New high-efficiency furnaces can cut heating
Sharon C. Laux, PhD
Housing & Environmental Design Specialist
Are you thinking about replacing your furnace? With rising fuel prices, you might want to consider purchasing a high-efficiency furnace. A high-efficiency model is defined as having an Annual Fuel Utilization Efficiency (AFUE) of at least 90 percent.
AFUE is a measure of the amount of fuel 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 90 percent-AFUE models, only 10 percent of the 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 purchased a 90 percent-AFUE model), your bill could drop to about $722 (assuming natural gas prices were 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 sits above the burner and transfers heat to air flowing through heating ducts. Even after 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 fuel efficiency primarily by running the hot exhaust through a second heat exchanger. This heat exchanger cools exhaust to the point that much of the water vapor (a major part of the exhaust) condenses. The exhaust is cool enough to be safe to exhaust out the side of the house through a plastic pipe. A small built-in fan ensures that the exhaust exits the house. Most high-efficiency furnaces have a second plastic pipe that brings a supply of outside air to the furnace so the furnace doesn’t have to draw air in through cracks in the house. This cuts down drafts in the house.
Now a new generation of energy-efficient furnaces offers even more savings. Called a two-stage burner, this type uses controls that allow the burner to burn at different rates and more efficient blowers that also operate at different speeds.
Traditionally, furnaces have had one burn rate that was set to provide enough heat on the coldest day. On warmer days the furnace would go on/off frequently. The latest generation of furnace 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 blower uses DC motors and can vary speed infinitely, using about a tenth of the energy used by traditional blower motors. In conjunction with the two-stage burners, this type of furnace provides greater comfort and a considerable savings in both electrical energy and natural gas. The new blowers are also much quieter and don’t produce a rush of air when they first start.
Independent evaluators estimate that these new ECM furnaces—using what is referred to as electrically-commutated motors—could save a typical home owner about 1,020 kWh of electricity and about 20 therms of gas per year, or $97.60 per year (figuring $.08 per kWh and $.80 per therm). Leaving the blower on all the time will save more money.
Savings also increase if the blower is used in summer with a central air conditioning system. The additional cost for this new generation of furnace is estimated to be about $500 more than an older, high-efficiency furnace. That means it would pay for itself in about 5 years. Looked at another way, you would earn just under 20 percent a year on your investment.
For more information on energy-efficient furnaces and other home appliances and lighting, look for the Energy Star label. These products have met guidelines set by the US Environmental Protection Agency and the US Department of Energy. Energy Star labels can also be found on newly-constructed homes.
For information about the Energy Star program, call 1-888-782-7937, or visit www.energystar.gov.
Adapted from an article by John Merrill, Housing Specialist, University of Wisconsin Extension.