Furnace Filter Replacement: Impacts of a Dirty Furnace Filter

Date Published: November 1, 2019

One of the most common things we recommend during a home inspection is to “consider furnace filter replacement at this time”.  Why is such a simple action so often overlooked? And what impact does having a dirty furnace filter have on the home? Often the simplest things can cause the biggest concerns.

In this article, we will look at the importance of the furnace filter in the health of both the furnace and the occupants of a home.  Then we will look at the impacts of a dirty filter.  Finally, we will explore the replacement of your furnace filter and items to consider.

Importance of the Furnace Filter

A furnace and forced air heating/cooling system operates by using a large blower fan to circulate air in a home.

When in heating mode, a gas heating element or heat pump unit will heat up while the fan blows air across the heating element.  The heat is transferred into the air which is then blown through a series of ducts in your home, then blown into each room at the registers in the floor, walls, or ceiling.  At the same time, return grills or registers are present that pull air from a room and back down through ducting to the base of the furnace.

furnace filter replacement example of a dirty filter

All forced air systems are a recirculating system.  The main blower fan operates at a certain cubic-feet-per-minute (CFM) of airflow.  That flow is split up between rooms based on the room size, and air is then pulled back through the return registers and reheated or re-cooled.

At the base of the furnace blower fan, the return air passes through an air filter.  The job of the furnace filter is to do just that – filter the air prior to passing through the furnace.  Dust, dirt, mold spores, and anything in the air that can be caught by the filter gets trapped and retained on the surface of the filter.

The benefit of using a furnace filter is to get those bad particles out of the air you are breathing and improve indoor-air-quality, as well as to keep those particles off the furnace components.

Impacts of a Dirty Furnace Filter

The longer a filter is installed and not regularly replaced, the more collection of dust and dirt there is on the filter.  The primary impact of a dirty furnace filter is the potential reduction in airflow across the furnace and the subsequent potential imbalance in heat/cooling flow throughout the home.

The best example we can provide is if you were to wear a paper mask around all day and have to breathe through the mask.  Over time as dust and dirt naturally build up on the mask, it would get harder and harder for you to draw in a full breath of air into your lungs.  The same is true of your furnace.  The dirtier your furnace filter is, the harder it is for your furnace to “breathe” which makes it harder to heat and cool your home.

Furnace filter replacement comparing old with new

At some point, if a dirty filter is left in a furnace for a considerable amount of time, the filter will begin to collapse inward from the pressure of the blower fan.  At that point airflow will work around the edges of the filter, allowing unfiltered air to blow through the furnace.

At that point, the dust, dirt and other particles in the air stick to the interior parts of the furnace and begin to build up internally.  This can cause wear on the components like the blower fan.  If you have air-conditioning, the interior cooling coil can begin to collect dirt and dust which will restrict the coil and inhibit proper cooling.

Furnace Filter Replacement

Needless to say, regular replacement of your furnace filter is critical for both your health and the health of your equipment.  It is generally recommended that your filter be replaced every 3 months.  This is a general rule of thumb, but a few factors can go into varying this schedule.

First, is the quality of the filter you are using adequate?  Some of the inexpensive, thin, non-pleated filters with a thin cardboard frame require more frequent replacement since they can only hold so much dirt, dust, and debris, before collapsing.  Other higher quality filters can go slightly longer between replacement.

At a bare minimum, we recommend replacing your filter 2 times during the heating season and at least once during the cooling seaso

Locating your Filter

The first step in replacing your furnace filter is to locate where it is!  Most furn