Reasons for Dirty Furnace Filters

Furnace filters become dirty during the heating or cooling season as air is pulled through the furnace for circulating through the house. A dirty furnace filter can cause parts to wear out on the furnace and increase energy consumption. Replacement furnace filters should be chosen that meet the needs for the amount of dirt in the air where the furnace is located.

The Facts

A furnace pulls air from the room where it is located, heats the air and a blower moves the warm air through the heat ducts in the house.  A furnace filter traps airborne particles, such as pet dander, dust mites, pollen, mold spores and bacteria from the air entering the system to prevent the dirt from entering the blower that could increase maintenance.

If the dirt from the air enters the blower, the blower could become clogged with dirt and require maintenance or replacement.  It is easier to replace a furnace filter than to replace worn parts in the blower assembly.


A furnace filter that becomes dirty in less than an average heating or cooling season may indicate poor air quality.  A furnace filter should be replaced annually at minimum, depending on the type of filter.

If the filter requires replacing more frequently, the source of the excess dirt in the air should be identified.  For example, a furnace located in a wood shop would become dirty from sanding dust; a furnace located in a house on a dirt road will become dirty more frequently due to dust from the road.

If a furnace is used in conjunction with a wood or coal stove, airborne ash and smoke residue also contribute to a dirty furnace filter.  When the furnace filter is located in areas where there is a higher than normal amount of dirt in the air, the filter should be checked and replaced as needed.

A dirty furnace filter restricts air flow, which can increase drag on the furnace blower, resulting in worn parts and an increase in operation costs. 


There are many types of furnace filters.  Flat filters are often the least expensive.

However, this type of filter is often the least effective at filtering dirt from the air.  Pleated filters have a larger surface area and are often more effective at removing smaller particles from the air.

Air filters have a standard efficiency rating of 1 to 20.  This is called a Minimum Efficiency Reporting Value (MERV) rating.

A rating of 1 is the lowest efficiency, and a rating of 20 is the highest efficiency.  Filters with a low MERV rating trap larger airborne particles such as animal dander and sanding dust.

Filters with a high MERV rating trap smaller particles such as dust mites and bacteria.  Consider purchasing furnace filters of a higher rating to increase the filtration of the air entering the furnace.


Many furnace filters work better when they have accumulated a little bit of dirt.  The particles that build up in the filter help trap smaller particles.

However, as the dander, pet hair, dust and other particles on the furnace filter increases, the air flow becomes restricted.  A filter is dirty when there is an obvious buildup of dirt on the filter, and it's then that it needs to be replaced.


During regular use, a furnace filter should be effective for an entire heating season.  However, if the filter is dirty before the end of the heating season, it can be vacuumed to remove the buildup of larger particles.

Some filters can be rinsed.  If the furnace is located where there is a large amount of dirt in the air, such as a home with multiple pets, a wood or coal stove, a wood shop or in a house with a dirt road, consider a secondary air cleaner to help reduce the amount of dirt, which will in turn reduce the amount of dirt accumulating on the furnace filter.

About the Author

Emily Patterson has been creating content for websites since 1996. She specializes in home improvement, natural body care and natural cleaning articles. Patterson holds a computing certificate from Penn State University.