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Community and Q&A

HVAC sizing / Minisplits versus Multisplits

alan72 | Posted in Mechanicals on

Climate zone 5


I’m interested to know the quick math on heating and cooling loads from any of the many experts who are gracious with their time (I’m talking to you Dana Dorsett). I’m at the point that I feel that I need a third party calculation….


Our build is not a “pretty good house” (unfortunately). Much less… it’s sort of a sprawling ranch -horseshoe-shaped around a patio. 


Single level 4300 sq ft house on a conditioned crawl space that is the foot print of the house. 


Exterior walls are 2×6 (16” oc) with about R9 rigid exterior foam. Fiberglass bibs in the cavities (about R24). 


Unvented roof – about R55


Windows – u factors around 0.18



Crawl  R15 – perimeter – no sub slab insulation

Air sealing details are planned – I’m not sure what we will reasonably get to.


Most roof overhangs are about 3’6” out from the exterior brick cladding


I attached a document with Manual J calculations.  Entire house, not room by room.


Any responses are appreciated. 

BTW – these are from a company that specializes in Mitsubishi heat pumps and Hydron ground source heat pumps.  I have read many articles here on GBA that question the wisdom of installing a GSHP in light of the performance of modern, newer air sourced heat pumps.  Their calculated heat load is 68k btuh and recommends a 5 ton unit due to the distance needed to distribute the air.  He said normally he would recommend a 4 ton unit for that load.






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  1. Expert Member
    Dana Dorsett | | #1

    I'm not an "expert"- don't even have a Wrightsoft license (though I've analyzed dozens of Wrightsoft reports for potential errors of commission/omission.)

    Right off the top: Where oh where in US climate zone 5 is the 99% outside design temperature -10F? What is your ACTUAL 99% outside design temp?

    The 17K of load for a conditioned crawlspace seems a bit on the high side, as does the total load. With 4300 square feet of uninsulated crawlspace floor that might be right if it's fairly conductive soil, but it's probably an overstatement. If playing "what if" scenarios with the load tool I'd be looking to see what adding R4-R8 to the crawlspace floor does for that number. Even as a retrofit it may be cheaper than upsizing a ground source system by a ton.

    The load/area ratio of 16BTU/hr per square foot of conditioned space is pretty high even for a code-min 2x6 framed house, let alone a 2x6/R24 + R9 c.i. type house with U0.18 windows. Even at -10F most code min houses would come in under 14 BTU/hr per square foot, and a better-than code house would likely be under 12, maybe even under 10. The 50,792 BTU/hr exclusive of the 17,386 BTU/hr crawlspace losses is probably closer to your actual total load at -10F, but without the full Manual-J it's hard to do better than a WAG at it.

    An IRC zone 5 code min window has nearly twice the heat loss as a U0.18 window, a code-min wall about a third less than your wall. The differences between all code-minimums and YOUR house adds up to quite a bit. Using an outside design temp 10-15F colder than the 99th percentile temperature bin just piles 15-25% on top of any other errors.

    It has the feel of a hastily done pro-forma Man-J performed by an HVAC contractor using all code-minimum inputs and conservative assumptions, not by an engineer using aggressive assumptions (per the instructions in the Manual) running the U-factor numbers on your non-standard assembly if it didn't show up in a Wrightsoft pull down menu.

    >"...recommends a 5 ton unit due to the distance needed to distribute the air. He said normally he would recommend a 4 ton unit for that load."

    The "... distance needed to distribute the air..." sounds like a Hydron(?) solution. With a Mitsubishi 4 ton unit the refrigerant lines can go quite a distance to multiple air handlers/cassettes/heads to keep any duct runs short. With a big U-shape it may work more optimally to install two or three MVZ air hander zones, one per wing and another in the middle rather than one huge air handler and a mile of trunk & branch duct.

    The 8C48NAHZ is good for 54K @ +5F as listed on the AHRI submittal sheets, and about 43K @ -10F (without actually looking it up), and may cover the load with 2-4 zones of MVZ or SVZ air handlers with appropriately sized auxiliary heat strip in the air handlers to manage the coolth of Polar Vortex disturbance events.

    1. Zdesign | | #2

      Dana, what happens when the build site doesn't fall into any of the set locations on the Outdoor Design Conditions. For example my build site is in Zone 5 in NY, 43 degrees North, and 1600' above sea level. The closest location on the table is 400' above sea level with a design temp of -2 but still 43 Degrees North.

    2. alan72 | | #4

      Thank you for the detailed reply.

      We are in SE Michigan - Northwest suburbs of Detroit. Oakland County.
      - 1% cooling temp 87 (F)
      - 99% heating temp 4deg (F)

      I attached data from Detroit and Flint. I'm closer to Detroit.

      Our elevation is 716' - if that matters...

      I'm thinking I need to hire a third party.


  2. Expert Member
    Dana Dorsett | | #3

    What county?

    Take a look at the elevation of the weather station used.

    If that's near Gloversville (zone 6, not zone 5) , subtracting ~3F per 1000' of elevation adiabatic cooling above the weather station would give you an approximated 99% outside design temp of -5F.

    If the weather station referenced in your case is the Albany Airport (which is in zone 5) , it would be a 4F subtraction since it's at about 300' of elevation, 1300' below your site (3F x 1300'/1000' = 4F), so using (-2F -4F =) -6F would be a reasonable guess.

    Sometimes the local topography is such that the valley floors can be colder than the higher elevations due to more night time radiational cooling in winter, so your actual 99th percential temperature could be as warm as 0F, but it's certainly not -10F. The contractor in Alan's case may have used -10F as a padded WAG to cover it (the opposite of "aggressive assumption") .

    The Albany airport is at 285' of altitude with a 99% design temp of -2F, but the weather station at the Albany City Office used for the county-wide design temp of +1F is at 275', only 10' lower than the airport, not 1000' lower which is potentially an artifact of an urban heat-island.

    The 43 degree latitude hardly moves the needle on heating loads, but is a factor in cooling loads due to the incident angles of the sun and the fraction of heat that is reflected off the exterior surface of the windows, as well as the amount of shading provided by overhangs.

  3. Expert Member
    BILL WICHERS | | #5

    I’m approximately in clarkston/Springfield. I can tell you my area gets a fair bit colder than predicted temperatures often, so we’re in a bit of an island of cold. If you’re north of M59 it’s probably worth using Flint for design temperature values. South of M59 you’re probably safe using Detroit values. Even though it’s the same climate zone, there are some pockets with pretty big differences from the average over the entire area.

    For Dana: the OP is in Oakland county. Flint is in Genesee county (immediately north of Oakland county) if that helps you help the OP. And you’re much more of an expert with this stuff than I am :-)


  4. KeithH | | #6

    At the risk of sounding like a fanboy, building my own load model using was extremely beneficial, not too time consuming, and demonstrated a significantly lower load than the pro manual J I paid for. The room by room detail of each construction detail might also prove useful to you in determining whether some envelope improvements would be superior to the upsizing your GSHP system. Give it awhirl. The worst that happens is that you wasted an evening.

  5. alan72 | | #7

    I decided that it would be worth it to just hire an independent firm to design our heating and cooling system. I'll post back with their design when it's complete. Thanks again.


  6. Deleted | | #8


  7. krom | | #9

    lets try this again...
    I've attached two versions of the same file.. Its a manual J spread sheet.
    Since XLS files aren't allowed on here, one has had the extension changed from .xls to .doc, simply change the extension back and it should work, the other is the same spreadsheet that has been compressed to a .zip file.

    1. alan72 | | #10


      Can u try to upload those again? I think the web site glitch the was messing up attachments may be fixed. Thx!

      1. krom | | #11

        Just tried again by editing the original post. It doesn't look to be working here, for some reason it shows 0 kb, and is pointing to my c drive...
        try this link I uploaded it to my dropbox

  8. alan72 | | #12

    As I mentioned above, I hired a third party engineer to do all the Manuals (J, D, S, and T). We are specifying Fujitsu mini splits. It's a large spread out ranch - there are 4 zones each with their own ducted minisplits. Currently, we have each indoor unit paired with its own outdoor unit.

    When is it reasonable to have outdoor units paired with more than one indoor unit - what suffers - comfort or efficiency?

    Another question - I have included an attachment - it's our common area (one of the zones) - we currently have a ducted unit here - would this space be reasonable for a wall unit (or wall units)?

    Thanks again,


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