Why is My Air Conditioner Filter So Dirty?

If it feels like you’re constantly having to change your AC air filter, it could be because:

• Your AC fan is set to “ON”
• Your air ducts have leaks
• Your home gets a lot of traffic

We'll explain how the 3 situations above dirty up your air filter. But before we do that, let’s determine what's "normal" when it comes to how often you should be replacing your HVAC air filter.

Think you’re replacing your air filter more than you should? Contact us and we’ll send a tech to inspect your air conditioner and make sure everything’s working as it should.

Generally, the higher the MERV rating on your filter, the more often you’ll need to change it.

What’s a “MERV rating”, you ask? Every air filter has a Minimum Efficiency Reporting Value (MERV), which tells you how efficient the air filter is at removing particles and pollutants from the air. The higher the MERV rating (ranging from 1 to 20), the more efficient the filter. You can find a filter’s MERV rating printed on the edge of the filter itself.

In addition to MERV rating, how often you should change your AC air filter also depends on the type of filter you have:

• Flat-panel fiberglass (MERV 1–4): Usually 1” thick and made of spun fiberglass, these filters should be replaced once a month.
• Electrostatic washable (MERV 1–4): These reusable filters trap contaminants between negatively and positively charged fibers and should be vacuumed or washed at least once a month.
• Pleated (MERV 5–16): Made from polyester or paper, pleated filters come in a variety of thicknesses (normally 1–6 inches) and should be also changed every 1–3 months.
• HEPA (high-efficiency particulate air) (17–20 MERV): Mainly used in whole-house air purifiers, these filters catch the most contaminants and should be changed every 12–18 months.

Note: The timelines above are just recommendations. You may need to change your filter sooner or later than what’s listed above. We recommend checking your filter every month—no matter what kind you have—and replacing the filter if it’s dirty.

Now that you know how often you should change your filter, we’ll get into what might be causing it to get dirty so quickly.

If you look at your thermostat, it has 2 fan settings: ON and AUTO. You’ll have to change your air filter more frequently if your thermostat set to ON.

See, when your thermostat is set to ON, the fan inside your air handler runs constantly. That means air is cycling though your air filter and pulling in particles and contaminants 24/7.

When your thermostat is set to AUTO, the fan only runs when your system needs to cool your home.

Solution: Set your thermostat to AUTO. This will prevent humidity, use less energy and reduce how often you’ll need to replace your air filter.

If you notice your home is excessively dusty, you can switch the thermostat to ON for 1–2 hours. You can also use the ON setting while you’re cleaning/dusting to catch the contaminants that get tossed up in the air. We don’t recommend leaving you fan on for longer than a couple hours because it puts a lot of wear and tear on your system.

If you have leaks in your ductwork, that means dust and dirt from your attic or crawlspace are getting pulled into your home. The dust/dirt then gets caught in your air filter once the AC pulls it in from your home’s air.

Learn more about the signs of leaky ducts in our article, “How Can I Tell If My Air Ducts Are Leaking?”.

Solution: Contact a professional to have your ducts sealed. This will make sure your ductwork doesn’t have any leaks, which will limit how much dust enters your home and reduce how often you change your air filter.

In short, the more traffic your home gets, the more you’ll end up changing your air filter. We’ll explain.

The more people and animals that live in or visit your home, the more dirt and outside particles they’ll bring in. That also translates to more hair, fur and dander floating around your home and getting sucked into your air filter.

Solution: Consider upgrading your air filter to one with a higher MERV rating or installing a whole-house air purifier. A purifier will improve your indoor air quality and drastically cut down on how often you change your air filter (since whole-house purifier filters are good for 12–18 months).