The forced-air heating system commonly known as central heating uses a system of ducts to deliver the warm air throughout the house. These systems have replaced most older-style furnaces, in which the air rose by convection. Central heating is somewhat ironically named, as the furnace does not have to be centrally located in the home to provide even heating throughout its rooms and floors.
Check the thermostat before you do anything else when troubleshooting problems with your central-heating system. Make sure that nobody has reset the thermostat, since this is often the simple reason why you notice the room is much hotter or colder than normal.
Go to your main service panel if the thermostat is not the problem and check for a tripped circuit breaker that serves the furnace. In older houses, look for a blown fuse. Reset the circuit breaker or replace the fuse with a new one. Turn the thermostat to a setting that should cause the furnace to start up.
Turn off the heating system at the thermostat if you notice that the heater is working, but it is not working as efficiently as usual. Check the air filter for dirt, dust and any debris that may be inhibiting air flow. A central-heating system cannot operate at peak efficiency with obstructed air flow. Replace a dirty air filter.
Go outside and check that the air flow obstruction is not related to the big condenser unit out there. Remove any impediments to proper air flow. Pull away any vines that are climbing up the outside of the condenser. Prune back any plants, including trees, that may be obstructing air flow.
Check the blower and motor of the furnace, located behind a removable panel on the bottom of the indoor unit. Locate the motor-bearing oil ports located at each end of the motor casing. Lubricate them with 20-weight motor oil.
Inspect the blower belt for signs of damage like cracking or splitting, and replace it if necessary. Press down on the blower belt to make sure it is at proper tension if there are no obvious signs of damage. The tension should allow you to press down about an inch. Locate the locknut on the blower-belt adjustment bolt. Loosen it with an open-end wrench if the belt is too loose or too tight. Turn the adjustment bolt clockwise to tighten the belt and counterclockwise to loosen the tension.
Make sure that the pulleys on the blower shaft and motor shaft are aligned accurately. Correct alignment allows the belt to ride straight and not bump up against the rim of either pulley. Place a straightedge alongside the two pulleys to check for alignment. Loosen the setscrew on the motor shaft and tap the pulley sideways very lightly until it is accurately in line with the other pulley. Tighten the setscrew.
Turn off the power to the furnace before attempting any repairs.