The three parts of the furnace that should be cleaned: the filter system, the blower and the motor.

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Cleaning a Furnace

Dirt is the biggest enemy of your furnace. It can waste fuel and drastically lower efficiency. In some cases, it can even cause the furnace to overheat [source: StartRemodeling.com]. Dirt affects all three basic components of your furnace, so cleaning is the most important part of regular maintenance. The three parts of the furnace that should be cleaned: the filter system, the blower and the motor.

The furnace filter should be replaced or cleaned at the beginning of the heating season and about once a month during periods of continuous use [source: Peterson]. To check the filter, take it out and hold it up to the light. If it looks clogged, replace it with a new filter of the same type and size regardless of the length of time it has been used.

A disposable furnace filter consists of a fiber mesh in a cardboard frame. The size of the filter is printed on the edge of the frame. An arrow on the edge of the frame indicates the correct direction of airflow through the filter. Air flows from the return-air duct toward the blower, so the arrow on the filter should point away from the return-air duct and toward the blower.

A permanent filter is usually sprayed with a special filter-coating chemical, available at hardware stores and home centers. Clean this type of filter according to the manufacturer's instructions, which are usually attached to the furnace housing. Here's how to replace a filter:

Step 1: Look for a metal panel on the front of your furnace below the return-air duct, between duct and blower system. The panel may be marked "filter," or it may form a lid or front of a boxlike projection on your furnace housing.

Step 2: Slip the panel off its holding hooks, or unscrew the panel from the box or furnace housing. On some heating units, filters are exposed; in that case, just slip the filter up and out of the U-shape tracks that hold it in place.

Step 3: Inspect and replace or clean the filter, depending on type.

Step 4: Clean the blower assembly, belts and pulleys to the blower and motor housing. Cleaning the blower is critical if your furnace has a squirrel-cage fan, because openings in this type of blower often become clogged with dirt. To clean your blower, remove the panel that covers the filter to gain access to the blower or panel on the front of your furnace. This panel may be slip-fit on hooks or held in place by a series of retaining screws. Access to the inside of your furnace blower is usually gained by sliding out the fan unit, which is held on a track by screws.

Next, we'll look at troubleshooting a gas furnace.