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How to clean furnaces and fix the blower

In a forced warm-air heating system, cool air from the house enters the furnace through the return-air duct and passes through a filter. A motor-driven blower forces the air over the hot metal pipes of a heat exchanger located on top of the burner or over electric heating elements. Heated air flows into ducts ending in room registers; some systems humidify the air. A fan and limit control turns the blower on and off and shuts off the burner if the air passing through the hot-air duct becomes too hot.

Once a year, have the furnace professionally cleaned and checked for leaks. An oil burner and its controls should also be serviced yearly; a gas burner, less often. Basic cleaning and maintenance between servicings will keep the furnace working smoothly.

Caution: Before cleaning or working on a furnace, turn off power to it by switching
off the safety switch, usually remotely mounted but sometimes near the furnace.

Cleaning the filter and the blower A dirty filter reduces air flow and heating efficiency. Once a month during the heating season, remove the filter and hold it up to a bright light. If you can’t see light easily through the filter, clean or replace it. Plastic or metal filters can be washed and reused. Hardware stores sell replacements for fiberglass filters. Install a filter so that the arrow marked on it points in the direction of the air flow.

Once a year, remove the blower access panel, and vacuum the area around the motor and the blower. Use a vacuum cleaner and a brush to clean the blower blades.

Adjusting the blower belt

In a belt-driven blower examine the belt for tension and wear twice a season. Check tension by pressing lightly on the belt midway between the motor and the blower pulleys. It should deflect by only 1/2 to 3/4 inch. A tight or worn belt causes blower noise; too much slack reduces air flow. To adjust tension or to replace a worn belt, see Drive belts.
Blower and motor pulleys that are out of alignment cause belt wear and power loss. To check alignment, place one edge of a carpenter’s square against the outer faces of the motor and blower pulleys. If they are not in a straight line at right angles to the motor, loosen the motor pulley’s inner setscrew with a hex wrench and move the pulley forward or back as needed. Adjusting blower speed

A slow blower delivers insufficient heat. A blower that runs too fast is noisy. To adjust blower speed, loosen the outer setscrew on the motor pulley with a hex wrench. Turn the outer pulley face clockwise to bring it closer to the inner face; this increases blower speed. Turn the outer face counterclockwise to decrease speed. Tighten the setscrew; adjust pulley alignment and belt tension.

On a furnace with a direct drive blower, turn off all power to the furnace and remove the cover from the blower junction box, mounted on the blower or next to the furnace box. A hot wire; usually red, will be attached to one of two terminals marked low and medium low; these control the blower when the furnace is working. A second hot wire, usually black, will be attached to the medium high or high terminal used for central air conditioning. To increase blower speed, disconnect the appropriate wire and switch it from low to medium low or from medium high to high; reverse the procedure to decrease speed. Oiling the motor and the blower If there are oil cups or grease fittings on the motor and the blower, lubricate them annually. Put a few drops of 20-weight nondetergent oil into the oil cups or two “pumps” of automotive grease from a grease gun into each fitting. Don’t overoil.