The average service life of a gas furnace is about 15 years, but the actual lifespan of an individual unit can vary greatly.

Replacing an old furnace costs between $2,000 and $6,000 based on a national average. Things like the type of furnace (electric, gas or oil), how efficient the unit is, size of the furnace, and the brand name impact not only the furnace price itself, but the installation cost, too.

A good rule of thumb is, no matter the age of the furnace, if the cost to repair is 50% of the cost of a new furnace, you should replace it. But for less expensive repairs, you should be less resistant to replacing the furnace the older it is.

A gas furnace should be serviced at least once a year in the fall. Annual maintenance, including a thorough examination and cleaning of the parts that commonly malfunction, like the air filter, the fan, the pilot light, and the heat exchanger, can usually prevent these problems.

A good rule of thumb is to change 1-2 inch filters every month, 4 inch filters every three months and 5 inch filters every 6 months. Signs your air filter needs changed: The filter is visibly dirty. Certainly if you can't see the material of the filter itself, it should be replaced.