Does an Old Furnace Need to Be Replaced?

Energy Efficiency: Old Furnace

• If your furnace is more than 15 years old, it has a lower annual fuel utilization efficiency (AFUE) rating than newer furnaces. The AFUE rating describes how much energy the furnace uses and how much of that energy is converted to heat. Many older furnaces only have an AFUE of 60 to 70 percent. For example, an older furnace with an AFUE rating of 70 only converts 70 percent of its energy into usable heat. The other 30 percent evaporates or dissipates, and 30 percent of the energy you pay for --- natural gas, propane, electricity or heating oil --- goes unused.

Energy Efficiency: New Furnace

• In 1992, the U.S. Federal Trade Commission required that furnaces manufactured after that year have an AFUE rating of at least 78. Manufacturers took additional steps. Most new furnaces have an AFUE rating of at least 80, and some of the new high-efficiency furnaces, such as condensing furnaces, have AFUE ratings that range from 92 to 97. This means that a furnace with a 97 rating only loses or wastes 3 percent of the energy it consumes, and 97 percent is converted into usable energy for heating.

Energy Efficiency Comparison

• Let's compare energy use between an old furnace with a 70 AFUE rating and a new furnace with a 92 AFUE rating. To make the comparison fair, let's say that both furnaces have a BTU capacity of 100,000 and both use natural gas, since it is often cheapest for home heating. The old furnace would require 130,000 BTUs because it loses 30 percent of its BTU capacity. The new furnace would only require 103,000 BTUs --- that is 27,000 fewer BTUs and nearly 21 percent less than the older furnace.

Natural Gas Cost Comparison

• Natural gas is measured in therms, and 1 therm equals 100,000 BTUs. To compare costs, let's use the same old furnace with the 70 AFUE rating and the new furnace with the 92 AFUE. If a home used 10 million BTUs of natural gas during a year, it would cost $1,352 to heat the home with the old furnace and about $845 to heat the home with the new furnace. The natural gas bill would be $507 higher with the old furnace, which is 60 percent more than what it would cost to heat the home with a new furnace.

Replacement Options

• Consider new furnaces with high AFUE ratings and those with variable and two-stage heating capabilities. These types of furnaces will heat your home efficiently and further reduce your heating and cooling costs. Retrofitting an old furnace may not be a good choice if your furnace is more than 10 years old because it will be costly to find and replace compatible motors, fans and blowers. In most cases, purchasing an entirely new furnace will be the best choice. New furnace prices start around $1,000 and can run as high as $5,000 and more as of the date of publication in August 2011.