I have a question about furnace size.
I am about to get the underside of my roof insulated with 6 inches of open cell foam. This is supposed to cut my heating bills by about 40%. (I have a Cape built in 1952 and Capes are notoriously leaky.) My question is whether this insulation change will be significant enough to warrant a smaller furnace.The house is about 1800 sq. ft. and has virtually no insulation in the walls (ok, there was originally 2 inches of rock wool insulation but that settles after 50 years, so call it no insulation!) I live in Rochester, NY which is pretty cold. It's 32 degrees now! The existing furnace is an 80,000 BTU single stage gas furnace and is 12 years old.
Will the added insulation make enough of a difference that a smaller furnace will be appropriate? If so, how much smaller would be right for this size house? What is the consequence if one has a 'too large' furnace? And how much too large does a furnace have to be to cause a problem or significant inefficiency? (I.e., is something like 20,000 extra BTU no big deal?) I am very lucky that I will actually have to pay only half the cost of the furnace and installation, so initial cost is less of an issue than it would normally be.
Also, one furnace guy told me that the choice between a 90% efficient single stage furnace or a 90% efficient 2 stage furnaces is mainly a comfort issue, not an issue of operating cost savings...Is that true?