We live in a 1920's brick 2 story in central Ohio. The house is just under 1600 sq ft with original windows in most of the house. Decent insulation, a gas furnace that is 20 years old,80% efficient at best. We are on a gas budget that runs around $130 a month. I am thinking of replacing the old furnace with a 95+ efficient one. How old is your furnace and what is the efficientcy rating? Maybe time for a new one. Also, I read in Popular Mechanics that too big of a furnace will actually cost more to run than one that is the correct size for your house. Have an expert in the heating field perform an analysis of the heating needs of your house.