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  1. #1

    Proper furnace size

    It would seem that many posters have a similar issue with respect to getting the proper sized furnace. I have had eight contractors out for estimates and they are evenly split between installing an 80,000 and 100,000 btu furnace. They all swear they have done an analysis and theirs is the "right" size. I don't want to freeze here in central Indiana when it gets below zero. The old unit is a 25 year old 156,000 btu beast that is way oversized and cycles very frequently but keeps the house warm. The house is an 1800sf 1950's ranch with little insulation, large original aluminum windows, and a partially finished walk out basement. My gut tells me that an 80'000 unit will be mostly fine but may struggle some in a prolonged cold snap so I'm leaning towards the larger unit but get it in a two stage so it will be more quiet and efficient and still heat well during extreme cold. There is not much price difference in size but the 2 stage is definitely more costly. All the quotes were for a 92 to 95% efficiency. Am I off base here or should I be looking at something else? All the contractors did come up with the same 3 ton AC unit so I think I am OK there. Any help would be appreciated. Thanks.

  2. #2
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,841
    Many things can change the final gain or loss. It's the old "garbage in = garbage out" situation. For example, in our area +4F is the standard for heating design. Doesn't mean everyone uses it though. Many will use 0F and some even -5F. Each answer will therefore differ and that's just the tip of the iceberg. Wall insulation R-values, window materials and types, door materials and types, whether the calculation was done to Manual 'J-7' or 'J-8' will all make a difference in the outcome. So ask the two whom you trust the most to share their design criteria, one who came up with 80mBtu and on that came up with 100mBtu. But at the end of the day, if you're more inclined toward the 100mBtu 2-stage model, that's good logic and if we assume for a moment that your home needs exactly the output of an 80mBtu 95% AFUE furnace, then a 100mBtu 92% furnace easily falls into the normally accepted range of no more than 25% over sized.
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  3. #3
    Join Date
    Jul 2004
    Location
    Charleston, SC
    Posts
    930
    Furnace sizing is not as critical, since an over sized furnace won't cause moisture/mold/humidity issues like an over-sized air conditioner. An over-sized furnace will simply cycle on and off more often, as you have already experienced.

    You may be in a scenario where the guys quoting 100k don't have a furnace that will produce the required output in a smaller unit. Furnace ratings are rated input X % efficiency = rated output. If your required output is 74000 btu, then an 80000, 92% won't do it. It's output would only be 73,400 btu per hour.
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  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,599
    Do it yourself. The load calc, not the furnace install. Click on the HVAC Calc tab on the menu bar above. You can get a homeowner registration for $50. Oversized is not good. It is noisy and less efficient.

    A 100% 95% furnace may want to move around 1500 CFM to avoid overheating and shutting down. I doubt a 1800 sq ft 50s vintage home has a duct system anywhere near that capability. With a variable speed blower that tries to achieve the needed airflow, it could end up sounding like a wind tunnel in there. Some furnaces if they cycle on their overheat limit a few times shut down and lock out.

    If you really need 80 or 100K worth of furnace, I'd try to get that huge heat loss down rather than throw a big furnace at it.

  5. #5
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    I agree with B. L. and Carter. I'm not so worried about oversizing a furnace. Anymore you can buy a 2 or 3 stage or even a modulating furnace.

    That said, why spend the extra money when it could go towards insulation, windows and making the home tighter.

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