 which at 900 square feet of floor would be ~6K of additional load to the first floor.

    If you do a more formal U-factor calculation on the first floor assume the air films below and above and below add up to about R1, and any rugs cabinets on top of the floor also have thermal resistance that can't be ignored. The 6K of floor losses are probably overstated.

    1. kjmass1 | | #2

      Ignore my infiltration- wild guess based on volume.

      That makes a lot more sense, thanks Dana. Is the 5cfm per window based on original wood double hung? I have mid-grade Harvey replacements. I'll properly measure them up but ballpark 24 windows *5*63*1.08 = 8,100 BTUs or ~1/2 that per floor?

      Totally missed the slab detail, thanks for catching that. The basement footprint is about 1100sf so call that 7,200 BTUs unheated.

      If I got those basement walls to R-21, I imagine the delta in the basement as well as the losses would be pretty small in comparison, down to 1-2,000 BTUs?

      I have a 12k on the first floor (part of 4 zone multi) that struggles in heating the entire floor (obvious now). With these revisions it looks like I'm around 16,000 BTU for first and second floors, and another ~15,000 BTU in window infiltration and basement losses. With the larger first floor than the second, and higher heating demand call it 60/40 on the first floor I could easily see a 20,000 BTU load for the first floor @ 9F. If can get those basement losses under control I'm probably in reach most of the season with the splits if not I could add another head to split the large area load.

      1. Expert Member
        Dana Dorsett | | #3

        If the replacement windows are feel pretty tight and the old sash weight pockets were reasonably sealed and insulated you might back off to 4 cfm per window as the ball park WAG.

        For a dozen 5cfm windows on the first floor you're looking at 60 cfm, 3600 cubic feet per hour, x 63F delta x 0.018 BTU/cubic foot per degree= 4082 BTU/hr of infiltration loss. If they're new-ish tight-ish replacement windows, at 4 cfm per window jit comes to 3266 BTU/hr of infiltration loss for that floor.

        Run the load numbers either room by room or floor by floor, since that's how you're zoning it. With a conditioned floor above it the first floor has no ceiling losses, and the WAG on floor losses we've already discussed.

        With the basement walls at R21 and air tight the basement probably won't drop below 55F, but it depends. Is there a gas fired water heater down there? Are there any uninsulated hot water pipes? I have enough standby and distribution losses from my hydronic heating system and hot water that my ~R17-ish insulated basement never drops below ~65F in winter, but it would be cooler at the wintertime nadir if I were heating with mini-splits.

        1. kjmass1 | | #4

          The windows definitely don’t feel drafty, I’ve been impressed with them. Only 4 years old. There is a nat gas DHW tank and uninsulated pex lines throughout the basement. Steam boiler and mains are the only other source of heat down there. Eventually we are going to finish it off but looks like I should have some good options when the times come.

          Appreciate the help.


  2. kjmass1 | | #5


    I updated my worksheet after measuring up the windows which came to about 6,100 BTUs. With a current 1st + 2nd floor load of 525BTU/degree hr, I thought yesterday (Boston) would be a good test to see if my 12k Fujitsu split centrally located on the first floor could keep up with the load overnight.

    Outside went 60F-38F in the morning, or ~24 degree hours (72 interior - 48F exterior, logged from ecobee data). 24 x 525 BTUs = 12,600 BTUs for the whole house, or around 8,000 BTUs just for the first floor. First floor dropped 3-4 degrees overnight, second floor dropped around 1.

    Am I asking too much from one heat source at these temps or square footage? I have additional multi-splits on second floor but am guessing I'll need another one on the first floor even with an insulated basement to get good heat distribution, unless I converted that split to a miniduct feeding first floor from the basement below.


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