The price of a furnace generally rises in step with its fuel efficiency. A furnace with a 90 percent AFUE might cost $1,000 more than a similar size unit with an 80 percent AFUE. But you can often recoup that additional cost through lower fuel bills over the life of the furnace, especially in regions such as the Northeast and Midwest, where winters can be harsh. How quickly you recover the investment depends on more than just AFUE. The electricity to run furnaces with different AFUEs can vary significantly. The climate where you live, how well your home is insulated, and your local gas and electricity rates also affect payback times.
Turn off the gas supply valve, typically located by your gas meter on the gas inlet pipe. Turn off the gas by rotating the valve one quarter turn with an adjustable wrench. When the gas is off,  the valve’s oblong stem points perpendicular to the inlet pipe. Then call your gas utility or the fire department from a remote location. Do not return to your home until you know it is safe.
High-efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main differences between a conventional and condensing furnace is the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these ways, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than does a conventional furnace; both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.
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