Air Filters      


blankFor the location of filters and different types of furnaces. (See figure on left).

Air filters are provided on all forced air furnaces to remove dirt and lint from heated air. (See figure on left). This keeps the fan, heat exchanger and air conditioning coil clean. It also helps clean the air of your home as air circulates through the system (Note the direction of the air flow).

Media Filters


The standard filter on most furnaces is a nominal 1"-thick media filter. (See figure on right). Usually, this filter is made of fiberglass. The filter should be changed when it is visibly dirty - usually every one month or two, depending on the quality of the filter and the amount of dirt in your home's air. Children, pets, plants, and ablankctivity tend to produce more dirt that finds its way into the heating system.

Be careful about the direction of the airflow through the filter. Filters are designed to be installed with one particular side facing the air stream. Most filters have directions or an arrow telling you which side should be installed toward the furnace. The arrow is the direction of the airflow and should be toward the base or the fan of the furnace.

Remember: the furnace filter is also used when you operate the fan and/or central air conditioning, so you should check on the filter during the summer, too.

Must Know/Must Do
Routinely Maintain the Furnace Filter

Maintenance is based on the type of filter, how often the unit is running (heating and cooling), and how you use your home. The three basic types of filters are media, electronic, and electrostatic.


blankI recommend that you try one of the pleated paper filters. These catch more dirt than inexpensive fiberglass filters. (See figure on left). Some even have a static charge to attract dirt. Others have a carbon filter content. Paper filters cost between $3 and $15 and can be found in most hardware stores. You will need to change this type of filter more often because it collects more dirt.


Washable filters can be made of foam or woven synthetic fiber. They are about as effective as inexpensive fiberglass filters. You can improve the efficiency of a foam filter by spraying it with a special filter coating; this oily/waxy spray helps the filter hold dirt better.

A big improvement over the standard 1" -thick filter is a pleated fiberglass or paper filter.

Often, the pleated paper filter is housed in a 6" -thick frame. The paper filter is very
fine, and it catches smaller particles of dirt and dust. This type of filter is normally changed once per year, and you replace only the paper element. (See figure on right).

A pleated fiberglass filter often is mounted in a throwaway paper frame. The entire unit is replaced about once a year.

Electronic Filtersblank

Electronic filters use electrically charged metal plates and wires that attract dirt. (See figure on left). These filters can remove very small particles from smoke and pollen which aren't caught by standard filters. If you have respiratory problems or are sensitive to dust or pollen, you may want to use this type of filter.

Electronic filters cost more than $600 to install. Maintenance involves washing the interior frame and metal plates and wires with detergent or running them through a dishwasher. Most electronic filters have a metal pre-filter that also must be washed. For more specific cleaning instructions, contact a heating contractor or the filter manufacturer. The references section includes contact information.

Electrostatic and Electronic Filters

Many types of washable filters have multiple layers of filtering material; vendors claim these layers contain an electrostatic charge that attracts and traps dirt more effectively than a standard media filter.

Several companies also make a 1" -thick electrostatic/electronic filter as a direct replacement for throwaway filters. This filter may have an electronic power supply and may require particular maintenance procedures.