University of Wisconsin - Extension

MADISON, Wis. — Wisconsin has been a leader in converting to high efficiency furnaces. According to staff of the Wisconsin's Focus on Energy program, 75 percent of the furnaces sold today are high efficiency models. A high efficiency model is defined as one that has an Annual Fuel Utilization Efficiency or AFUE of at least 90 percent. AFUE is a measure of the amount of the fuel that a furnace uses that is turned into useful heat.

If you have a furnace with the combustion products going up a chimney instead of out of the side of the house through a white plastic pipe, chances are you have a furnace with an AFUE of 65 percent or lower. That means that 35 percent of the fuel you pay for literally goes up in smoke. In the high efficiency (90 percent AFUE) models, only 10 percent of fuel is wasted. If you have a heating bill of $1,000 with your current furnace (assuming it has an AFUE of 65 percent) and you purchase a 90 percent AFUE model, your bill could drop to about $722, if natural gas prices are at 80 cents a therm. Since gas prices are expected to rise your savings would likely be higher.

In a traditional furnace a primary heat exchanger sets over the burner and transfers heat to the air flowing through the heating ducts. Even after the combustion products pass by the heat exchanger they are hot enough that they need to be sent up a chimney. High efficiency furnaces attain higher AFUEs primarily by running the hot exhaust through a second heat exchanger that cools it so that much of the water vapor that is a major part of the exhaust condenses. The exhaust is then cool enough to safely exit the house through a plastic pipe with the aid of a small built-in fan. Most high efficiency furnaces have a second plastic pipe that brings a supply of outside air to the furnace so that the furnace does not have to draw air in through cracks in the house. This cuts down on drafts.

Now there is a new generation of energy efficient furnaces that offer even more savings. They use controls that allow the burner to burn at different rates and blowers that are much more efficient and also operate at different speeds. Furnaces have traditionally had one burn rate setting so that on the coldest day the furnace would provide enough heat. On warmer days the furnace would go on and off frequently. The latest generation of furnaces is designed to run more of the time but at a lower speed when temperatures are warmer. Traditional blowers are run by AC electric motors that are very inefficient, particularly when they start. They are also only capable of running at one speed. The new generation of blowers have DC motors and can vary speed infinitely. They use about a tenth of the energy of traditional blower motors and, in conjunction with the variable rate burners, provide a greater degree of comfort and a considerable savings in both electrical energy as well as natural gas.

The independent evaluators working with the Focus on Energy program estimate that these new furnaces, using what is referred to as electrically commutated motors or ECM furnaces, could save a typical home owner about 1,020 kWh of electricity and about 20 therms of gas per year. At $.08 per kWh and $.80 per therm, this amounts to about $97.60 per year. Savings will increase if the blower is left on all the time. Savings will also increase if the blower is used in the summer with a central air conditioning system. The additional costs for this new generation of furnaces are estimated to be about $500 over that of an older high-efficiency furnace. That means that it would pay for itself in about five years. Looked at another way, you would be earning just under 20 percent a year on your investment. As an incentive to encourage consumers to buy these new furnaces the Focus on Energy program is offering a $150 rebate to homeowners. To find a heating contractor in your area who is participating in the rebate program call (800) 762-7077.

For more information on saving energy in your home, visit Wisconsin's Focus on Energy program Web site at http://www.focusonenergy.org .

If you have a housing question, you may send it to John Merrill, 1300 Linden Dr., Madison, WI 53705, or visit the UW-Extension housing web site at http://www.uwex.edu/house .