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Community and Q&A

Cool Calc 2.0

Fretted | Posted in General Questions on

Does anyone know how reliable this software is and does it give you valuable results?

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  1. Expert Member
    Peter Yost | | #1

    Hi Rick -

    It could be that our GBA HVAC hotshots have not weighed in on your question because Cool Calc has come up a number of times before in Q&A.

    A good place to start would be this GBA blog by Martin Holladay:


  2. Expert Member
    Dana Dorsett | | #2

    I've not used it personally, but have seen output from that tool (not sure which version) used by relative newbies that had insanely inflated heating load numbers. In those off-the-charts erroneous reports the wall construction was pretty different from current standards, but whether the error was due to the tool or the user wasn't clear. I can't really condemn or vouch for it.

    With any load calculation tool garbage in == garbage out. But the graphs & charts are pretty enough!

  3. monkeyman9 | | #3

    I may have been the newbie haha. :) So, it works decent if you are careful and run so you fully define everything. It will try to automate some things (and my house is nowhere near efficient).

    I ran cool calc and did a manual J speedsheet with a manual J book.

    BUT, there are some bugs I noticed (reported a couple). For one it automatically put some windows and doors on some walls. I had to manually delete and check custom. Had to save in the right order to get it to stick.

    Now, those issues may very well be fixed at this point. Some of them I didn't notice til later. Like I left hot water heat pipes exposed (which I swear I deleted) which made my loads very high.

  4. AndrisSkulte | | #4

    I'm another newbie with it - I modelled my 1960's cape, entered the numbers, and it did report 22% higher heating BTU's than my fuel use suggested, but it's way better than using contractor rules of thumb for sizing mechanicals. If it was missing something from a dropdown, I submitted a bug report, and they responded promptly. I'd love to compare like for like against the well known commercial grade products like WriteSoft. (hint hint, maybe a future GBA article?) ;)

    1. Jon_R | | #5

      > and it did report 22% higher heating BTU's than my fuel use suggested,

      As it probably should - because average load and design load are different things, even when normalized to HDD.

      1. Expert Member
        Dana Dorsett | | #6

        There are several known error factors in a fuel use load calculation, some of which can be corrected for, others not. But it puts a firm stake in the ground as a reference point.

        The errors in a Black Box load calculation software tool aren't usually knowable to the user.

        A 22% difference between the tool's output and a fuel use load calc is a bit higher than I usually see when comparing an aggressive Wrightsoft Manual-J to the fuel use- ~10-15% is more typical, but I don't have large data sets to compare.

        But even if reality turned out to actually be the calculated 22% higher than a fuel-use load calc implies, sizing the heating equipment output by ASHRAE's 1.4x recommended factor still has ~15% margin at the 99% outside design temp, which is enough. If the fuel use load number turnd out to be exactly correct, upsizing by 1.4x from the 1.22x calculated by the tool is still only a (1.22 x 1.4 =) 1.7x oversize factor, which is the oversizing factor presumption in AFUE testing of fossil-burners- the equipment would still hit it's efficiency numbers, even though it's not as comfortable as perfectly sized due to the (1 / 1.7 =) ~60% duty cycle at the 99% outside design temp. For comfort it's better to be running a 100% duty cycle when it's cold out.

        Though a 1.7x oversize factor can cut into heat pump efficiency slightly, it's not falling off an efficiency cliff yet, though 1-2 stage heat pumps could be less comfortable than a perfectly right-sized heat pump. With modulating heat pumps the duty cycle at cold temperatures would be 100% at design temp, even at 1.7x oversizing.

  5. Jon_R | | #7

    > ~15% margin at the 99% outside design temp, which is enough

    I roughly agree. For example, 2014 had 24 continuous hours of cold here that required ~22% more than design day BTU to keep up. With only 15% margin (equipment size more than design day load) I would have been about 7F too cold. Fixable with a couple of small electric heaters.

    Some very interesting numbers could be derived from a months worth of daily temp, wind speed, fuel use and run-time data.

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