You may also need to consider your home's vents and chimneys when you are purchasing a new furnace. Replacing a low-efficiency furnace with one that has an AFUE of 90 percent or more will require installing a vent that meets the special needs of a high-efficiency furnace. Furthermore, if other appliances (a gas-fired water heater, for example) share a vent or chimney with the old furnace, they, too, may need new or modified venting. All this work can easily add hundreds of dollars to the installed cost of a new furnace.

Note that high-efficiency models have more complex controls and thus tend to need more repair than other models. We think that's partly because very high-efficiency furnaces tend to have more components that could break down and are more likely to use new designs that are not yet tried and true.

REPLACE OR REPAIR? If your furnace falters or fails, a few simple procedures may save you the cost and trouble of seeking professional help: If you're getting low airflow, check the air filter on the furnace; a clogged filter could cut airflow to a trickle. See if there are loose wires or a malfunction in the thermostat. For an electronic thermostat that runs on batteries, try changing them. Are fuses burned out or circuit breakers tripped? If so, power may have been cut to the fan or circuit board. If those steps don't work, call a heating contractor. Despite the improved efficiency of most new furnaces, it's generally more cost-effective to repair a furnace than to replace it. However, if a key component such as the heat exchanger or control module fails, you're probably better off replacing the furnace, especially if the unit is more than about 15 years old (furnaces typically last an average of 15 to 18 years).