Modern furnaces that depend on heat though a series of ducts and air ports to spread heat throughout the house have a powered air compressor that’s powered by an external motor. Generally, this type of fan is called a squirrel cage due to its uncanny similarity to an exercise wheel in a cage. If the belt between the motor and the blower breaks, it can be replaced with a brand new belt in about 15 minutes with some basic hand tools.
Switch off the circuit breaker to the furnace. Confirm the power to the furnace is off by setting the thermostat’s fan setting in the house to the On position, and check to see that the fan engine did not come on.
Remove the front access panel of the furnace by removing the screws that mount the panel to the unit using a screwdriver. Lift the panel off of the unit and set it aside.
Loosen the two mounting bolts securing the engine to the furnace using a socket set. Then push the engine toward the compressor marginally, and remove the damaged belt.
Put a new blower belt above the blower pulley, then above the engine pulley. Pull the motor with one hand, applying tension to the belt, and tighten the two mounting bolts to the motor’s base.
Check the tension of the belt by pushing on the belt between the two pulleys. With proper tension, the belt should deflect approximately half an inch. If the belt is either too loose or too tight, loosen the motor mounting bolts and adjust the stress before tightening the bolts once more.
Replace the access panel and tighten the mounting screws. Then turn the breaker, and analyze the operation of the blower.