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  1. #1
    Join Date
    Jun 2008
    Posts
    58

    hvac calc ? seems to low

    I purchased the HVAC calculator and I seem to have a problem the my loads do not seem to add up. If anybody has a good grasp of the program and wants to due a quick calc. It would be appreciated. Thank you.

  2. #2
    Join Date
    Dec 2006
    Location
    Dallas
    Posts
    189
    A bible story comes to mind... something about teaching a man to fish...

    What leads you to believe the loads aren't adding up?

  3. #3
    Join Date
    Sep 2006
    Posts
    1,253
    If you read the instructions and the web site it states that you will likely think the result is too low. Many if not most residential systems are over sized, so if you are comparing to what you currently have it will seem too low. The other thing you should do is switch off the cooling load shown in tons. The number shown when that is switch on can be misleading. have the cooling load only shown in BTU of sensible and latent.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,743
    Why don't you think its right.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Jan 2002
    Location
    East Grand Forks, MN
    Posts
    1,373
    Give us some info about your Load Calcs!

    What's the Approximate house size and type and location, i'm curious.

    I'm willing to guide you!

  6. #6
    Join Date
    Jun 2008
    Posts
    58
    Thank you guys for your responses. I will try to put this as easy as possible.


    Contractor has 2.5 ton and a 75k furnace up stairs and a 2 ton 45 furnace downstairs. Im looking to upgrade my system but I first wanted to find out if they even have the unit size right.

    My settings were

    I live in southern il so I used the Scott, belleville selection, I added the 1200 for the kitchen and added 4 people for the 1st and 2nd floors.
    Temp 92, 6, degrees summer grains of moisture 111, daily range medium
    Inside tems 73 and 68 winter, 55% humidity.

    The house is 2 story aprox 2550 sq feet. Brick and vinyl siding avg. construction. with a basement. Front of house faces south.
    Measurements all windows double pain low e glass, R 13 insulation and basement is underground concrete floor no insulation

    basement
    S wall 42'4"
    E wall 41' 32"-20" low e window
    N wall 42'4"
    W wall 41 4'x4' low e window

    first floor
    S wall 42'4" metal insulated door 3x6 (2) 3x5 low e double panel windows
    e wall 41' 3x5 window
    n wall 42'4" (3)3x5 windows (1) 6 ft sliding glass door) (1) 3x3 window
    w wall 41'

    second floor
    S wall (4) 3x5 windows (1) 3x4 window
    e wall (2) 3x5 windows
    n wall (1) 3x5 (1) 4x4 window
    w wall (1) 4x4 window

    roof is vented R 30 insulation with light colored shingles.


    My calcs came out with


  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,743
    You didn't list your location.

    I don't see a people load on it.
    What infiltration rate did you use.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  8. #8
    Join Date
    Apr 2007
    Posts
    6,285
    For Scott AFB, Belleville I see:

    453 ft elevation, 38* lat. N., 10* 99% DB, 93* 1% DB, 77* coincident wet bulb, 46 grains for 55% RH.

  9. #9
    Join Date
    Jun 2008
    Posts
    58
    I used .5 and 1.2 winter that was "average" it said

  10. #10
    Join Date
    Jun 2008
    Posts
    58
    I am getting ready to pull the trigger here and still need some advice about my manual J calculation. The only that I am going to change is that I am going with spray foam insulation. Any help on the calculation would be appreciated.

  11. #11
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37
    I know this is off topic but seemed important to mention....My concern with spray foam....regardless of the foam people's claim of 'open cell', I have yet to see a sample that allows water to pass through like a sponge. If you spray the inside of your roof deck and down the road the roof develops a leak, you'll never know. The water will continue to soak into the decking, rotting the deck and the trusses before you ever know there is a problem. With ordinary blown in, at least you'll see a stain on the ceiling alerting you to a problem. I've had to replace a few roofs in my time that still looked pretty good but leaked....

  12. #12
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37
    I seem to have developed an opposite problem from acnoob. When my house was built, the energy rater and A/C contractor sized my unit at 3.5 ton (2000 sq ft home in FL, meeting state energy code). As time has passed, install of solar shades, HRU, plantings on west side of house, solar attic vent, another 12" of attic blown in, I'm beginning to suspect the unit is now oversized. Talking with the energy guy, he pulled out his calcs he stated that they did indicate 3.5 ton but was close to 3. I'm sure they'd go on the large size to avoid a customer who couldn't get his house cool on a hot day. Energy bills are low but the cycles seem shorter to me now that when we first moved in. Any sense in me paying to have a recalc done now? And if they indicated a 3 ton, what if anything should be done? Lower the fan speed, downsize the condenser or even downsize just the compressor or would just leaving well enough alone be better? If there is a chance to further reduce my energy use, I'd do it but how would I determine that?

  13. #13
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    Is your duct in conditioned space? The spray foamis great it will often times give back the duct exposure, knee walls and sky light shafts for example. The house should be able to be considered tight for infiltration with foam also.

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