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  1. #1
    Join Date
    Nov 2007
    Posts
    14

    Question furnace sizing and BTU calculation

    We are getting a new furnace, Trane XV95, and our contractor estimated we need 66,500 BTUs, which means we need the 80K BTU furnace. I was disappointed that we weren't able to qualify for the 60K furnace, as we spent a lot of money this summer getting our home insulation improved. My question is, what can we do to reduce our BTU requirements so we only need the smaller furnace? Our house is only 1450 square feet, in upstate NY.

    1) do the calculations take into account the temp we keep our house? We generally keep the thermostat at 67, and only turn it up to 69 if grandmother is visiting.

    2) How specific are the calculations? For example, we didn't have our ducts sealed, or the crawl space walls insulated. Would doing that bring down our BTU needs?

    Any other things we can do to bring down the BTUs? Thanks.

  2. #2
    Join Date
    Oct 2005
    Location
    MN
    Posts
    133
    How did the contractor come up with the 66,500? Did he actually do a manual J, use the size of your old furnace to determine the BTU's, or take a guess based on his experience?

    The CORRECT way is to get a manual J done to determine the size you need.

    The manual J will take into consideration all the various factors to determine the heat loss of your home, and size the equipment correctly.

  3. #3
    Join Date
    Nov 2007
    Posts
    14
    The contractor put a lot of data into a computer, so I assume it was "J" (I will ask), but didn't ask me about the temp we keep our house, and didn't do a blower test (we had significant work done to seal air leaks.)

    And does correct duct sizing enter into the calculations? Thanks.

  4. #4
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,897
    Goodman and Amana offer 70k BTU 95% AFUE units that provide over 65.5k btu output.

    Some other manufacturers offer 70-75k input units that should be sufficient.

  5. #5
    Join Date
    Nov 2007
    Location
    Chicago
    Posts
    76
    That Trane has two stage heating. Most of the time it will run at the lower stage. I doubt there is any significant savings between the cost of the 60 & 80,000 BTU unit and cost of operation. With 95% efficiency and 80,000 input you are getting 76,000 output. I would say, assuming your contractor's calculations are correct, that is the correct size furnace.

    Correct duct sizes will not affect the size of the furnace, but would affect the distribution of the heat it provides.

    As far as inside temperature, I am sure they just assumed 70degF for heating mode.

    You will see significant usage saving because it is variable speed and two stage heating, but most importantly make sure you get a good programmable thermostat and program it to lower the setpoint during sleep and unoccupied modes.
    Last edited by hvacconsultant; 11-18-2007 at 02:47 PM.

  6. #6
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    biggest heat loss is thru ceilings
    yes, insulate crawl walls, keeping 4+ inches of masonry visible to watch for termites, etc
    keep crawl vents closed when humidity outside is high
    all duct seams & joints should be sealed
    all elec boxes & penetrations should be sealed
    be sure space between window & door frames & the structural frames are insulated --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,389
    Where is the heat going? That is A LOT of furnace for such a small space. We get below zero and that would heat 2500+ in our area in a typical house. On high fire, the blower needs to move a lot of air which could be rather noisy. He needs to check his calcs carefully.

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