There are signs that your furnace may be need replacing.  First off what is the age of your furnace?  A furnace generally lasts 16 to 20 years.  Are your furnace  bills going up and have you had to have repairs done on it over the past 2 years?  Does the furnace make strange noises, is your flame yellow instead of blue and does your furnace keep the house comfortable?  Is your house dry and dusty and has your family been sick more often than usual?  If you answered yes to these questions you probably could use a new furnace.