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  1. #1
    Join Date
    Jul 2007
    Location
    Chicagoland
    Posts
    474

    Furnace size VS. Man. J

    After calculating with man. J (on paper) a house shows 70400 (71* HTD). I am looking at goodman 90+ equipment. The 70 would undersize by 4000, the 90 would over by 14. Man. J says stay tight to results. Opinions?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,702
    Go with the 90.

    Nothing worse then finding out that the gas company is suppling gas with a 100 BTU lower content then you thought when its at design temp outside. That could put you at 10,000 BTUs light.
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  3. #3
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,702
    Quote Originally Posted by mayguy View Post
    I'd go less. I did that on mine, and it keeps up just fine when it's -25˚ out.
    Shows that your calc was inaccurate.

    Not knowing how inaccurate his calc is, and in which direction, undersizing could mean he has a 10 cooler house at design.
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  5. #5
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    it is based on output

    If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
    I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.
    Do it right the first time.

  6. #6
    Join Date
    Jul 2005
    Posts
    717
    Quote Originally Posted by 21degrees View Post
    If the furnace is 70,000 x 90% = 63000 output and a 90,000 x 90% = 81,000.
    I would go with 90,000 just incase R-valve was high on something. Have a small safety factor.
    ************************************************** ********
    My thoughts exactly. Go with the larger one and get the benefit of shorter, but efficient, comfortable cycles, like my furnace with only about 2 six minute burner cycles an hour even at design temperature.

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