I am getting a number of quotes for a new furnace, for a home I just moved into last fall. My current furnace is a VERY old 100kBTU, and is extremely over-sized. My house is 1200 sq. ft., plus an un-finished, un-heated basement.
Most contractors seem to be quoting me furnaces of either 60 kBTU or 70 kBTU, but based on my gas bill and a couple calculations I don't think I need anywhere near that large. I requested a Manual J from them, and one provided it, showing a heat load of 49.3 kBTU for a 0 F day.
I don't have a lot of data for my house, not having lived here long, so I talked to my best buddy next door who has the same floor plan house, plus a conditioned/finished basement and a ~300 sq. ft. addition on the main floor. Based on his gas consumption, local monthly average temperatures, and baseline summer gas consumption, I am getting a heat loss for his house between 0.30 and 0.36 kBTU/h per 10F temperature difference, assuming his 10+ year old furnace is 80% efficient. For a design -10F day (I am in Michigan) that works out to a heat load between 24 and 29 kBTU.
Should I just bring up this point with the contractors? It seems like most furnace manufacturers don't make gas furnaces below 60 kBTU, so it doesn't seem like I have a lot of options anyway. Any other suggestions?