Every furnace that circulates warm air also builds up a certain amount of dust particles that travel through the venting or ductwork system into the input return vents located on interior walls. These input "returns" usually bring the circulated warm air back into the furnace process. The warm air that's returned contains these dust particles. They travel from the plenum into a small chamber at the base of the furnace and are trapped by a fabric filter. Most people refer to this as a "furnace filter".

Thanks to modern technology, there are now several types of fabrics available from which furnace air filters are made. Depending on the manufacturer's recommendations for furnace specifications, standard fabrics can range from typical fiberglass, coated surface, HEPA or other specialty fabrics with varying micron degrees of opacity. It's important to compare efficiency ratings of these fabrics with the manufacturer's suggested rating. The purpose of the density of the fabric is to insure clean air is circulated through output vents. This is also reliant upon cleanliness of the ducting as well. Ductwork that is heavily clogged tends to require more frequent changes of filters.

Depending on the size of the furnace, there are fairly standard sizes for most furnace systems. The size of a furnace is dependent on the overall size of the area to be heated. Therefore, furnace filters could be as large or as small as the size of the furnace. Filter sizes can easily be determined by checking the furnace installation and operations manual. The manual may even specify a specific type to be used.

The volume of furnace and/or central air conditioning use makes the difference in how often filters need to be changed out. It is simple matter, in most cases, of sliding the old filter out and inserting a fresh, new filter. Generally, people tend to put a new filter in at the beginning of a cold season and again at the beginning of a hotter cycle of weather. However, most furnace manufacturers prefer them to be changed out when there is a visible amount of dust particles on a wider part of the fabric surface. The dust particles collected do not represent a health threat and can be disposed of according to local disposal ordinances. One very important note to remember about changing out filters is to do so when the furnace is not operating. Removing the filter while the furnace is in operation can damage the blower motor.

Maintain a healthy heating and cooling system as well as ductwork by having a regularly scheduled "seasonal tune-up" performed. This insures a safe system as well as better system performance. Regularly scheduled maintenance helps preserve the furnace operation for long wear. The cost of these filters range quite a bit from a  fiberglass furnace filter to a HEPA filter. HEPA filters also trap allergens. They are manufactured from a special fabric that is set in a frame and are pleated for more efficiency. HEPA filters require less frequent change outs. Usually, one HEPA can remain effective for 6 months. However, these are more expensive. People who live in areas of where there are high levels of allergens find that HEPA filters reduce chronic dust problems as well as lung ailments.