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Thread: Correct btu?

  1. #1

    Hmm Correct btu?

    I know a lot goes into furnace sizing. That said, I'm replacing a 17yr old,
    100,000 btu Bryant. It's probably 70% - 75% efficient. My home is two floors, about 2,400 sqft. I live in Cincinnati, OH and we can get some pretty cold stretches. I'm putting in a Rheem 90% + modulating and have two reputable quotes. On says a 90,000 btu will do just fine. The other says a 90,000 is 'maybe just enough' and suggests going to the next step, a 105,000btu. Any suggestions?

  2. #2
    Join Date
    Mar 2004
    Grottoes VA
    A 100,000 btu at 70% means a 70,000 btu output.

    A 90,000 btu Rheem has 86,000 btu output.

    A 75,000 btu Rheem has 72,000 btu output.

    The 75,000 btu Rheem will put out more heat than your current furnace.
    Karst means cave. So, I search for caves.

  3. #3
    Join Date
    Sep 2002
    South Dakota
    A furnace as old as yours is far less than 70% efficient. More like 50 to 60% efficient.

  4. #4
    Join Date
    May 2006
    Philadelphia, PA
    Did anyone perform heat loss calcs?
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  5. #5
    Join Date
    Sep 2002
    I don't know
    Quote Originally Posted by NormChris View Post
    A furnace as old as yours is far less than 70% efficient. More like 50 to 60% efficient.
    80% efficient units with inducers existed 17 years ago. So did natural draft furnaces with dampers and electronic ignition. (65-70% seasonal, 75-80% steady state)

  6. #6
    Join Date
    Apr 2005
    Make them perform a heat loss calculation, that is the only way to know if the furnace that you are getting is the rigth one. If they dont perfom a load calculation they are guessing and you going to have an oversize or undersize furnace.

    I hope this help.


  7. #7
    Join Date
    May 2000
    Indianapolis, IN, USA
    First, the boys saying LOAD CALC are the only ones making sense!

    Second, AFUE doesn't always tell what the output is. You may have a unit with AFUE of 60% but in steady state operation, the output is around 80% of the input. Look at the rating plate! So just because he has an older 100K furnace, doesn't mean the output is any less than 80K.

    You can take a 50s vintage relic and once it is warmed up, the house is getting 80% of the heat the beast is making. But it is still likely wasting 40-50% of the heat you pay for. Why? Because AFUE looks at all losses and oldies lost a lot of heat up the flue as they warmed up and as they cooled down. The more oversized and the more cycling, they more they lost. But once at temp, they weren't too bad. So below 80% AFUE, pretty much figure output at 80% anyway.

    Just on the guess aspect, 2400 sq ft decently built house in Cincy, 90K max. But as the wise men before me have said, only a load calc will tell for sure.

  8. #8
    Thanks, all. My old Bryant was pretty much their basic at the time. Never any problems with it, but I'm sure it's efficiency is low. In any event, I'll make sure to have the load calc done. Thanks again!

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