In the United States, forced-air furnaces are installed in nearly 60% of all new single-family homes. Federal regulations require them to have a minimum annual fuel utilization efficiency (AFUE) of 78%, but a new generation of modulating-condensing furnaces tops out at more than 95%—a significant improvement.
In a conventional furnace, the burner and blower kick on when the thermostat calls for heat. When the room reaches the set temperature, the furnace shuts off. Modulating-condensing furnaces, or “mod-cons” as they’re sometimes called, have several ways of increasing efficiency.
First, a modulating gas valve varies the amount of fuel the furnace burns depending on how much heat is required. Second, models with variable-speed fans can run faster or slower to regulate the amount of heat delivered to the house much more precisely.
Third, the “condensing” part refers to a heat exchanger that captures residual heat in combustion gases leaving the unit. In old-style furnaces, this energy goes right out the flue. Because flue gases are cooled as energy is extracted from them, a masonry chimney or double-walled stovepipe is not needed to vent a mod-con. Plastic (PVC) pipe will suffice, which reduces installation costs compared to a conventional furnace. An additional benefit to mod-cons is their size—some models are small enough to be hung on the wall of a mechanical room.
These furnaces are sealed-combustion units, which means they draw in outside air for combustion rather than using air from inside the house. This eliminates the potential for backdrafting, a dangerous condition in which lowered indoor air pressure, from a bathroom fan, for example, draws combustion gases into the house. It also decreases the infiltration of unconditioned outside air needed to replace combusted air.
Variable-speed motors for fans use less energy than single-speed units. The American Council for an Energy-Efficient Economy warns that some furnaces can use more than 1,200 kWh of electricity per year. The most efficient Energy Star models use roughly 150 kWh per year.
High-efficiency furnaces can be twice the cost of conventional units, according to the National Association of Home Builders’ Home Innovation Research Labs. For example, the labs said a complete Rheem modulating heating system with an AFUE of 94%, including ducts, could cost $4,000, compared with a Rheem conventional system with an 80% AFUE furnace at about $2,200. (Prices can differ sharply in different areas and with different manufacturers.)
According to the American Council for an Energy-Efficient Economy, it’s possible to calculate how much money you’ll save by replacing an older heating system with a more efficient one. For example, if you replaced a 65% efficient gas furnace with a 90% efficient model, you’d save $27 per $100 of your existing fuel bill, the ACEEE says. If your annual fuel bill is $1,300, the savings would be $351 per year. A chart for calculating these savings for a wide range of heating efficiencies is available at the ACEEE’s website (bit.ly/HeatSavingsCalc).