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Sizing a Minisplit System

1910duplex | Posted in Mechanicals on

Hi all,

so I hired an out-of-town engineer to do a Manual J, but forgot that the blower door number I had was before we sprayfoamed attic/top of balloon framing plates, replaced basement back door that was not meant to be an exterior door, and went from no insulation in attic to r-34ish (and polyiso strips on rafters).

Still, their manual J isn’t far off the estimate of ‘Out with the Old, in With the New.”

Their estimate for my heating upstairs, half my finished square footage, is 30,468 btu/hr (before ducts are considered; currently I have radiator heat)
My estimate for whole house is 45,883 using the 1.4 multiple, using actual natural gas use data (including hot water/stove draw; without that, it’s about 42k). It’s a duplex, with no west windows, most windows on east.

One of the HVAC quotes I have for a ducted minisplit to serve the upstairs is for a Daikin unit whose submittal data says it can provide between 5,500 and 24,000 in cooling and 6,700 and 26,000 btu per hour in heating at 47 degrees

Here’s my question. I have radiator heat, and am keeping it for now, though one day I might go to an all-electric house. We are not putting any minisplits on first floor right now (though we’ll see what it feels like having to close up the first floor because the second floor minisplits are running.)

The manual J estimates I need 25,462 for cooling, combining sensible and latent (latent is about a third of the load in this 4a Washington, D.C. climate).

So is that Daikin an appropriate size, do you think?

Also, is it important to have a register in the hall near the bathroom for future-proofing, if I do want to use it for heat one day? I don’t care about it for cooling now, though the sales guy said it could help cool the downstairs, since it would be at the top of the stairs. (Bathroom has a radiator).



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  1. Expert Member
    Dana Dorsett | | #1

    That model is capable delivering 27,600 BTU/hr of cooling @ 95F, almost 32K @ 82F. See:!/product/25264

    The 1% outside design temp at the DC airports are ~91F with a coincident wet bulb temp of ~75F, so it should be handle all but the most torrid-sticky days. Sometimes it's better/more efficient/more comfortable to use a dehumidifier to offload some of the latent load rather than upsize a modulating heat pump and ONLY run it in a less efficient "DRY" or "DEHUMIDIFY" mode. A standalone dehumidifier fully inside the house will convert the latent load to sensible load and should only be used when the AC can't handle it. Most of the time the AC will handle it on hot days even in standard cooling mode, but it's fine to set it to the less efficient drying mode on days when it's too sticky.

    For heating it's nowhere near big enough to support a 30.5K load at your 99% outside design temp of 16-20F. It's max output @ +17F is only 17,250 BTU/hr. The fall off in capacity is an indication that it probably isn't using an enhanced vapor injection compressor of ilk used by most cold-climate mini-splits. The 2-ton cold climate Carrier/Bryant (= Midea) 2 ton ducted mini-split that comes close:!/product/36688

    A 3 ton Carrier/Bryant/Midea would be more appropriate, assuming the load can't be reduced, and still modulates low enough in cooling mode to run most of the time- it modulates even lower than the 2 ton version across temperature, heating or cooling:!/product/30285

    Unlike the Daikin, those units can be mounted vertically in an upflow orientation, which gives you some options.

    Fujitsu has a similar line of mid-static compact duct cassettes, eg:!/product/25352!/product/25351 < their 2.5 tonner modulates lower than their 3 tonner, and is only 300 BTU shy of your calculated load @ +17F.

    With load numbers as big as yours it sounds like either a very large or uninsulated antique house (in a historical district?) with only single pane windows or something? (With tight code min new construction a 3 ton cold climate heat pump should be able to handle ~3500-4000 square feet of conditioned space, often more.) If you're stuck with antique single pane windows even historical buildings can be retrofitted with hard coat low-E single pane storm windows that take down the window heat gains & losses by more than half.

    Are the ducts going to be in an attic, above the insulation layer? (A bad configuration, but sometimes unavoidable in retrofits.) If yes, do NOT add unnecessary registers for potential later use- that just makes the house & ducts more leaky/less efficient. If they are all fully inside the thermal and pressure boundary of the house it's fine to install a duct & register, but also install a balancing vane right at the take off to the plenum, turn the balancing vane all the way to the off position, keeping the static pressure in the plenum a bit higher.

  2. 1910duplex | | #2

    Yes, antique house, 1910, original windows in all but the now-enclosed sleeping porch and front porch, but it does have new low-e Larson Gold storm windows over most of the windows now (and replacing a couple more 1970s era storms with those this year.

    It is not large at all -- just 1300 sq ft on two floors (plus unfinished basement and walk up unfinished attic and those porches)! But no insulation in the stucco-clad walls.

    The attic is insulated at the roofline, so the attic temps only vary by a few degrees from the second floor. So that hallway register is okay then?

    I am surprised you're recommending the Carrier/Midea, I had thought y'all felt those aren't as high quality as the Fijitsu/Daikin/Mitsubishi. Also, the link says that's a non-ducted version.

    The other equipment suggestion from this same contractor is the fujitsu AOU30RGLX.

    Another bid suggested Mitsubishi. SUZ/SEZ-KD18NA Mitsubishi 18,000 BTU Low Static Ducted Heat Pump System
    Mitsubishi SUZ/SEZ-KD18NA

    Since we're definitely not going to go natural-gas-free until we replace the hot water heater with a heat pump water heater, and I think we're nine years away from needing to replace the hot water heater, I guess we could upgrade to a cold-weather system in 10 or 12 years. Coldest night of the year has been 25 degrees, though we did hit 22 in February 2020.

  3. 1910duplex | | #3

    bump for additional thoughts?

  4. kyle_r | | #4

    Could you provide a floor plan with room by room loads? Is each floor 1,300 sqft or total? Either way I can’t see how your load numbers aren’t orders of magnitude high. Do you have the details of the original Manual J calculation?

    It sounds like you want to put in a ducted mini split in your conditioned but unfinished attic to provide air conditioning for the upstairs and maybe some day heat. I would suggest having another Manual J done by an HVAC design firm, not installer.

    I would bet you could put a Fujitsu 18k RLFCD unit and have more than enough capacity for heating and cooling for the second floor (and probably the first floor). Although I’m sure it would be easier to handle the first floor with its own equipment in the future.

  5. 1910duplex | | #5

    Here's the floor plan

    Only 599 sq ft upstairs. (though I think that doesn't include the enclosed 12 by 5 ft back porch)

    The manual J was done by an HVAC designer, not installer. The manual J is not fully accurate because I gave them a blower door number that was done a few years ago before we did some pretty significant improvements, such as 2 inches of closed cell spray foam at attic roof deck/spray foam to airseal top of balloon framing, and then R23 rockwool below flash at roofline, along with polyiso furring along rafters & drywall over eave walls... also, replaced a wooden backdoor in basement that was not even supposed to be an exterior door with a real back door with insulated core, and replaced a piece of plexiglas in one of the spots where there should have been a basement window with an actual basement window.

    That said, doing the calculations suggested by Dana based on actual natural gas use, along with boiler efficiency and degree days for the billing period, I used 30,109 btu per hour for both floors (backed out use for hot water heater by subtracting therms used in August). For that month, it was set at 63 overnight, 66 in morning, 70 degrees about 12 hours a day. With ASHRAE 1.4 multiple, that adds up to a 42k heat load for entire space. We keep it at 68 degrees on thermostat (which means 66 degrees upstairs) when it's just us, but we're foster parents, and when we have kids, we're required to keep at 70.

    The upstairs bedrooms do tend to be about two to three degrees cooler than first floor with current boiler/radiator heat, this was a subject of a Q&A spotlight, fwiw

    The room by room Manual J, heating first, then cooling; upstairs only as I'm only adding minisplits upstairs at this point, for cooling.

    1,444 for the bathroom
    11,309 for north facing bedroom 189 sq ft (including wall of closet)
    8,219 for east facing bedroom 158 sq ft
    9353 for east bedroom that borders on south-facing sleeping porch, unconditioned, 109 sq ft
    hallway 145, 93 sq ft

    cooling load:
    646 bathroom
    6066 north bedroom
    5875 second bedroom
    4837 small third bedroom
    68 hall

    The plan is for a ducted minisplit, yes, with ducts in the attic. Attic does not have heating or cooling, so I guess technically it's semi-conditioned? Yesterday, a sunny day when the high was 89 here, it ran six degrees hotter than that front north bedroom.

  6. kyle_r | | #6

    I would start by searching around!/product_list/
    It’s a database with tons of mini split specs.

    I would get quotes from Fujitsu, Mitsubishi, and Carrier installers. I would have to go through the database again, but I believe the 18k Fujitsu would cover your load plus give you the lowest modulation in case your actual load is much lower than your Manual J.

    The Fujitsu and Carrier can be mounted vertically, but in your case with the attic, probably not a concern. The Carrier has a pan heater, but again in your location probably not a concern.

  7. 1910duplex | | #7

    Thanks, I have gotten quotes from Fujitsu, Daikin, Mitsubishi installers; tried to get one from a Carrier installer, but he never submitted. The company that does Daikin or Fujitsu recommended 24k or 30k because that's what the Manual J says. If I want to get off natural gas in the next 15 years, however, I do need the larger unit for my heat loads. (and I would need to add a ductless downstairs)

    The company that does Mitsubishi recommended 18k. SUZ/SEZ-KD18NA Mitsubishi 18,000 BTU Low Static Ducted Heat Pump System

  8. kyle_r | | #8

    The 18k Fujitsu I mentioned can do 21k BTU/hr at 17F outdoor temperature.

  9. 1910duplex | | #9


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