Furnaces consist of two main operating systems: one for generating heat; the other for distributing it. In a hot air system, a blower forces air through ducts that open into each room. Fixing a malfunctioning blower may be less daunting than you think. After all, blowers are basically just large fans.
Before every heating season:
Clean the fan blades. Most blower fans are of the "squirrel cage" variety; each blade needs to be cleaned completely for optimum efficiency. Use an old toothbrush to do the job.
Check the belt that connects the fan to the blower motor for cracks and other wear and tear. If you see cracking or fraying, replace the belt. Take the old belt to a hardware store so you can find the exact type.
Lubricate the motor. Most have oil ports located above the axles so that the inner bearings can be lubed. Apply two or three drops of 20 weight non-detergent or dedicated bearing oil.
If the furnace blower suddenly quits running, there are several likely culprits.
The reset button may have been tripped if the motor had overheated from constant use or something had plugged up the fan blades. Unplug the power; make sure the fan spins freely; push in the reset button; and then turn the power back on. Your problem may be solved.
If the motor is running but the fan is not turning, the belt may be broken. Take it to a hardware store to find its match. To install the belt, place it on the motor pulley first, then turn the fan by hand as you press the belt onto the fan pulley. It should almost self-feed back on.
If the motor is sluggish or sqeaking, oil the bearings. Place two or three drops into the oil ports.
Eventually, the motor may simply burn out. You'll notice a burning smell or the motor will simply refuse to run no matter what you do. At this point, you have little choice but to replace the motor.
Detach the motor from its mounting and disconnect the belt. If the fan is directly attached to the motor (without a belt), remove the fan first and then pull out the motor.
Take the motor to the hardware store so that you can make sure to buy a replacement with the correct horsepower and RPM rating. You shouldn't have trouble finding one that suits your furnace.
Most motors come with two types of bearings: sealed sleeve bearings or oil-ported ball bearings. Sealed bearing motors will last for years, but they can't be lubricated. With proper lubrication, ball-bearing motors may last even longer.
Once the motor is back home, reinstall it the way it came out. Perform regular maintenance every year, and your furnace blower will last a lifetime.