How to Tell if I Should Change Furnace Filter?

One of the most inexpensive parts of an HVAC system is also the most vital. That would be the furnace or AC filter, something you’ll find on every modern HVAC system. Replacing filters on a regular basis is the best way to keep your system running at peak performance.

It’s also something that can be challenging for a variety of reasons, which we are going to discuss in this guide. Whether you are curious about how often to replace a furnace filter or their location, we have answers to all your AC and furnace filter questions.

Where are Furnace Filters Located?

Before you can think about changing the air filter in your home or the type of filter you need, the first thing you should do is familiarize yourself with filter locations. Where the filters are located depends on your home and the type of air handler that’s installed.

In apartments or condos, the filter could be tucked away into a closet. This is especially common for vertical HVAC units which are installed in areas where space is limited. The location of filter access varies depending on the design as a downflow furnace has an intake at the top, and discharges at the bottom.  Some of these units, along with older models, have filters that slide into a slot on the HVAC system.

In larger homes where horizontal HVAC units are installed, you can usually find the furnace filter in the garage, basement or attic. They can also be in more convenient areas, however, like this large return plenum you see in the photo below. It pulls in a lot of airflow, even if the placement isn’t exactly ideal in this instance.

large return plenum

It’s important to remember that some systems can have multiple filters in both the supply and return plenum as well. If you can’t find a filter, keep looking as it could be in a very unlikely spot including a hallway wall or ceiling. External filter racks can also provide access to hidden filters near furnaces like this one from Alpine Home Air.

How to tell if I should change my Furnace Filter

As mentioned, a bit of dust in the area around your filter could be a sign to replace the filter, but there are other things to consider as well. All the air that passes through an HVAC system goes through these filters, and what happens indoors can have a direct effect on how long they will last.

The most obvious sign of when you need to change an air filter is when it simply looks dirty. With cheap fiberglass filters, that can be hard to discern unless you see visible dust, but on “white” pleated filters, the change can be far more noticeable.

Have you noticed the airflow coming through your vents isn’t quite as strong as it should be? That could be because of a filter that’s become clogged. A clogged or bad filter will cause your system to work harder than it should, which could lead to unnecessary wear and tear. Conditions like allergies and asthma can also be aggravated when the air quality indoors decreases due to a dirty or poorly made furnace filter.

How often should I change HVAC filters? 

Replacing filters is something anyone can do, and as you can see, it’s not difficult to tell when one is dirty. Actually knowing when or how often to change a furnace filter is a different story, however. Whether it’s located in a return air duct or on the air handler, knowing when to pop in a replacement filter is key.

As a rule of thumb, most HVAC professionals recommend changing the air filters in a home once every 60 to 90 days when using a high-quality pleated filter. Fiberglass filters may need to be changed once a month and some premium 4-inch models can last between 4 to 6 months. Keep in mind, the longer the filter goes without being replaced, the less effective it will be.

Do you have pets in your home or people who smoke? If so, you’ll want to change your air filters more frequently because of dander and the pollutants that come from smoke. Homeowners that have washable filters in their HVAC system will want to refer to the manufacturers guidelines on cleaning these sturdy filters.

Choosing an AC or Furnace Filter for your Home

When it’s time to choose a clean new filter, there are two things you’ll want to remember – the size and rating. Furnace filters are typically sold in two thicknesses with 1-inch and 4-inch air filters. Thicker versions come with a premium price tag, while thinner 1 inch filters can be fiberglass filters or pleated filters.

Ratings are measured using one of three scales, and the numbers tell you how effective an air filter is at blocking dust and contaminants. 3M Filtrete filters use MP and Honeywell uses FPR while everything else essentially falls into the MERV category. You can find some of the best filters for furnaces and more buying tips in our guide.


Knowing when and how to change a furnace filter can be difficult at first, but will become routine once you understand filter sizes and know where they are located in your home. Keeping the filters changed on a regular basis not only ensures clean air quality indoors, but also keeps your HVAC system running at peak efficiency throughout the year. 

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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