An annual inspection of your unit can ensure that the various potential problems mentioned above are detected early, and before they require complete replacement. During this inspection, for instance, a professional might find small cracks in your heat exchanger or a faulty seal in your blower motor. Some manufacturer's warranties even require these annual inspections to maintain validity. Typically, your furnace inspection will cost between $80 and $150 depending on the area in which you live and the professional you work with.
Put simply, any home that uses air pushed through ductwork for heating purposes takes advantage of forced air heating. In other words, this term refers not to your furnace, but to the delivery method of the heat throughout your home. Forced air heating is possible with electric or gas furnaces, or a heat pump. Any issues with this delivery system that aren't directly related to the furnace likely have to do with the duct work, which costs between $35 and $55 per linear square foot to repair, or the air handling unit.
In addition to the information below, see these two articles for the general care and maintenance of your air conditioner: Preparing Your Air Conditioner for Summer and How to Replace Furnace & AC Filters. Most noteworthy, you should replace the filters at least twice a year, before the heating and cooling seasons. For information on furnace problems, please see Furnace Not Working.