What is a Furnace Air Filter?

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  • Written By: Ken Black
  • Edited By: Bronwyn Harris
  • Last Modified Date: 25 February 2020
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A furnace air filter is device that is meant to clean the air as it passes through the furnace. There are a number of reasons furnace air filters are useful and important. Not only do they increase efficiency, they also increase the safety of operating furnaces and may affect the health of those who are in the home.

Just like lint and other materials that can build up in a dryer or stove vent and be highly flammable, materials that build up in an air duct can also cause a fire. Duct fires can travel fast and the ducts provide a natural conduit for fires to travel along. An air filter can help remove solids that may otherwise get caught in the ducts and ignite.

A furnace air filter can also have some health benefits as well. One of the harmful health effects associated with circulating air is the ability of that recirculated air to pick up allergens and put suspend them in a room. These can especially be irritating in the winter, when the relative humidity makes the air much more dry than at other times of the year. Some furnace air filter models are designed specifically to remove microscopic allergens from the air.


A furnace air filter is usually made out of one of three types of materials: paper, fiberglass or foam. Paper and fiberglass filters are often the cheapest but need replacing more often. They are designed for one-time use. It may be possible to remove foam air filters from furnaces and vacuum it or wash it with water. Thus, those filters can be reused.

During the coldest times of the year, it is recommended to check a furnace air filter at least once per month. For those who are not used to dealing with air filters, the best way to tell if it is dirty may compare it to regular filter. If a substantial amount of light is blocked looking through the used filter compared to the new one, the time has come for a replacement. Foam filters simply require an inspection to see if they are dirty. A dirty furnace air filter can lower the efficiency of a furnace making it work harder to force air through.

When looking at a furnace air filter, it is important to choose one that is sized to fit your particular heating system. Some heating systems may require a different type of filter made of a different material. The manufacturer's recommendations should be followed exactly. Otherwise, the effectiveness of the furnace may be diminished.


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