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Ducted Minisplit Sizing

woodguy00 | Posted in General Questions on

Planning to build in NW Arkansas, Zone 4.  Running the plans for a 1830 ft2 single story over a hillside walk out basement / conditioned crawl space through the Better Built NW Manual J calculations with no fudging, I’m coming up with a heating load of 27,500 and a cooling load of 15,100. The various Gree or Midea variable ducted minisplits have  either 24K or 36K capacity.  Which direction would you go?  Slightly undersized for heating (2 ton)or significantly oversized for cooling (3 ton).

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Replies

  1. paul_wiedefeld | | #1

    Two ton with the smallest resistance element you can find! You might still be oversized with the two ton, manual J's are often high.

  2. Expert Member
    Akos | | #2

    BTU/sqft is not the best measure, but 15BTU/sqft in Zone 4 means there are is a lot of low hanging fruit in the design for a new build. You should be close to or under 10. I would take a closer look the output from the Betterbuilt tool and see where you can trim losses.

  3. onslow | | #3

    woodguy,

    I will second Akos in urging a revisit to the parameters you fed into the program. A very rough investigation of design temps for Madison county Arkansas shows a heating target of 15F, Fulton county the lowest at 14F. Assuming a square-ish foot print of 42x43' and 9' of wall exposure with a total of 400sf of glass, I blended the walls and ceiling sf to R24 and windows at U.35 ( a bit off to current code) to get ~6300 btu for walls and ceiling load, 7000 for windows at delta T50. So 13,300 total and ~7.25 btu sf.

    I have attached an Energy Star chart link your reference. Too big to attach. It can guide you to local weather resources names if you want to get finer details. So unless you have some very odd features in your design, I can't see how you would see almost double the load unless you leave a door open.

    With more specifics it might be possible to get more specific ideas of where to tighten up.

    https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20%282019%20Ed%29%20-%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf

  4. woodguy00 | | #4

    Thanks for the replies and advice.
    I believed I was using accurate info on my home input
    -1830 ft2 on main, 1830 basement
    -R44 ceiling, R21 walls, 9' walls, three doors
    - 484 ft2 of .28U windows which included a 96 ft2 8x12 double patio door on N side
    - ducts in conditioned space. .2 ACH winter, .15 ACH summer infiltration
    - 13 degree heating target, 93 cooling
    - 25 CFM ventilation

    Output winter loads show:
    Windows 6700
    Doors 700
    Walls 9700
    Ceiling 2300
    Floors 2800
    Ventilation 1500
    Infiltration 4300
    Total 28000 of which 8700 was from the basement/crawlspace

    Thoughts?

    1. Expert Member
      Akos | | #7

      They really went out of the way to make the exterior wall to floor area ratio extra high. I understand not wanting to live in a box house but there is also no need for so many jogs in the wall. I guess it is not as bad as some of the U or Z shaped floor plans.

      Without exterior insulation, your framing factor is so high that it doesn't matter what you insulate the walls with as most of the wall area will be lumber. All those transitions need extra care to air seal (or extra leaky if not taken care of properly). Exterior insulation is hard to justify in warmer climate, so not saying you should go for it but I would take a bit of care and try to reduce the framing factor where possible. 24" OC can also help.

      If you are really married to this design, then it is what it is. I would take extra care with bed2 as it has 3 exterior walls. Make sure to size the ducting here correctly as it needs more heat than the rest of the house and most likely will be the longest run.

      I don't know which direction the house is facing, even if you are not doing it now, it is a good idea to make the house mostly PV ready. For best efficiency you want the south and west sections covered in PV. Best to avoid things like low plumbing vents or chimneys that can create a lot of shading. With a hip roof, widening the ridge can significantly increase the usable area without effecting the look of the house.

  5. onslow | | #5

    Woodguy,

    Sorry didn't mean to sound like I was scolding, just my quick back of the envelope calculations pointed to a high estimate being provided by the software you were using. For a quick guide to how well you are doing already, a published "quick guide" to heating requirements proposes that CZ 4 homes need to have 45-50 BTU per sf. They shall remain nameless, but it does help explain the crazy numbers people say they have gotten.

    I started a re-run my guesstimates using a new higher Delta T of 57, the difference between 70F and your chosen 13F design temp. I would normally try to use a 65F interior value to help adjust for the over sizing problem created by frequently unrealistic design temps seen in many jurisdictions. It has always struck me as unhelpful at best to force system designs to accommodate conditions that may only encompass a week or two. Then I remembered you had posted an attachment.

    Oh, dear.

    If you are going with this plan, I can see why the numbers are different from what I was getting. The total wall perimeter is about 209 ft not 170 ft though the outdoor exposure footage is reduced to 149 ft thanks to 60 ft being shared wall with the garage. This does put your exterior wall to glass area relatively heavy on glass. The effect may be more pronounced on your cooling load depending on final orientation of the house. More on your 9000 BTU wall number later. The window number seems low.

    The balance of my calculations/guess, which did come in only slightly lower than yours is further rendered moot by the floor plan. I own a house of very similar floor plan and can see that this one also has the furnace and water heater located in the garage. The plan as shown is suited for either slab or very short crawl space construction. The crawl space version does allow for duct work to distribute below the floor deck, but it will almost certainly be planning for a vast duct-o-puss of flex to live amongst trusses. Even with the amount of cellulose I have, the duct work is not really protected well.

    More to the point is your inquiring about using ASHP for your build. Not sure what you were intending. One head in the great room, kitchen, dining may be successful for heat though others familiar with high humidity environs can guide you better for A/C. The complex ceiling variations might be a significant problem. A ducted ceiling unit can provide good results with short runs and clustered rooms. ASHP use is laudable, but this is a tricky home plan. The bedrooms are scattered and isolated, especially #3/office. The 11' entry ceiling and the coffered living room ceiling will generate truss profiles that will greatly impede easy duct work. What happens over the garage area will greatly influence the attic area over the living floor space. Placing an even layer of R-44 in this attic will take care. Summer sun will not be your friend.

    An air handling unit being fed by an ASHP would mesh with the existing plan more quickly, but I would not want to attempt a load calculation remotely. I am not sure if any of the available energy calculation software can really compensate for the duct runs and attic locations realistically. Another fly in the ointment is your apparent plan to have a partial basement with walkout due to a sloping lot.

    I was at first astounded by the 9000 BTU wall number, but finally realized it was combining first floor wall load with basement wall loads. Same for the floor load number. I am going to guess that you are planning on R-10 under slab with R-10 on the foundation perimeter. You will certainly be better served by including the crawl as conditioned space, especially if going with duct work below the first floor.

    All in all, this is a much more tricky analysis problem than first appeared. Your total load number is pretty close to what I got through the minimum case, which did not include a garage space. The summer heat load you are facing with the extra ~600 sf of roof area needs to be look at closely. The shared wall area with the garage will slightly reduce your heat load. The protected portion of the foundation crawl wall will gain you a bit of relief as well.

    There are other features to look at such as where will you be putting in stairs to the basement and final orientation of the house relative to trees, if any. One thing to remember when entering R values into software programs is whether the software automatically de-rates the entered value based on framing factors and such. Your R-21 wall entry may mean R-21 batts in a 2x6 wall or some combination of framing and exterior foam performing as a whole wall R-21. IF the software is assuming framing factors your effective whole wall R value will be lower. Thanks to the irregular perimeter profile and frequent windows I suspect your framing factor will be higher than for a box done with advanced framing.

  6. woodguy00 | | #6

    Onslow,
    Thanks for your detailed reply. It is very helpful. Never thought you were scolding - I'm just confused that you had shown much better numbers. Let me provide a bit more clarification on my plans
    - I plan to put the air handler and all ducts in the conditioned basement/crawlspace. I am thinking a traditional ducted air handler and a horizontal discharge outdoor unit like the Gree Flexx. I'm thinking to put one register and return in the basement to condition the air down below.
    - No internal steps to lower level. It is common here to build this way and just excavate and use a portion of the lower level for a shop area or storage. Balance is just crawl space
    - I plan to insulate the perimeter of the crawl with R10 foam where it is block and R21 batts in the 2x6 framed sections.
    - I'll use TJI or similar I joists for the floor system. This should give me room for a central trunk and flex runouts up between the I joists
    - nothing will be in the attic except insulation. I will spec energy heel trusses with R49 in most areas. The areas near the perimeter will be a little less thus the R44 assumption I used
    - I plan to use 2x6 walls with R21 batts. 99% of builds in this area still use 2x4! I agree that the window percentage is a bit high for a high performance house but damn, I like them!
    - In my calculation I used 210 feet of outdoor exposure for the main floor and 210 for the lower level. I didn't realize to cut it back where walls are protected by the garage. I also cut back the exposed footage on the lower level to 80 feet which I estimate to be the above ground exposure
    - with the change above I'm now showing a heat load of 23K of which 5K is the basement/crawl load

    Is this making more sense? I appreciate the help!

    https://hvac.betterbuiltnw.com/Site/Results.aspx?BuildingId=12149

    Note: I tried to attach my results printout below but it's showing another participant's file and I can't get rid of it.

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