Why Is My Furnace Filter So Dirty?

Dust and dirt are a natural part of life. No matter how hard you scrub your counters or how many times you wipe off those ceiling fans, more dust will always, always accumulate. 

The good news? Your furnace works hard to remove dirt and debris from the air so you don’t have to. By sucking in the cooler air when it moderates temperatures, your furnace pulls in a lot of that lingering debris that floats in your home’s air.  

As a result, homeowners must remember to change their furnace filters, as they tend to get dirty fast. But how much dirt is too much? What does a dirty home air filter look like, anyway? And why is your furnace filter turning grey?

These are all great questions. In this post, we’ll dive into the ins and outs of furnace filters and what to do about it the next time you notice your filter accumulating too much dirt. 

Why Is My Furnace Filter Always Dirty?

While many homeowners understand how important a furnace is, few understand just how vital a filter is – or how hard it works. 

Your filter is what catches dirt before it impacts your indoor air quality and stops it from building up inside your air ducts or clinging to your furnace system. 

While it depends on the size of your filter, in general, you should change your filters twice a year. However, sometimes homeowners must change filters once every few months. 

Why does this happen? A few factors can impact how dirty your filter becomes, such as: 

  • Construction in your home
  • Increased people in your home 
  • Poor outside air quality 
  • Allergy seasons 

Still, other aspects can affect how fast your filter dirties. Pets, fan settings, and poor air duct sealant are among the top three reasons why you may see a filter become clogged faster than usual. 


We love our animals. They provide immense joy and endless entertainment. Unfortunately, they shed a lot. Between their hair and their dander, having pets significantly increases the amount of dirt and dust in your home’s air. 

If you have pets (especially if you have more than one pet), it’s more than likely your furnace filter will have to be changed once every few months. 

Fan Settings 

Many homeowners don’t know that their fan settings impact how much dirt accumulates on the furnace filter. 

When your thermostat has your fan set to “on” instead of “auto,” this means that your system’s fan is constantly filtering out your air, even if your furnace is not pumping out hot air. 

On the one hand, doing this means that your fan is constantly circulating and improving your indoor air quality, which means that your air is much cleaner and healthier. 

On the other hand, it also means that your fan is constantly removing dust particles, which can create a dirty HVAC filter at a much faster rate. 

If you notice your furnace filter turning grey within a month or two of installing it, inspect your fan settings to see if this is the cause of the problem. 

Poor Sealant 

Your air duct system is a complicated network of tubes and ducts that run throughout your entire home – including those dusty, forgotten rooms like your attic or basement. 

When air ducts are not properly sealed, the air from inside these spaces can be sucked into your system and cling onto your furnace filter. 

As a result, your furnace filter will become dirtier much sooner than it should. If you suspect your air ducts have leaks, it’s best to call certified technicians at TSS Home Comfort for the best furnace repairs around.

How to Know When a Furnace Filter Is Dirty  

You may be wondering – what does a dirty furnace filter look like? 

Homeowners should be able to immediately recognize when their filter is dirty because it will have grey or black dust and debris clinging to the sides. You’ll be able to see visible clumps that become thicker and thicker as your filter becomes dirtier. 

Though some homeowners know what filters look like when they are dirty, many people wonder which side of the furnace filter should be dirty.

Your filter should be dirty on the side that air enters the furnace from, which is the side that faces outwards to your home. 

If it is dirty on the other side, this is a big indicator that your furnace has internal problems and requires repairs. Alternatively, it could mean that you installed the filter the wrong way. A furnace repair technician can get to the bottom of the issue for you.

If you sense that your furnace needs repairs, it’s important not to delay in calling for assistance. 

Contact TSS Home Comfort Now

Changing your furnace filter every few months is a sign of good homeownership. But sometimes filters can become dirtier than average. 

For homeowners who wonder, “Why is my furnace filter so dirty?” It may be time to consider furnace repairs. For example, if you notice that your furnace filter is black after one week, or if you have to change an air filter every two weeks, that’s an indicator that your furnace requires repairs.

Call TSS Home Comfort for the best furnace repairs around. Pick up the phone today to book your appointment!