sizing a gas furnace replacement based on previous gas usage

I have a 27-year old horizontal gas furnace with a 5-ton a/c installed in the attic. When the building was constructed 27 years ago the sizing was done by a guy saying, "This will be big enough." The furnace is 140k btu/hr input and 112k btu/hr output. Now, it is time to replace the furnace and a/c. The 5-ton a/c seems to be the right size based on the history of the building, but I have no idea if the present furnace is oversized. Is it possible to use the history of gas usage as a way to size the new furnace? For example, the most gas I have ever used during the past 27 years in the coldest winter month was about 200 CCF/month, which according to the gas supplier's data equals about 200 Therms. If I convert the 200 therms into BTU, this comes to 20,000,000 BTU. Assuming 30 days in the month, then the usage is 667,000 BTU/day, or a little under 28,000 BTU/hr.
Are my calculations crazy? Can anybody give me some help here on sizing the new gas furnace. If possible, I need to use a smaller furnace than my present one. The access to the attic is not big enough to get a new unit, the same physical size as the old one, up there. Don't ask me why, that was a mistake made 27 years ago. If I can use a furnace with about 90k btu/hr output then I will be OK because the size of the cabinet is small enough to get it up into the attic.
Thanks for any help.
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adequate for job, pick your equipment based on these calcs and no other.
If the hole to the attic is too small make it bigger.
--
Bob Pietrangelo
snipped-for-privacy@comcast.net (home)
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On Fri, 16 Nov 2007 10:03:19 -0800 (PST), barefoot

Dont install your new 5 ton AC in the attic. Thats not a good place for it. It should be outside.

He should have been using the rule of thumb method because everyone has those.

Sounds right. Thats how we all size them........"history"

Nope, you are a genius in my book. Going from a 140k furnace to a 20K furnace will save you a heap of money......plus, it will match your 5 ton AC perfectly.

That method works too. Use a furnace the size of the hole in the attic.

Your welcome. Id say you have this job about wrapped up real well. Congrats
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Thanks for your reply Bubba. You have confirmed my long held brlief that there is at least one jerk in every forum or discussion group on the internet
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On Sun, 18 Nov 2007 09:53:13 -0800 (PST), barefoot

What, only one? You really are clueless if thats all you found. Kinda sucks eh? I mean, you know, with what all you paid for that answer, you really got kinda screwed. I'll have my pitty party for you tomorrow. Bubba
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