Furnace Filter Problems

Dirty Furnace Filter


The furnace filter is most definitely the most ignored part of routine home maintenance. As a home inspector near Buffalo, I try to inform my clients about these filters.

These systems make use of a square or rectangular filter that should be installed in the return air duct (also called a “plenum”), near the base of the air handler portion of the heating or cooling system.  

Many people refer to the air handler as the furnace, but some homes near Buffalo and Western NY use the air handler only for air conditioning (along with a boiler for heat), and a growing number have installed geothermal heat pumps.  Neither of these are furnaces, but they still need air filters. But to keep things easy, we’ll refer to the filters as “furnace filters”.

Furnace filters are often forgotten about, and when I inspect a house for a buyer or seller, sometimes I think I’m the first person to have looked at the filter in decades.    

The main reason why filters are installed is to protect the blower motor on the furnace or air handler from accumulating dust, debris, dog fur and anything else that might get sucked into the cold-air return vents of the HVAC system.  These things can become wound around the fan, or pulled into the motor and lead to overheating and unwanted resistance. Blower motors can cost hundreds of dollars to replace, so make sure that you replace your filter on a regular basis!

In recent years, as awareness and concern over indoor air quality has increased, so has the interest in using air filters to stop the movement of small particles such as dust mites, pollen and other allergens.  

Not all furnace filters are of equal quality, unfortunately.  The Minimum Efficiency Reporting Value, or MERV rating, was established in 1987 to help consumers understand the efficiency of furnace air filters. Filters with a MERV 13 rating can trap particles that are from 0.3 to 1.0 micron is size, which can include viruses, bacteria, tobacco smoke and mold.  Lesser MERV ratings are more common, and HEPA-rated filters are also available which provide the best filtration commonly available.

What are the most common problems with furnace filters?

Between shady HVAC contractors with low standards for the quality of their installations, and lax maintenance by homeowners, there are several issues that we frequently find when we inspect heating and cooling systems for our clients.

The filter is loosely installed in the furnace plenum

Lots of times when I start the HVAC inspection for my clients, I find a filter that is sort of just resting in the slot of the furnace plenum, with obvious gaps where air can find its way around the filter.  

If air can get around the filter, then it will bring dust, dirt, pollen, cat and dog fur and other junk with it.  The filter should always firmly rest in the plenum, and should require gentle force to remove. 

A good home inspector should take note on their report when filters are found to be loose.

This image shows a furnace filter “rack”, which provides an air-tight cover for the filter. This type of installation is preferred, since it improves the efficiency of the system.

Furnace Filter Rack

I would say that the #1 cause of loose filters is simply that the filter is the wrong size.  If your filter is loose, measure the thickness of the opening, width and height. Then go to the home center and buy a filter that matches those dimensions.  There’s a slight chance that your size isn’t stocked at local stores, and you may need to order online. 

The plenum filter opening doesn’t match a common filter thickness

Experience as a home inspector has taught me that there are varying qualities of HVAC contractors, and that a few of them in the Buffalo area take shortcuts when it comes to furnace filter accommodations in the plenum.  

The best way to make space for a filter is to use furnace filter “rack”, which sits inline between the air handler and plenum.  This rack provides for a tight fit the filter, and the best ones include a flap which uses a magnet to create an air-tight seal.  

Some contractors however just cut an opening in the plenum, without any care for its impact on furnace efficiency, air movement or the ability of the filter to actually filter air.

When a furnace filter slot doesn’t match a common filter size,  I recommend to my home inspection clients that they have a quality HVAC contractor install a furnace filter rack in place of the junky slot. 

The filter is installed backwards

Furnace filters are designed to restrict air flow as little as possible, while also filtering bad stuff out of it.  The air flow direction should be clearly noted on the side of filter. The arrow should always point toward the main body of the furnace or air handler.  

The homeowner has never changed the filter…

Furnace filters should be changed at least once every six months, or more frequently. If you have dogs or cats in the home, then perhaps more often. Filters can be purchased for modest $, and you can save if buying in bulk. Get a bunch of them and set a reminder on your calendar to replace the filter!

Wrap up

Frequent furnace filter changes are important for the longevity of your home’s furnace blower motor and for keeping the air in your home clean. High-quality furnace filters can filter out bacteria, viruses, pet fur and mold.

Make sure that you replace your furnace filter no less than every six months!

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About the Author:

Bradley Beck

Bradley is owner of Alto Home Inspection, LLC.  He lives just south of Orchard Park, in Western New York’s Southtowns, and inspects homes throughout the Buffalo area, and is NY Licensed Home Inspector #16000086029, NY Certified Mold Assessor #MA01313.