Warm air furnaces use a filter to capture airborne particles to reduce wear on the fan and heating coils. They also improve indoor air quality. The filter should be periodically replaced. A dirty filter puts more strain on the heating system, and this can lead to a premature breakdown.
How frequently to replace the filter depends upon a few factors. First of all, always follow the manufacturer's recommendations. Typically, inexpensive fiberglass filters should be replaced every one to three months, depending upon use and environmental factors. High-efficiency filters are typically good for 3 months to 1 year. Some units have washable, reusable filters. These should be rinsed out monthly.
Inexpensive fiberglass filters capture larger particles but they are not effective for pollen and other allergens. To get the greatest filtration benefit from your heating system, you should use high-efficiency air filters. High-efficiency (HE) filters are typically pleated, dense weave material. Compared to the relatively open-weave of fiberglass and electrostatic filters, HE filters capture more airborne particles as well as smaller particles. If you have any question about what filter to use, check with the manufacturer's website for information on choosing the right filter.
Replacing a filter is usually very easy. Purchase a replacement filter of the proper size. If you aren't sure what size to choose, take the old filter with you to the store. The filter may be located at the cold air intake, a large rectangular vent near the furnace or it may be located on the furnace itself. Change the filter when the heater is off. Generally, a removable panel provides easy access to the filter. Simply open the panel, remove and dispose of the old filter, position the new filter to fill the opening and then close the access panel.