Signs and Symptoms Of A Clogged Furnace Filter

Signs and Symptoms Of A Clogged Furnace Filter

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It likely doesn’t cross your mind very often but your furnace might be the single most important appliance in your house.

It’s easy to assume that it’s just going to continue working properly day after day for years to come, you might forget the need for maintenance until it breaks down, which is far more expensive to handle.

More often than not, you never have to worry about your furnace keeping you cooled in the summer and warm in the winter months, but think about what will happen if there’s a snowstorm outside and your furnace breaks. What will you do?

Because it is so important for your health and comfort, you need to pay close attention to your furnace to make sure it’s in working order and to understand what a dirty furnace filter can do.

In fact, you might be thinking your furnace is in good shape, it could have a clogged filter or worse.

If this is the case, it will not be long before the efficiency is ruined and other parts of the furnace start to fail.

You’ll need to understand how your furnace works to be in tune with issues that can arise, and that means being knowledgeable about clogged furnace filter symptoms you should look for between filter checks and cleanings or replacements.

How a Furnace Filter Works

So, the furnace filter’s main responsibility is to ensure that the air going into the system doesn’t cause damage to the fan or other components.

However, there is a bonus to figuring out if your furnace filter is showing signs of being clogged. As with air purifiers, this filter allows you to breathe cleaner air in your home.

It’s not manufactured for this purpose, but it can certainly help you maintain a happy, healthy home that doesn’t cause allergies or asthma to act up.

Knowing more about your furnace and furnace filter can make it easier to spot issues and create maintenance schedules that keep everything flowing smoothly.

Depending on your environment in the home, your furnace filter may easily get clogged or extremely dirty, both scenarios causing problems with your airflow and leading to anything from poor air quality to having to replace your whole furnace. It’s vital to set up a regimen so that you’re checking and replacing your filter at normal intervals to avoid such hassles.

Why Do I Have a Black Furnace Filter?

Close up dirty car air conditioner filter isolated on white - deep focus image

The furnace and HVAC system is the center of your home and the appliance that offers you the greatest comfort. But like so many things that are out of sight, the parts and pieces that need maintenance are often forgotten.

To keep your furnace in good working order, you must pay attention to your furnace filter. This is the most essential piece of your entire HVAC system, and if left unchecked, you’ll run into several problems with your furnace and air conditioning, leaving you asking, “Why is my furnace filter black?”

More on the Topic

There are plenty of things that can go wrong with your furnace filtering device, especially if you haven’t checked it in a while.

However, going in and finding that it’s turned black could be alarming. You may ask yourself, why does my furnace filter look black, and what do I do about it?

Let’s first consider the way a filtering screen works, and then we can discuss what could have happened that turned the filter black, as well as ways you can deal with it and rescue your furnace and HVAC system from failing.

Furnace Filter Maintenance

Furnace filter maintenance

However, you have to perform regular maintenance on your furnace filter, or it can cause more harm than good. If the filter isn’t cleaned or replaced regularly, it is likely to get clogged. This reduces air flow into your furnace, which causes the fan to work harder. That can lead to the fan burning out.

In addition, it can eventually put enough stress on your furnace as it attempts to pump warm air into your home (and you’ll notice the difference in the lack of comfortable warmth) that the entire appliance fails. Just as detrimental is the excess energy wasted trying to continue to operate, as it costs more.

Symptoms of a Clogged Filter

Your furnace filter should be cleaned or changed at least every ninety days, and if you find that your electric bill is abnormally high, you should check it. Additionally, if you experience more difficulty getting your home to the desired temperature, or that your system turns on and off rapidly, be sure to check for a clogged filter. Keep in mind that all of this is true of a filter on an air conditioner, as well.

But what does it mean when you find that your furnace filter is black? How should you address the issue? Let’s take a look at the causes of the problem so we can discuss possible solutions.

Common Causes of a Black Furnace Filtering Screen

Close up dirty car air conditioner filter

If you’re asking yourself, “Why does my furnace filter get black really quick?” then there could be several reasons why. There are four major things you should check that could be amiss in your home if you find that your furnace filter is black, especially if it hasn’t been installed and ignored for longer than recommended:

  • Dangerous Levels of Carbon Monoxide: In quantities, carbon monoxide is poisonous and even fatal, so it’s important to know if you happen to have a high concentration of carbon monoxide in your home. A black furnace filter could be a sign that gas or exhaust fumes are leaking into your home, leading to a concentration of the colorless, odorless gas.
  • Black Mold: Is your home particularly humid? Do you run your air conditioning frequently without shutting it off? This could lead to condensation of moisture on your furnace filter, which then mixes with the dirt and debris caught on the filter. This can cause the growth of black mold, which appears black and can also be detrimental to your health when you breathe in the spores.
  • Excess Dirt, Debris, and/or Soot: The air filter is meant to capture these particles. However, several issues can arise based on the amount of work your filter is doing. If you haven’t regularly checked and cleaned or replaced your filter, the filter could be so clogged it appears black. In addition, if you have an unusually high amount of particles in your home (lots of dust, pollen, or pet dander) or a smoker in your home, this could lead to quicker clogging that appears black. Also, using a fireplace and having soot in the air could blacken the filter.
  • Air Flow Problems: It could be that your entire ventilation system has become clogged, which means that excess particles are getting into your air when the air can actually flow through. In addition, it could be leaking. Both of these issues could cause your furnace filter to turn black.

What Do I Do About It?

Several solutions may need to be in play, depending on which of these problems has caused your furnace filter to turn black. Here are some steps you can take to fix the problem, as well as avoid future issues.

  1. Check for a carbon monoxide leak: Have your home tested to see if there is carbon monoxide in the air. Not only is this essential to determining if it’s the cause of your black filter, but it’s essential to avoid health problems for you and your family. If a leak is found, have it repaired immediately.
  2. Install a carbon monoxide detector: Like a smoke detector, these devices can literally save your life. When you have a buildup of carbon monoxide in your home, the alarm will sound, and you’ll be alerted to the hazard. This will also keep your filter from being damaged.
  3. Get your ducts cleaned: You should have your entire HVAC system cleaned on a regular basis to avoid clogging and mold buildup. This can go a long way in assuring you don’t have problems with your filter.
  4. Consider a dehumidifier: If your home is particularly damp, consider adding a dehumidifier so that you don’t have to concern yourself with the problems caused to your health and your home by black mold.
  5. Clean and replace your filter: Never forget to perform regular maintenance on your furnace filter. You should check it at least every thirty days, or more often if you have pets or a smoker in your home. Clean and replace your filter at least every ninety days, or more frequently as needed based on the condition of the filter when you check it.
  6. Dust and vacuum more frequently: Even if you maintain your home diligently, if you happen to live in a home with excess dirt and debris (especially homes with large families, as this can lead to shedding skin and hair in large volumes), consider upping your game and dusting and vacuuming more frequently to avoid clogging your filter.

You wouldn’t hesitate to have your refrigerator checked if it wasn’t cooling your food. The same should be true of your HVAC unit, and a black furnace filter is a sign of serious trouble. Check for:

  • Carbon monoxide
  • Black Mold
  • Excess debris
  • Reduced airflow

Any of these problems should be addressed right away. Was this answer helpful to you? Let us know in the comments.

Consequences of a Dirty Furnace Filter

Front view of a dirty furnace filter taken out

Having a dirty furnace filter has a negative impact on a lot of different levels. Only by understanding the consequences of not cleaning or changing your filter, can we get it into our system that this should be treated just like any other mandatory house chore.

Blocks Proper Heating

First off, a dirty filter will prevent your home from getting heated as it should. The blower fan that’s located inside the furnace is responsible for pushing warm air, and will find it difficult to operate at normal capacity if the filter’s dirty surface restricts airflow. Of course, this means that your home won’t receive the amount of heat needed during the cold season.

Uses Extra Energy

You will leave the furnace running for a longer period if you feel cold, which results in a higher heating bill. But that’s not the only bill that’s bound to go up.

There is a direct correlation between the speed of air recirculation and the energy your furnace consumes. If your filtering unit is clogged and dirty, the furnace will work extra to pull the air needed through a surface that doesn’t allow air to pass through, which will result in more energy consumption.

Causes Damage

Because an air filter’s main responsibility is to keep the furnace protected from contaminants, not having a clean filter can damage the system and shorten its lifespan.

Dirty filters cause stress among the components of the furnace. Even more, because the furnace can give back heated air at normal capacity, this could cause overheating of the system and, eventually, failure altogether.

Let’s not forget that it’s also a filter’s job to clean the air from contaminants and put it back into circulation. When the air filter is dirty, not only does it fail to decontaminate air, but there are also chances of the particles trapped in the filter being released back into the air.

Bad for Allergies

Woman in short hair blowing her nose on the tissue on white background

If you or your family suffer from airborne allergies to dust and dander, then this could cause more problems with your breathing and air quality. Especially if you have asthma or other respiratory problems.

Frozen evaporator coils

Consider that, in the summer, air passes through the filter over the evaporator, or cooling, coils. Without enough airflow due to a clogged filter, not enough air blows through to evaporate the condensation on the coils, so the coils then freeze up. This will eventually cause your entire system to break down.

Unhealthy air

Aside from no longer being able to properly remove debris and allergens from the air, some of the clogged material could blow back through the system and stir up into the air you breathe, making the air unhealthy. In addition to this, the debris that comes loose from the clog could go on to cake on the fan blades, eventually damaging the fan so that it, too, will have to be replaced.

Furnace failure

Eventually, the clogged filter will lead to the entire furnace shutting down and failing, which will cost a very large sum of money to repair or replace.

All of this can be avoided with not only proper care and maintenance but also by paying attention to symptoms that could mean you have a clogged furnace filter.

Does a Dirty Air Filter Affect Air Conditioning?

When it comes to having a nice, comfortable home where you want to relax, the environment is essential. And the most important appliance in your home to ensure the convenience of a comfortable environment is your HVAC – including your furnace and central air conditioning.

However, if you don’t perform regular maintenance on the system, you could end up with significant problems at the worst of times.

Consider turning on your air conditioning when the heat hits and the start of the summer and finding it isn’t working properly. One of the biggest culprits is the filter. But does a dirty air filter really affect air conditioning?

Checking Your Furnace Filter

Featured Image - How to Clean Furnace Filters

There’s a lot of advice on how to tell when it’s time to change your furnace filter. Truth is, you can tell if a filter is dirty by examining it, but there are also other methods.

Look at the Home First

The first sign is the one visible in your home. If you notice more pet dander, more dust, or a decrease in the quality of the air you’re breathing, that’s one of the first signs that it’s time to change your furnace filter.

If you sense a burning smell inside your home, it could be a sign that the blower fan is overheating because the dirty filter doesn’t allow airflow at optimum capacity. Another sign could be a high-pitched noise coming from your furnace.

That also means that the airflow is restricted. You can also tell if you have a dirty filter by examining how the furnace functions. If the furnace is taking longer than usual to heat your home (and assuming the outside temperature isn’t excessively low), that could also be a sign that you have a clogged filter that’s preventing the furnace from functioning in normal parameters.

White Sheet Test

Some people have invented a new way of testing how clogged a furnace filtering unit actually is. The “white sheet” test implies hanging a white sheet of paper a few inches apart from one of the vents and leaving it there for about an hour. If, after the hour has passed, you notice that the sheet is black or gray, that’s a sign that there’s dirt accumulated on the surface of the filter and it’s time to consider putting in a new one.

Old Fashioned Way

Of course, there’s always the traditional method of checking your furnace filtering unit. Simply turn off the unit, slide out the air filter from its slot, and hold it against a source of light. If the light is still able to penetrate the surface of the filter, that means that it’s still usable.

However, a lot of experts question this method, as there might be some light visible through the surface of the filter, but barely enough for us to assume that airflow is still kept at good levels.

Extra Tips

Oh, and while you’re checking your filters for dirt, also make sure you look for any cracks, bends, scratches, or other signs that the filtering unit may be in any way damaged. A damaged filter can compromise the airflow, but it can also prevent the surface from working properly and actually have the blower fan draw contaminants past the screen and into the system.

Keep in Mind

thinking man and woman

The most important thing to remember here is that even though there are some general guidelines on how to tell if your furnace filter is dirty and needs replacing, there are some factors that vary from one household to another.

For example, a lot of furnace manufacturers recommend changing the filtering units once every three months, but in some cases, it can get dirty way faster than that.

For instance, if your home is near a road with intensive traffic, there are way bigger chances of dust getting inside the house and your filters getting dirtier faster. The number of pets you own can also cause the filter to clog fast, as pet dander is one of the many contaminants that a good air filter can block.

Also, if you’ve recently renovated your home or hosted a party or some out-of-town guests, you might want to check the filtering unit afterward. When a higher number of people wanders around the house, the number of airborne contaminants also increases.

A Look-Back

To avoid issues with your air conditioning based on a dirty air filter, there are plenty of things you can do, including proper maintenance. Things to remember include:

  • Checking your air filter every 30 days to ensure it’s not clogged
  • Changing your air filter (or cleaning a permanent/reusable air filter) at least every 90 days to avoid dirty filters
  • Watching your electric bill to see if your air conditioning might be pulling extra energy and costing more
  • Listening to see if your air conditioning is cutting on and off rapidly, a sign you may have a dirty air filter
  • Keeping your home vacuumed and dusted, reducing the amount of dust, dander, and other particles flowing through your air intake
  • Not smoking in your house, can actually speed up the process of creating a dirty or clogged air filter

Prevention and maintenance aren’t free all the time, but they are much less costly and a much less stressful project than repairs and replacements. Take the time to care for your filter because a dirty air filter most definitely affects your air conditioning.

Replacing a Furnace Filter

Woman replacing the filter

When you’ve purchased the type, size, and MERV-rated filter you want to install, you’ll find the task is actually fairly simple, in most circumstances.

  1. Turn off your furnace system completely until you are finished making the exchange. You don’t want any chance it’ll kick on while you’re working.
  2. Open the door where the furnace filter sits, roughly in between the air intake and the furnace. The old filter should slide out without resistance, so you shouldn’t need any tools to accomplish this. Typically, it’s fitted perfectly into the narrow hole.
  3. To avoid releasing dirt, dust, and allergens into the air around you, be sure to properly dispose of this old, clogged filter. Place it quickly but carefully into a bag, avoiding bumping it or dropping clumps of debris.
  4. By contrast, if you’ve got a permanent one that isn’t old enough to replace, rather than a disposable type, don’t throw it out. Instead, vacuum it with the brush tool or at least the extension hose from your vacuum. Then, rinse it well using water only, without soap or solvents. These can damage and further clog the furnace filter and should not be used for any reason. You then need to allow it to dry completely before putting it back into the filter slot.
  5. Before returning the furnace filter to its slot or installing the new one, you should also vacuum out the slot itself. Debris and dust can settle here and cause excess clogs. Again, use the brush if you can, or at least the hose extension, to get the best results as you treat this area.
  6. The filter, whether new or reused, should have a sticker or marking on it to show which direction the airflow is supposed to go. Just ensure the arrow is pointed toward the furnace. Otherwise, installing the filter the wrong way can quickly cause the system to fail, since debris will immediately clog that filter.
  7. Now that the filter is back in place, you can close the compartment door and turn the unit back on, making sure all the parts are working properly and that you have the heating or cooling you need throughout your home.
  8. Don’t disregard regular maintenance, including checking monthly to make sure you don’t have clogs or excess debris to remove. This will also help you identify any other issues you may have with the filter. Also, create a schedule that reminds you to clean or replace the filter again in 90 days to avoid additional problems that could lead to costly repairs and replacement of parts.

The overall process of replacing the filter shouldn’t take more than fifteen minutes with the ease of the task, even if you have a permanent filter you’re cleaning.

The only additional time required in this case is to let the filter dry completely before you return it to the slot, and you can move on to other things while you wait for this to happen. It’s also nice to know you won’t need any tools other than a vacuum and a rag no matter what the process.

Ratings – What Does MERV Mean?


Filters are rated on the MERV scale. ‘MERV’ is an acronym for the minimum efficiency reporting value, and those values go from 1 to 16. The MERV rating tells you where on the scale of efficiency standards the filter ranks.

The bigger the MERV rating is, the more allergens and particles the device can remove from the air. Higher numbers indicate fewer releases of allergens into the air that’s filtered via the unit. But, you should avoid the highest end of the scale, since these are more likely to clog and can also make your furnace work harder to pull air through even a fresh filter due to the density of the filter material.

Most of the time, you should be content with a MERV rating of around 9 or 10 to maximize the efficiency of filtration without lessening the overall airflow and causing your energy bill to skyrocket with the way the furnace has to work harder. However, check with your manufacturer to determine the recommendations for the most MERV rating for your particular furnace to keep performing properly.

Even More Rating Systems

A few of the larger chain home improvement stores have created a rating system of their own for air filters, and while these tend to fall in line with the MERV system, you need to check with the manufacturer to ensure that any rating you choose is still compatible with your heating and cooling system.

That’s because, even if the numbers seem to line up, there is fluctuation and variation in the crossover of ratings. If you’re concerned you may make a wrong decision, simply choose a brand that uses the traditional MERV rating to ensure you get the right product for your needs.


Making sure your furnace filter is clear and doesn’t cause problems due to a clog is essential to the continued efficiency and overall health of your furnace system.

Even though it’s a small and relatively inexpensive piece of a much larger puzzle, the filter is the component that is most likely to cause furnace failure, and a bit of attention can go a long way in ensuring you’re not left out in the cold or brewing in the heat when your furnace stops working. Look for symptoms of a clogged filter, even between inspections of the filter itself, so you catch the problem early before any damage is done.

Continued maintenance is key to making sure your furnace filter stays efficient and effective. Take the small amount of time required to check the filter monthly and replace or clean it every 90 days so that you don’t end up with a clog that damages the fan, the emergency switch off, or any other component that could lead to complete system failure. It’ll save you headaches and a lot of cash in the long run.

It’s easy to assume that it’s just going to continue working properly day after day for years to come, you might forget the need for maintenance until it breaks down, which is far more expensive to handle.

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