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What Does A Dirty Furnace Filter Look Like?

By January 4, 2023April 9th, 2024No Comments
New furnace filters will ensure the system works for a long time.

Getting to know your furnace is a good idea. If you purchase a home that has a furnace, you should get to know the ins and outs of how the furnace works. No homeowner wants to be caught in the cold due to a malfunctioning furnace. One of the easiest points of maintenance on a furnace is changing the furnace filter. What does a dirty furnace filter look like? Well, there are typically 4 stages of dirty when it comes to a furnace filter.

Let’s take a look below at what each stage of dirty means for your furnace filter and how we can really tell if it needs to be replaced.

What Are The Stages Of Dirty Furnace Filters?

Believe it or not, there are various stages of dirty furnace filters. For the most part, we can break down the dirty levels into four stages. Let’s take a look below at what those stages might be.

    1. Brand New – This means the filter is fresh out of the packaging and has zero dirt on it. The filter will look very white and clean.
    2. Working – This means the filter is between 0-2 months old and is beginning to develop a color change. The filter is likely a grey or brown color and you can begin seeing a thin layer of dirt on the filter.
    3. Older – This filter is 3 months old and definitely looks grey. There is dirt all over the filter and you can notice the accumulation. You can actually remove the dirt particles with your fingers at this point.
    4. Replacement is Needed – This filter is at least 4 months old. The filter cartridge is now a dark grey color. You can notice dirt particles, hair collection, mold spores, and even some insects collecting on the filter. It is time to replace this cartridge.
Make sure that you know where your furnace filters are located so that you can replace the dirty ones.

How Do I Know If The Filter Is Dirty?

Aside from observing a dirty furnace filter, there are other signs that can indicate you have a dirty furnace filter. In order to avoid a build-up of carbon monoxide in your home, you will want to ensure that your dirty furnace filter is replaced. Let’s take a look at some other signs that your furnace filter is dirty.

    • Sickness – If you notice that you and your family are getting sick more often, it could be due to dirty filters in your furnace and AC unit. Be sure to change them out right away so that you have high indoor air quality circulating through your home.
    • Energy Bills are Higher – If you have a dirty furnace filter, the furnace will need to work harder and longer to ensure that the proper amount of heat is being dispersed. A dirty furnace filter can obstruct the airflow and thus make the furnace work harder to pull in air and recycle it back to your home.
    • Dust – If you notice that dust is building up at a faster rate on the surfaces of your home, it is likely a cause of a dirty furnace filter. As air circulates, it will drop particles through the home. The only way to keep the particles out of the air is to have a filter capable of trapping them. If the filter is too dirty, it will not grab as many dust particles and remove them from the air.
    • Little Heat – When you turn your heating system on and you notice that little or no warm air is blowing through the vents, it is likely a blocked or dirty furnace filter. This could cause your heating system to work harder.
    • Burning Smell – Smelling a burning scent coming from your furnace is likely due to a dirty air filter. Keep in mind that a burning smell could also be due to a needed repair. Change the air filter first as a way of troubleshooting the issue. Dirty air filters clog up the unit and prevent airflow and cannot pull the necessary air to heat and re-circulate.
    • Failed Test – Hang a white bedsheet approximately 6-8 inches from one of your heater vents. Leave the sheet up for one to two hours and watch as the dirt and debris particles are blown from the heating system onto the white sheet. Additionally, if the white sheet turns grey or black, you definitely need to replace your furnace filter.

Other Recommended Maintenance

Now that you have an idea of what a dirty furnace filter will look like, you can begin reading up on where your furnace filter is located. Consequently, you cannot possibly change a furnace filter if you do not know where it is located.

Next, if you pull the furnace filter out of the unit and notice that it is black, this is a clear indication that you need to change the filter. Be sure to read up on why your furnace filter is black so that you can take the necessary steps for keeping it from getting that bad.

Lastly, if you are having problems operating your furnace, there are a few reasons why it won’t turn on. Make sure that you understand all of the reasons why a furnace might not turn on. Dirty furnace filters could be one of the reasons.

Always replace a dirty furnace filter as soon as possible.

When Do I Call A Professional?

Having a furnace in your home might require a little more maintenance than your AC unit would. Be sure to call on your local HVAC professional service technician to assist with maintaining your HVAC unit. If you aren’t sure who to call, reach out to your local home inspection team. They can recommend a reputable HVAC professional service technician in your local area. Also, they can inspect your furnace to ensure that its parts are working properly.


Replacing a furnace filter should occur every 90 days. Depending on the type of system that you are running and if you have pets in the home will determine if you need to change the filter sooner. Consequently, reach out to your local home inspection team and they can inspect the furnace and your furnace filter to ensure all parts are working properly. Reach out to ACE Preferred Inspections for a furnace and full home inspection in the low country of South Carolina.