Inspect and Replace

So many variables enter the equation of how often to replace the furnace filter -- the type of filter, living habits, the amount of airborne dust in the home -- that examining filters each month and replacing or cleaning them as needed is best. Filters should be replaced or cleaned at least every three months or at the start of each season.

Factors to Consider

Whether a family has indoor pets or smokers in the home can result in the necessity of changing filters more often than every three months. The same is true for homes built near dirt roads or where other environmental conditions produce high levels of dust and dirt. Once a filter becomes so clogged with dirt that it is no longer easy to see light through it, the filter needs to be replaced. Furnace filters only remove a small portion of the dust in homes, so cleaning the house with a vacuum that includes a HEPA filter can help improve inside air quality.

Types of Filters

An inexpensive, fiberglass filter in a cardboard frame does a good job of removing dust that can harm the furnace, but it does little against allergens and larger particles. Washable filters have a plastic or metal pad that may be hosed off and then reinstalled. Pleated polyester filters trap smaller particles, and many are electrostatically charged to attract tiny particles like bacteria, soot from smoke and smog, mold spores and the particles on which viruses ride.

Clogged Filters

The tight sieve between fibers in a polyester filter make it very efficient at pulling dust, dirt and harmful particles from the air. The same tight sieve leaves little room for keeping filters in place too long. Particles cling to the filter and quickly restrict airflow to the furnace, decreasing its efficiency and increasing heating bills. Low-cost fiberglass filters are more forgiving, and failure to change them exactly on time is less of an issue. Dirt clinging to these filters actually helps them capture more small particles. Even so, once light no longer penetrates through them, they need to be changed.