CoolCalc vs loadcalc

Discussion in 'HVAC Heating & Cooling' started by WestMIchigan, Mar 31, 2021.

  1. WestMIchigan

    WestMIchigan New Member

    Joined:
    Oct 2, 2020
    Location:
    Michigan
    I ran both programs and came up with wildly different answers. I am finishing my basement and adding heat to a bathroom that the current HVAC doesn't handle well. Both more or less agree on the cooling load but are off by a factor of 2.5 on the heating load. I have a master bathroom with lots of north facing double pane low e window area relative to the exposed wall space with about 88 sq feet above what will be a finished heated basement. The walls are 2x6 with some unknown amount of spray foam in them. The basement is a mix of exposed and underground poured concrete walls. As an approximation one room is 15x15 with two interior walls, one south facing wall completely exposed but under a deck and the other about half exposed and half underground. Each wall has about 12sq ft of low e double pane horizontal slide windows. The final area is roughly 33x14 with 3 interior walls and only about 8 feet of exposed wall. The exposed wall has one 36" door and another 12sq ft window. The exposed basement walls are again 2x6 with unknown amounts of spray foam with 2.5" of XPS on top of that. The buried portion of the 33' wall is 2.5" of XPS. Everything is covered with 1/2" drywall.
    CoolCalc says 9617 BTU heating while loadcalc says 22986 BTU. This is for a house in the Grand Rapids, Michigan area. The contractor is saying a 2 ton 3 head Fujitsu minisplit with a 7k head in the upstairs bathroom, a 7k head in the downstairs bedroom, and another 12k head in the large family room. That seems like overkill but if loadcalc is correct it isn't far off. If CoolCalc is correct then I am incredibly oversized.
     
  2. breplum

    breplum Member

    Joined:
    Oct 2, 2004
    Occupation:
    Plumbing and heating contractor
    Location:
    San Francisco Bay Area
    That is wack that the contractor is not doing the load calcs to base the sizing upon. I bought the software just to do a handful of systems per year.
    I have only personally used Wrightsoft for load calcs and my suite has expired.
    Aside from combing through your data to find the discrepancy, do it again with, maybe https://www.loadcalc.net/.
    Or getting a energy consultant tech to provide the loads.
    Maybe your "contractor" has someone?
     
  3. Sponsor

    Sponsor Paid Advertisement

     
  4. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Last time I had to do this, Wrightsoft's trial version would let you do everything except set your outdoor design temperature, it was fixed. But if you look up the outdoor design temperature yourself, you can just scale the sample outputs by the ratio of delta T to get the answers you need. Once you've settled on all the little details that will change the room by room BTU/hr numbers, you can even manually enter a heating load for each room to account for the discrepancy. Then I believe you can use the duct sizing tools.

    Cheers, Wayne
     
  5. WestMIchigan

    WestMIchigan New Member

    Joined:
    Oct 2, 2020
    Location:
    Michigan
    In my post I mentioned I used loadcalc, it is even in my thread title. Not sure what you were asking me to do since I already did that and gave the results in my original post.
     
  6. WestMIchigan

    WestMIchigan New Member

    Joined:
    Oct 2, 2020
    Location:
    Michigan
    Thanks for the reply. I am pretty sure I was able to set the outside temperature but I can double check. I am using a ductless mini split so while a good suggestion the duct sizing tool isn't really applicable here.
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609

    The LoadCalc number is crazy if you're talking about less than 900 square feet with much of it with below-grade insulated walls.

    Two tons of cold climate multi-split is overkill. A half ton head in the downstairs bedroom might be overkill too- what was the load for just that room only?

    There is no WAY you should be installing a ductless head in a bathroom. Even a half ton head is LUDICROUSLY oversized- too oversized for comfort or efficiency. It will just short cycle itself (and the multi-split compressor) to death, and may even overheat/overcool just on the bypass refrigerant flows when other heads are calling for heat/cool, even while nominally "off", without the blower running. The head will also likely have condensation internally during the summer from that bypass refrigerant, and will spit some moisture back into the room every time it cycles on. Run the load numbers for just the bathroom and consider adding that much mesh radiant floor &/or heated towel rack instead, and run a low cfm continuous bath fan (some of the Panasonics have a 20cfm minimum speed mode) during the summer to manage the cooling loads.

    It would be easier to understand your layout if you list the room loads as reported by each of the tools, and indicate which are fully above-grade rooms, and which are mostly below-grade. List also how much of the room loads are attributed to infiltration + ventilation.
     

Share This Page