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.