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 furnace filters are located in the blower cabinet of the furnace itself.  At times, the HVAC installer may have created a special access location for the filter immediately adjacent to the furnace unit, allowing for easier access and replacement.

Some homes have multiple filters that are located in wall return registers or ceiling return registers, allowing access in a more accessible location than going to the furnace itself.

Furnace filter replacement furnace slot

If your furnace filter is located within the furnace blower compartment, this will require removing the furnace equipment’s front access panels.  Prior to removing any panels, we recommend that you first turn off the furnace at the thermostat and wait until it completes the heating or cooling cycle and the unit shuts down.

Then, for safety, there is normally always a shut-off switch near the furnace to remove power from the unit.  Turn that off prior to opening the covers for electrical safety.  At that point, you can remove the cover(s) required to access the lower blower compartment.

Once open, the filter is typically slid into the compartment on the side or underneath the furnace where the return air inlet opening is located.  The filter slides in and covers the opening.  There is usually some mechanism for holding the filter in place so pay attention to how that is done.

Size of the Filter

Once the filter is located, you need to determine the size of the filter so you can purchase replacements.  All filters are marked with a length x width size.  Some also have thickness markings also.

For example, a 14 x 25 x 1 filter would be 14″ wide, 25″ long and 1″ thickness.  Take a look around the furnace and see if the HVAC installer wrote or marked the proper size of the filter for reference.  We have come across many furnace filters that are the wrong size for the furnace during our home inspections.  So make sure the filter appears to fit into the proper location.

If you had a home inspection completed, typically the furnace filter location and size are noted within the report.

Style of Filter Selected

Now that you know your furnace filter size, it’s time to go shopping for a replacement filter.  And if you have ever gone to your local big-box hardware store, or looked online, you know the simple act of purchasing a replacement furnace filter just got really complicated.

Just like most things in life – there are LOTS of choices.  Selecting which type of furnace filter to use can be moderately overwhelming.  There are pleated and non-pleated filters and different MERV ratings on filters.  MERV stands for the Minimum Efficiency Reporting Value and is a standard for determining how well the filter traps particles.

The higher the MERV rating, the better the filter is in blocking particles and keeping them out of your home and off the equipment.

Study the selection of filters and make your choice based on the level of protection you desire.  If you are uncertain, a good quality pleated, mid-priced filter is the best way to go, in our opinion.

Proper Installation

The most important part of installing a furnace filter is ensuring that you have it placed with the direction of airflow going in the right direction.  All furnace filters were designed to have air move through them in one direction.

There is usually an arrow on the filter that indicates the direction of airflow.  The blower motor pulls air through the furnace, so just think about where the filter is and which way air will pull through the filter, then install it with the arrow pointing toward the blower motor.

If there is a clip or retaining system to hold the filter in place or a slot the filter runs through, just make sure things are seated and positioned and secured properly.

Who knew there was so much to consider regarding furnace filter replacement? The smallest things can cause the biggest concerns.  But for the health of the occupants of the home and the long term health of your equipment, ensuring that you replace the filter regularly is an important part of your home maintenance plan.

At Scott Home Inspection, during our standard home inspections, looking at a furnace filter is one small part of what we do to help ensure the health of you and your equipment.

Colorado’s Top Rated Inspection Company:

Scott Home Inspection is Colorado’s highest rated inspection company offering home inspections, energy services, and Boulder Rental License Inspections. Click your service to learn more.

Home Inspection Services
Energy Services
Rental Inspections

About the Author: George Scott

George founded Scott Home Inspection in 2006, and has grown the business into a multi-inspector firm serving the Colorado Front Range, from Fort Collins down to Colorado Springs. As an ASHI Certified Home Inspector and Certified Energy Rater, George is an excellent resource to help with inspection and energy-related requirements.

Recent Posts

2802, 2024

The Importance of Attic Ventilation

In this day and age, it's all about building energy efficient homes. We put a lot of emphasis on insulation, especially in attics. Sufficient attic insulation is [...]

2302, 2024

Home Inspections in Lafayette CO

About Lafayette Lafayette, CO is a quaint city situated in the southeastern region of Boulder County. It was originally established in 1889 by Marie E. [...]

Ready For A Great Inspection Experience?

Experience quality with the top rated Colorado based company for home inspections, energy services, and rental inspections.



Visit Our Ask Your Inspector page to see all our posts, find great Colorado contractors, and ask us questions about your home. 

Ask Your Inspector