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
Thanks for any help.
Get a Manual J load calc done, get a manual D done to make sure duct size is
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.
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.
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