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  1. #1
    Join Date
    Feb 2010
    Posts
    79

    Using the HVAC Calc

    I spent sometime today trying to figure out this free trial. I want to see how over sized my furnace is and how undersized my dicts may be. Recent posts suggest to start with the Heat gain and lost calculation. I plugged in dimensions for the first and second floors. However, I'm not sure if I have the ceiling type correct for the first floor. I chose under a ventilated attic, but that is what I selected for the second floor. What should I use in that spot?
    I also wasn't sure what "over a conditioned space" was for flooring. I selected that for both the 1st and 2nd floors. My 1st floor is over a basement that maintains around 54 degrees. Would a conditioned space apply?
    The final results display total for heat gain and loss for ceilings, doors and windows. If I did everything correctly.

    total CFM 1,200

    heat loss heat gain

    ceilings 14,371 36,199
    windows 7,058 10,591
    glass door 1,305 2,306

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,171
    Don't enter a ceiling for the first floor. Since you have a second floor that is conditioned.

    Over unconditioned basement for first floor's floor.

    In HVAC Calc, you don't enter any surface area that doesn't have a heat loss or gain, with the exception of the floor.
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  3. #3
    I'd like to do a calc for my house also. But I don't want to spend $400.00 for a one use. Are there any free software options?

    Thanks,

    AC Researcher

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,171
    The home owner version is only 49 dollars.
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  5. #5
    Join Date
    Feb 2010
    Posts
    79
    I made the changes to the 1st floor ceiling, but I'm stilll not sure about the floor and basement relationship. The options are either over condition space or over unheated basement. I went with the unheated basement. It does have supply vents and reads 54 degrees.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,171
    Most below grade basements will hold 50 to 55 without heat.

    I don't heat my basement, and it holds 50 except for the coldest days. And if I fixed the the hole where my cable and phone come in, along with the water hand pump hole, it would probably hold on the coldest days too.
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  7. #7
    Join Date
    Nov 2005
    Location
    Kansas
    Posts
    4,423
    Quote Originally Posted by turfgrass View Post
    I made the changes to the 1st floor ceiling, but I'm stilll not sure about the floor and basement relationship. The options are either over condition space or over unheated basement. I went with the unheated basement. It does have supply vents and reads 54 degrees.
    unheated basement is correct

  8. #8
    Join Date
    Feb 2010
    Posts
    79
    I added the new calculations, but don't understand the difference between when reporting the heat loss and gain. You can either select "sort by heat loss" or "sort by heat gain".

    sort by heat gain

    sensible gain 11,271
    Latent gain 690
    Total heat gain 11,961
    Total heat loss 16,707

    sort by heat loss

    sensible gain 11,412
    Latent gain 0
    Total heat gain 11,412
    Total heat loss 22,805

    I need to calculate the furnace's CFM rating in order to determine the duct cfm calc.. Would this be printed on the furnace?

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,171
    Quote Originally Posted by turfgrass View Post



    I need to calculate the furnace's CFM rating in order to determine the duct cfm calc.. Would this be printed on the furnace?
    No.
    You determine furnace CFM by the allowable temp rise for the furnace.
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  10. #10
    Join Date
    Feb 2010
    Posts
    79
    I found a calculation for furnace CFM but this could be just a general rule of thumb. Take the furnaces btus and divide by 10,000. Then multiply by 100 for CFM

    Furnace spec sheet
    btuh input 140,000
    btuh output 114,000

    140,000/10,000x100= 1,400 cfm

    so, 1400 would be used to figure the duct sizing?

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,171
    Quote Originally Posted by turfgrass View Post
    I found a calculation for furnace CFM but this could be just a general rule of thumb. Take the furnaces btus and divide by 10,000. Then multiply by 100 for CFM

    Furnace spec sheet
    btuh input 140,000
    btuh output 114,000

    140,000/10,000x100= 1,400 cfm

    so, 1400 would be used to figure the duct sizing?
    That method would make for a bad install since to would cause a temp rise above allowable limit for most gas furnaces.
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  12. #12
    Join Date
    Feb 2010
    Posts
    79
    sorry, i have a forced hot air oil furnace. Still a bad install?

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,171
    Quote Originally Posted by turfgrass View Post
    sorry, i have a forced hot air oil furnace. Still a bad install?
    Using that rule of thumb, it would/will be.

    You really want 145 to 155 supply registers.
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