Your neighbor lives in a 2000-square-foot older home heated by natural gas. The current gas heater was installed in the early 1970's and has an eciency (Annual Fuel Utilization Eciency, or AFUE) of 65%. It is time to replace the furnace, and your neighbor is trying to decide between a conventional furnace with an AFUE of 80% which costs $2,200 and a high-eciency furnace with an AFUE of 95% which costs $3,600. Your neighbor has o ered to pay you $100 if you assist in making the appropriate decision. Considering the weather data, typical heating loads, and the price of natural gas in your area, make a recommendation to your neighbor based on convincing economic analysis.