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Old 12-04-2009, 04:46 PM   #1
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furnace sizing


i need to replace my furnace. i have a G.E. input 135,000 BTU's, 24 years old.
recently had an energy audit completed, they recommend replacing my furnace with a AFUE of 92% or higher.
live in anchorage, alaska. 3791 SF home.
one contractor tells me they do not make a high efficency furnace large enough to handle my needs and wants to install an American Standard 140,000 80% AFUE.
another contractor wants to install armstrong G2D95CT100X 95% efficient.
i am concerned, Will the armstrong will keep my house warm.
If my current furnace of 135,000 BTU's is 60% efficient according to my energy audit, then i need 135,000 x 60 % = 81,000 BTUs output.
Is this correct? any comments? thanks for your suggestions.

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Old 12-04-2009, 05:10 PM   #2
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furnace sizing


Your furnace may have an AFUE of 60%. But its steady state will be much closer to 75 to 80%.

They do make 120,000BTU input 90% plus furnaces.

So 135,000 X 80%efficiency equals 108,000 BTU output.
120,000 X 90%efficiency equals 108,000 BTU output.

When it was at your areas coldest, or design temps. Did/does your current furnace run non stop. Or does it cycle on and off.

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Old 12-04-2009, 05:40 PM   #3
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it cycles on and off. does not run continuously.
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Old 12-04-2009, 06:09 PM   #4
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Then you don't need a furnace that has as much output as the one you currently have.

So the 100,000 BTU input 92% may still be larger then you need.
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Old 12-04-2009, 07:23 PM   #5
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furnace sizing


Quote:
Originally Posted by dmkkwk View Post
it cycles on and off. does not run continuously.
How many minutes on, how many off?
92,000 BTU/hr running at 10 min on, 2 minutes off is putting out 92(10/12) = 77k BTU/hr.

Last edited by Yoyizit; 12-04-2009 at 07:28 PM.
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Old 12-05-2009, 04:36 PM   #6
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furnace sizing


Right now it is 19 degrees F. here. The furnace i have is not running much at all. Within the next few days i will try and keep track of it running, timing the on and off periods. The critical time is when it is minus -20 degrees for 10 days straight. I guess that is when i really need to know how much it is running. i will be back with more info.....
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Old 12-05-2009, 08:23 PM   #7
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furnace sizing


Quote:
Originally Posted by dmkkwk View Post
Right now it is 19 degrees F. here. The furnace i have is not running much at all. Within the next few days i will try and keep track of it running, timing the on and off periods. The critical time is when it is minus -20 degrees for 10 days straight. I guess that is when i really need to know how much it is running. i will be back with more info.....

You can buy a hand full of units at 95% eff that may meet your needs. Here is a spec sheet for a goodman that you could install yourself if you wanted. It's not as hard as you might think.

http://www.goodmanmfg.com/Portals/0/.../SS-GMVC95.pdf
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Old 12-05-2009, 09:08 PM   #8
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furnace sizing


I would and I use Beentheres 80% rule. The input is one number but we heat our houses with the OUTPUT, not the input BTU'S. look at the output rating on the model # info inside your unit. Efficiency has to do with air leakage up the chimney in the off cycle etc. Not a factor with a high efficiency unit. It goes down to -40F where I am and we can get 30 mph winds. A 110,000 BTU high efficiency will be okay where I am. I would go one further and buy a 2 stage furnace. It will fire at 70% on low fire and use that for quite a bit of the heating season so the final size is not critical. We have sold some of the Armstrongs (Lennox owns them) and they are pretty good units. The AmStd is way oversized and will cost a LOT for $$ in fuel over several yrs in your climate. If your guy sells Lennox a G61, 110 would be a good choice (2 stage and 95% efficient) and is eligible for your rebates. The 100 Armstrong may be a bit small but it all depends on how good your windows, doors and insulation are. They make a 125 in that one.

Last edited by yuri; 12-05-2009 at 09:20 PM.
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Old 12-06-2009, 08:35 AM   #9
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furnace sizing


Have a heat loss done with the loss numbers you acquired. That tells you the btu's required to heat your home. Than choose the proper size unit from there. I always use the Gross output or DOE output which are the same.
See link FAQ about why a heat loss is required
http://www.comfort-calc.net/faq.html

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