Why Does My Furnace Filter Get Dirty So Quickly

It is relatively easy to replace a furnace filter and the job won’t take you too much time. But, let’s be honest here, no one wants to be doing that more often than once a month…

The homeowners who have noticed that their furnace filter started getting dirty too quickly might have leaky ductwork, a dusty home, a filter that is either too cheap or has a MERV rating that is too high, and a fan that is set to ‘on’.  

What will happen, if you simply leave the filter dirty, and is it worth upgrading your filter?

Let’s get these questions sorted out once and for all.

How Dirty Should a Furnace Filter Get?

How to know when the time has come to change your filter?

Well, if the system has been working properly, the filter should definitely be dusty with grey, ash-looking material on the side of the filter that was facing the duct.

By the way, if your little helper doesn’t look that way, then there might be a problem.

  • All these months the filter might have been installed incorrectly. That’s why it failed to filter out any dust from the air.
  • Your AC might not be strong enough to pick up dust. In such a case, you should call a professional as the system might be damaged.
  • Some homeowners can accidentally install the filter upside-down. You have to make sure that the arrows point toward the fan, otherwise, the filter won’t be completing its main job.
  • Finally, a cheap, poor-quality filter might fail to catch any dust at all, so upgrading to a better model is definitely something worth considering.

How Often Should You Change Your Furnace Filter?

How often you would have to change the furnace filter will depend on quite a few factors, starting from the thickness of the actual filter and ending with the frequency of usage of the heating system.

Below are the general recommendations.

1-2-inch filtersBetween 1 and 3 months
3-4-inch filtersBetween 6 and 9 months
5-6-inch filtersBetween 9 and 12 months

Why Does My Furnace Filter Get Dirty So Quickly?

If your filter tends to get dirty more frequently and you end up constantly changing the thing, it doesn’t necessarily mean that something is wrong.

Yes, there are a few issues that you can try fixing. But bear in mind that if you have a pet or a large family, you would have to change the filters more often than the homeowners that live alone.

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Leaky Ductwork

According to ENERGY STAR, an average home loses up to 30% of air through duct leaks.

These holes make the system less effective, drive your bills up, and, ultimately, allow more dust to get into the unit.

The truth is that leaks usually appear in the spaces that are not well-maintained, for example, attics, crawl spaces, and garages. The chances are high that these spaces are the dirtiest ones in your house, right?

The leaks in the duct become entry points for all the dust and the next thing you know – the particles from the dustiest spaces in your house start circulating around the living room.

Ideally, you would want to hire a professional who is going to inspect the whole ductwork and fix any leaks.

Dusty Home

Keeping to your cleaning schedule is incredibly important to the quality of the air in your home.

Pay extra attention to such dust collectors as carpets and drapes.

By the way, simply wiping the dust with a dry cloth won’t really do anything, so always use a damp cloth or a sponge to make the particles stick to the moisture.

Tip: some candles produce soot that is going to end up clogging the furnace filter. If you are a candle lover, then pick the ones that have braided wicks and that don’t contain petroleum jelly.

Your Filters Have a High MERV Rating

It goes without saying that cheap filters will fail to get the job done properly.

Some owners decide to go for the most advanced option on the market and get a filter with the highest MERV rating that is usually quite expensive. But such a decision might also end up doing more harm than good.

A high-quality filter is going to get dirty much faster than a poor-quality one as it does an amazing job at filtering out even the tiny particles.

But if your filters have the highest MERV rating, they can restrict the airflow in the unit, so make sure to consult a professional when choosing a high-performance filter.

Fan Is Set to ‘On’

Having your fan to ‘on’ all the time is not a bad thing. But bear in mind that in such a case the device is going to be constantly re-circulating the air in your house.

As a result, the filter is going to get dirty faster.

Try to set the fan to ‘auto’ (you’ll find this setting on the thermostat). This will make the thing work only when the furnace is on.

Pet Hair Problems

If you have one or more furry friends at home, then you have to be ready to change your furnace filter a little more often.

As soon as the little guys shed their hair, it will get sucked in the return air vents.

Vacuuming every day would certainly help, but it’s definitely a bit too time-consuming.

Extreme Temperatures

Have you experienced a surprisingly cold winter this year?

Then don’t worry, if your filter got dirty much faster than it usually does. After all, the chances are high that your furnace was on 24/7.


This sounds disgusting, but mold spores also circulate inside your furnace and in your room, if your house has such an issue.

Some of the signs of mold include a musty smell, signs of water damage, itchy and watery eyes, condensation inside the windows, and, of course, actual mold growth.

You would have to try and find the place where the mold is growing and get rid of it.

Tip: to prevent future mold growth, you should keep the humidity level in your house below 60%.

A Large Family

Finally, your furnace filter might be getting dirty extremely quickly simply because you have a large family living in one house.

In a nutshell, the more people there are in the house, the more often you would have to change the filter (especially, if one or more house members smoke).

Do HVAC Air Filters Help with Dust?

They certainly do.

However, the majority of ‘basic’ HVAC filters do a great job at removing only the large particles. Such as dirt, hair, and fur, for example.

You should definitely upgrade your filter to one with a better rating if you want it to be able to trap tiny particles.

Air filter technology is evolving. Today, you can find filters that offer ‘electrostatic filtration’.

These guys can trap up to 98% of particulate in the air. They, basically, create an electrical field that catches the dust and then holds it there.

However, do bear in mind that the filter has to stay electrified, in order for it to work its magic. 

What Will Happen If You Leave Your Filter Dirty?

So, let’s imagine that your filter gets dirty too quickly and you just can’t seem to find a spare minute to change the thing. What is going to happen in such a case?

It is quite obvious that the filter won’t be able to filter out the air that is circulating through the furnace, so the air in your house will become dustier and there will be a constant build-up of allergens.

But there are a few other problems that might occur that are not as obvious:

  • A dirty filter will cause the on and off cycles of your furnace to become shorter. As a result, the unit will be constantly turning on and off which leads to uncomfortable temperature changes, increased heating bills, and noise pollution.
  • The furnace will start to heat up the room unevenly. Restricted airflow results in hot and cold spots which certainly will reduce the overall comfort level.
  • A dirty air filter won’t be able to ‘save’ the components of the furnace from the dust buildup. If you don’t want to deal with serious repairs, then keeping the filter clean is the easiest preventative measure that you can take.
  • Overall, a system with a clogged air filter is less efficient. But, at the same time, the unit would require more energy to keep the house nice and toasty.You’ll end up paying more for your heating bills for the temperatures that are, possibly, not even warm enough.

To Sum Up

A dirty air filter does not always mean that there is something wrong with your furnace.

However, replacing this part of the system is incredibly important not only for the members of the household but also for the ‘health’ of your HVAC unit. 

So, always try to keep an eye on your filters and change them whenever there is such a need.