Furnace size
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Thread: Furnace size

  1. #1
    Join Date
    Oct 2005
    Posts
    26
    Plan on getting a Duel Fuel setup install and was wondering what a "ballpark" size furnace I might need. I have estimates that show 90,000 - 110,000 BTUs (old unit was 114,000 I think). I know I need to use the actual load calculations but right now I just need a quick estimate. House near Memphis, TN built in 1976 approx 1700 sf.

    Thanks

  2. #2
    Join Date
    Oct 2004
    Posts
    3,400
    Looks like you already have a "ballpark" figure.

    There is no way to tell which size is correct without a local heat loss calculation.


  3. #3
    Join Date
    Oct 2005
    Posts
    26
    I guess my question should have been does the 90K-110K look like a decent ballpark figure. For some reason I have been trying to make sure that my A/C unit was properly sized and haven't given much thought to the Furnance sizing.

  4. #4
    Join Date
    Oct 2005
    Posts
    26
    Current Furnace seemed to keep temps adequate in the winter and it is around 17 years old so I assume its efficency was only around 65% or so.

  5. #5
    Join Date
    Jun 2003
    Location
    Florida
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    9,548
    Don't skimp on the calculation.....you'll be sorry. Best money you could spend,not only for furnace comfort but for fuel usage.
    If everything was always done "by the book"....the book would never change.

  6. #6
    Join Date
    Mar 2004
    Posts
    1,042
    Sounds big to me, sight unseen. There is an overwhelming tendency to oversize furnaces, often by large margins, but it's awfully hard to tell without seeing the place. If you have, or had, more than three tons of cooling, it may be that the furnace had to be oversized so it would have enough blower capacity to meet the demand for airflow in cooling mode (this is typically only a problem in hot climates, houses with a lot of solar load, or systems that only serve an upper floor).

    If the old furnace was 114,000 input, what was its output? We can get a handle on that based on its efficiency rating or at least its age. If the old one was enough to do the job, you want the same (or less) output from the new one. Assuming that efficiency of the new furnace is more than efficiency of the old one, which is almost always the case, you want the input of the new one to be significantly less. That suggests to me that you at least want to be on the low end of your 90-110k range. And don't consider getting a two-stage furnace the solution for oversizing concerns; that's not a very good band-aid.

    If the old furnace never ran for more than a few minutes at a time, even in the coldest weather, you can pretty much be sure it was oversized, so subtract some more capacity from your guesstimate. The stakes are awfully high for just guessing like this, though. What happens if you end up guessing wrong?

  7. #7
    Join Date
    Oct 2005
    Posts
    26
    thanks wyounger

    I really never have paid that much attention to the furnace cycles but as I recall the cycles did not seem to be extraordinarily short. Currently cooling is a 3.5 ton condenser w/4 ton coil and had a hard time keeping temps under 80 in the hot part of the summer. The new system will be a 4.0 ton unit (heat pump). As I mentioned I was paying more attention to the cooling size versus the heating and 2 different load calcs showed the heat gain(?) at over 43,000 btu and for some reason never looked at heating needs.

  8. #8

  9. #9
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,131

    Lightbulb Oversizing

    Our house has an 88,000 BTU furnace that is at least 2X the size needed. It never runs more than 20 minutes in any hour even when temps are in the teens. House is 1587 sq ft, built 2001. The A/C is also oversized, has a 3 ton that runs about 1/2 the time when temps are in the mid 90's, it even cycles when temps hit 100+. Heat load calcs show a 2 ton A/C with a 40,000 BTU furnace. Room temps were uneven, I had to do some ductwork changes. The sad thing is the seller was a HVAC contractor !!

    The worst thing is once the wrong equiptment is installed you are stuck with it until it wears out in 10-20 years because it's not cost effective to replace equiptment just because it's mis-sized.
    Some contractors don't consider oversizing a problem because they don't get the blame for high utility bills, however if the HO is cold or hot the HVAC contractor is the first one to get a call...

  10. #10
    Join Date
    Aug 2005
    Posts
    40
    gtownguy,
    I'm a homeowner in Charlotte area. Read my thread "On time for properly sized furnace". Contractor put in an 87K BTIH furnace in my house (approx 1714 sq ft) and its way oversized. Make sure you get the load calculation done. I was like you more concerned about the cooling side and didn't pay any attention to the sizing of the furnace. Just my two cents..

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