A furnace should be replaced once it is 15 years old, according to Energy Star. Other symptoms indicating the need for a furnace replacement include frequent necessary repairs, problems with humidity and temperature variations between rooms. Such symptoms could be a sign of improperly working equipment or ductwork.
An improperly working furnace can be expensive, uncomfortable or even dangerous. According to Bob Vila, a furnace can be professionally rated by using the annual fuel utilization efficiency method of evaluation, which ranks a furnace based on its efficiency. If a furnace scores under 80 percent, it is probably more cost efficient to have it replaced. Finally, a faulty furnace can be dangerous to its occupants. Each year there are cases where people become sick or even die from carbon monoxide poisoning, which is a by-product emitted by the burning of natural gas.