We came across this article from the American Council for an Energy-Efficient Economy and thought it would be helpful to some of our readers!

Decide Whether to Replace Your Existing System

If your furnace or boiler is older than 20 years, chances are it is a good investment to replace it with a high-efficiency model with the guidance of a good contractor. Also consider a replacement now if your system is one of the following:

• Old coal burner that was previously switched over to oil or gas
• Old gas furnace without electronic ignition. If it has a pilot light, it was probably installed prior to 1992 and has an efficiency of about 65% (the least efficient systems today are 80%)
• Old gas furnace without vent dampers or an induced draft fan (which limit the flow of heated air up the chimney when the heating system is off).

If your furnace or boiler is 10–20 years old, and you are experiencing discomfort or high utility bills, hire a highly-qualified home performance or heating contractor who can help you evaluate your existing system. Often it will be more cost-effective to improve house insulation and air-tightness, repair or insulate ductwork, or tune up your system before purchasing a new one. 

(Original article: http://aceee.org/node/3065#replace)