A single-stage Westinghouse gas furnace operates at the same capacity whether it's an extremely cold winter day or on the mild side, while a two-stage furnace operates at a lower capacity on milder days without sacrificing comfort. The two-stage furnace is usually more energy efficient.
The most efficient Westinghouse single-stage furnace on the market has an 92 percent efficiency rating, based on the Annual Fuel Utilization Efficiency, or AFUE, ratings. Some older models already in use may only have a 50 to 60 percent AFUE, making them more expensive to run. A patented control board increases the lifespan of the igniter, often making the furnace usable for more than 20 years.
Two-stage gas furnaces come in variable and fixed speed models. The most efficient is the two-speed variable gas furnace with the 95.1 percent AFUE rating. That means that over 95 percent of the energy used to power the furnace is converted to heat. The two-stage furnace idles more on mild days and is quieter than the one-stage.
Both types of furnaces are compatible with a variable speed blower that circulates air more efficiently. The blower compensates for dirty air filters, duct work and air registers, keeping the occupants comfortable until these issues can be taken care of.