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Heat load Calculations

KevinHughes Member Posts: 3
edited November 2022 in THE MAIN WALL
I have been using "Rule of thumb for years in Denver Colorado. It was the way i was taught. But i was also taught to measure the duct work to make sure it can handle the BTU's and CFM for heating and Cooling. I Have never had any problems un till now when I have to do Manual J software for energy rebates. I have no problem using software but want to be assured and comfortable it is correct.
the problem is my rule of thumb will match the heat loss but not my heat gain. This is because my Adtek Mobile software is adding the basement square footage to the heat gain. and Thus doubling my heat Gain?
The formula i use is as fallows. I calculate only Above Ground Square footage only. If there is a portion of a basement or walk out, I use a % of the basement in the calc.
for those who are curious for sizing a furnace i get the total above ground square footage say 28x40=1120 multiply by a BTU factor 35.40.45. if home was built 1920's to 50's i use (50). and 50's to 1985 i use (45) and 1985 to present (40 or 35) 28x40x=1120x35=39,200BTU's Output then Divide by the Efficiency of the furnace to get the Input of the furnace.
28x40=1120x35=39,200 output BTU's / .95=41,263 input BTU's
for your duct work you need 2 square" per 1000 BTU's heating and 6 square " per 1000 BTU's Cooling
this is for the trunk line. if your system is in the center of the home and the duct branches off in 2 directions the add the 2 ducts to total your Area and divide by your cooling or heating factor.
Yes there is a lot more to sizing ducting so just get a good look over the complete ducting and returns to make sure nothing has been chocked down any where.
OK as For cooling i take total Above ground square footage and divide by 600. so 1120 / 600 = 1.8 tons to i go with a 2 ton ac but my software is telling me i need 3 tons. Am i not using the software correctly or is there a huge margin for sizing an ac with software. the software is nearly doubling my square footage telling me i need 34,746 it was higher at first i had to remove decimals to get adtek mobile to work. I love Adtek's Accuload and will probably buy the pc version. the old ac was a 3 ton new home owner never used ac. am i using or inputting some thing incorrectly or should i go with it. we have a lot of basements 8" concrete wall in a heavy clay to sandy clay. frost line is bought 3ft max.


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    Do learn to use at least one of the programs (apps) which are available for calculating heat loss. These take into account the actual construction and exposure of the spaces -- usually on a room by room basis -- and are far more accurate than any square foot based calculation. Since they give a room by room result, you can also use them to properly size your ductwork -- or radiation, if you get into hot water or radiant work at all.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,472
    In the time it took you to type that question you could run a heat load calc with one of the free ap, or buy a program. :)

    Here are a few freebie heat load programs.

    I imagine you can find cooling programs online or from your supplier.

    More and more permits require a load calculation sheet to permit and get an inspection.


    Also fine one at

    Burnham load calc
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Zman
    Zman Member Posts: 7,605
    I definitely think that you should find software that works for you and stop using "rules of thumb". I suspect that there is an incorrect setting in you software that is messing you up the calcs.

    I did a calc for a poorly insulated 1951 brick ranch with newer windows in Denver. The above ground main floor came in at 26 btu/square foot and the basement (2' out of the ground with no insulation was 13 btu/square foot. The furnace is sized for about double the heat load and runs about 1/2 the time on design days. This, to me validates the heat loss.

    I seriously doubt there is a house in Denver with a total heat loss over 30 btu/ft.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 15,833
    If the basement is below ground how does that double your heat load?

    (This is because my Adtek Mobile software is adding the basement square footage to the heat gain. and Thus doubling my heat Gain?)

    You need to calculate it correctly, basements typically have no heat gain if below grade.

    Rules of thumb are great when they work, no substitute for calculations

    Example: In MA where I am the cooling load is (usually) 1/2 the heat load
  • KevinHughes
    KevinHughes Member Posts: 3
    I tend to get too detailed in to things. Hot Rod but thats what's made me good at what i do. i normally use rule of thumb on my first visit. if i close the sale, then ill return for heat load and material list. i think it does just come down to getting to know the software you are using. My office is all Apple products. they just last longer and easer to use. problem in theres not much software that works on apple. so i use the mobile sights. I have used wright soft with a emulator but was using windows ME. So I have found Adtek to be a more user friendly and a better software to get Employees up and going on. So What soft ware are you guys using?
  • hot_rod
    hot_rod Member Posts: 22,472
    I hear you on the procedure. Nobody want to spend the time on a detailed load calc and design andd not get the job. Some contractors charge for that on the front end, you just adapt to your market.

    I really like Siggys heat load calc program as it is generic. It doesn't try and sell or build a parts list. You can customize stuff like basement walls, indicate how much is above grade, etc.

    It is windows based and would require Parallels to run on Mac.

    Shortly Siggy will have his CAD program available to use browser based, no need to run Windows, something like 100 bucks a years for a seat. It will have electrical components also.

    Perhaps the HDS load calc will morph to that platform also.

    If you have time tune in this Thursday to see a preview of that at our Coffee with Caleffi webinar. Siggy will discuss system documentation, the topic of the next Idronics.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Hvac Calc works fine for the majority of my work.
  • Tinman
    Tinman Member Posts: 2,808
    I use Elite software, Uponor's, Siggy's, and others. If you don't want to get a Windows based laptop you can get the Manual J written forms from the ACCA store. I still do some manually. Old habits, I guess.
    Steve Minnich
    EBEBRATT-Ed Member Posts: 15,833
    I like doing written forms because you can see the calculations and find mistakes, it gets my antique brain thinking the right way. Especially if I haven't done any calcs in a while.

    Like the software though for speed though. No doubt about that
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    EDSTech said:

    Heat load calculations...

    The post is over 6 years old

    There was an error rendering this rich post.