Furnaces will run without a furnace filter, but the furnace and air quality in the building are at risk. Dirt buildup in the furnace can cause it to malfunction, and dirt and dust particles that float around the air can cause serious health issues for occupants.
Furnaces that are run without filters will have dirt particles settling inside the furnace. Dirty furnaces must work harder to provide heat. During the struggle to heat the building, the furnaces wearing out the parts inside that keep the unit running. The blower motor, coils, igniter and contacts inside the furnace must remain clean for the furnace to be efficient and not cause a fire.
Dirt particles that aren't trapped by an air filter will not only make a furnace dirty inside, but the particles will continue to float around in the air. Floating dirt particles can aggravate respiratory problems in those who have a weakened respiratory system. Even people without health issues may experience eye watering, sneezing, coughing and other issues related to the air quality in the building. Danger, mold spores and dust are just a few dirt particles that can float around in the air if the furnace doesn't have a filter installed.
Fiberglass and pleated furnace filters are disposable. The fiberglass filters will capture large dirt particles but not the small dirt particles that are less than 0.3 microns. The pleated furnace filters will capture large, medium and small dirt particles in the air. These filters are used once and discarded. Filters should be changed once a month when the furnace is in use.
Reusable filters include the washable filters and the electrostatic filters. The washable filters will catch small dirt particles if you spray a sticky substance on the filter grid. The electrostatic filters capture dirt particles using static electricity, but you don't need any electricity. The special filters are electrically charged. When fit into a furnace, the static captures the particles. These filters need to be washed and dried when dirty.