No furnace will keep functioning efficiently without regular maintenance. Ideally, you should have a furnace inspected and serviced annually by a qualified professional before the heat season begins. If you do your own maintenance, have it inspected professionally every 3 to 5 years.

The first maintenance chore is to clean or replace the furnace filter once a month. You should also start every heating season with a thorough cleaning, repeating this step as often as needed. Before cleaning it, be sure the furnace is off and cool. On a forced air type, open the fan compartment and vacuum away all lint, dust, and debris. Then clean out the fire box or burner compartment. Use the brush attachment to remove soot and other debris from the walls of the chamber.

Some walls are made of fragile fibrous materials, so vacuum carefully. If possible, remove the flue connection and vacuum it and the flue. If you have a forced air system, service the blower. Check the fan belt for tension; it should "give" about a 1/2-inch. Moving the motor bracket adjusts the fan best. You can also change the controls on the blower so it cycles the air at lower temperatures, allowing the blower to work even when the burner is not.

If the furnace is oil-fired, oil the bearings in the pump motor. Locate the fuel filter in the main oil line and change it once a year. Every 2 to 5 years the burners should be checked for proper adjustment, so that they mix the right amount of fuel and air for efficient combustion. For oil furnaces, this requires testing the smoke emissions.

Black smudges around the inspection door to the burner compartment are a telltale sign that an adjustment is needed. Even though gas burners can be adjusted by watching the flame, this should be done by a professional.

How heat is distributed. A forced-air system blows heated air from the furnace to the living area through a system of supply ducts to room registers. The openings of these registers can usually be regulated by means of small levers that increase or decrease the amount of heat flowing into a room while the furnace is on. A separate set of ducts, called " cold-air return ducts", returns the air from the living space to the furnace.