By having the furnace cleaned out, you will get what is called “gas scale build-up” out of the heating elements of the furnace such as the heat exchangers and combustion chamber. This substance looks like rust, and will plug up the components of the furnace, causing them to overheat and eventually crack.
The other part of the furnace that is vital to clean is the blower and blower motor. As the blower and blower motor become contaminated with dust and debris, the blower motor begins to lose efficiency and have to work longer and harder. Eventually, the blower motor will burn out if there is too much contamination. In fact, many furnaces that have not been properly cleaned and maintained, will have to replace their blower motor long before the entire furnace needs replacement.
Most experts will agreee, you should have your forced air furnace and air ducts cleaned every year for safety and efficiency purposes, as well as extend the life of the appliance.