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HVAC Discussions - Heating, ventilation and air conditioning.
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Posted by barefoot on November 16, 2007, 1:03 pm
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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