The problem: While gas-fired furnaces can last 20 years or more, ones made before 1992 are only 55 to 78 percent efficient, compared with up to 97 percent for today's.
How to spot it: If your furnace has a pilot light, it's likely more than 20 years old and only about 60 percent efficient. If this telltale sign isn't present, ask an HVAC pro to inspect the furnace and assign it an annual fuel utilization efficiency (AFUE) rating based on its age. An AFUE of 80, for instance, means that 80 percent of the fuel burned is converted into heat for your home.
How to stop it: Replace an old furnace with a properly sized modern unit with a high AFUE. Manufacturers now display the rating right on the furnace so that consumers can easily compare the efficiency of various models. Expect to pay $2,500 to $4,000, including installation. On the high end are ultraefficient furnaces with a rating of 97, such as Trane's XV95. Its variable-speed fan motor adjusts to provide a consistent flow of warm air, making your home more cozy and saving you extra cash over the long haul.
The payoff: Cut your heating-fuel bill by more than 30 percent by replacing a 60-percent-efficient furnace with one that's 97 percent efficient.