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Dirty Furnace Filters Can Negatively Impact Your Home & Your Health

The side effects of a filter that’s too full of dust to do its job are numerous, and can even affect your health. We take a look at some problems it can lead to.

Dirty Furnace Filters Can Negatively Impact Your Home & Your Health

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Fire & Ice Team


May 3rd, 2023

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Home air filters are small and - compared to the cost of the rest of your heating and cooling system - cheap.

Yet they protect your furnace and air conditioner or heat pump from breaking down. They affect your energy bill, repairs to your HVAC system, and the air you breathe.

Even folks in the heating and cooling (HVAC) industry are guilty of a cardinal sin: They don’t always check on their furnace filters when they should. (We’ll use these terms interchangeably: filter, furnace filter, air filter.)

Whatever you call it, the filter in your heating system is vital. Yes, your system will run without one (please don’t try it), but it will cause instant damage to your furnace.

And it’s easy to ignore or forget. Your system can technically run with a dirty filter, whether it’s winter or summer. And that’s a problem. It’s like driving with old oil in your automobile engine. It’s all great until it’s not. And when it’s not, it can go very bad in a hurry.

There’s a reason manufacturers want you to change the oil in your car every 5,000 miles or so. And there’s a reason we’re making the point that a smooth-running HVAC system has a filter that’s not clogged.

Ninety percent of the time, a dirty filter is the root cause of some sort of damage to the machinery.

This article will go through some of the issues a dirty filter will cause. By the end of it, we hope you’ll see the value of replacing filters when they get so dirty, they stop doing their job.

A Dirty Air Filter Can Damage Your Heat Exchanger

Your furnace heats air by blowing it over a heat exchanger, which is made of metal.

If your filter is clogged, there won’t be enough air to blow over the heat exchanger. Extra heat will build up inside the exchanger itself, and the stress of the overheating can cause the heat exchanger to crack.

When your heat exchanger is cracked, it has the potential to allow carbon monoxide (CO) to leak into your home’s air. Carbon monoxide is one of the byproducts that a gas furnace creates when it's heating. In a system that functions normally, that gas is vented away from your home.

According to the Centers for Disease Control and Prevention (CDC), common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. If you breathe in a lot of CO, it can make you pass out or even kill you.

A Dirty Filter Will Damage Your Coil

An evaporator coil is the component of your heat pump or air conditioner that absorbs the heat and moisture from the air inside your house. The coil typically sits on the top or rests at the bottom of the furnace.

Keeping the coil clean is important. If the coil is dirty, it’s a sure bet that dirt is coating the rest of the HVAC, because return air has to go through your furnace before it gets to the coil.

A clogged air filter can cause the evaporator coil to get caked with dirt, mold, or bacteria. This interferes with the heat exchange process, making your air conditioner unable to control the indoor temperature or humidity.

The evaporator coil provides the perfect place for microorganisms to multiply. That can lead to mold or mildew. And when the blower is operating, those particles circulate through the ducts. They will either gather in the ducts and multiply, or they get blown into the indoor air.

If a good filter keeps your coil clean, it’s also keeping your furnace clean. The system will last longer and require less maintenance.

A Clogged Filter Can Increase Your Heating and Cooling Bill

A clean system is an efficient one. Clean works better than dirty.

If dirt gets on the blower motor, that affects the blower wheel. When as little as 1/10th of an inch of dust and other debris accumulates on your blower wheel, it can reduce your furnace's efficiency by 20%.

Dirty air can find its way around your clogged air filter, and then it goes through the inside of your furnace, sticking to the coils and making them work harder. Which makes the unit work harder, which is costing you money.

Replacing a dirty, clogged filter with a clean one can lower your air conditioner's energy consumption by 5% to 15%.

The harder the system has to work to pull air throughout your home, the more energy it requires and the higher your energy bills will be. This inefficiency intensifies as the filter becomes increasingly clogged.

Simply put, anytime your system is pushed to work harder, it's costing you dearly. You’re also getting less conditioned air for the money.

How Does a Dirty Filter Affect Indoor Air?

Some people dislike air conditioning because they say that it increases their allergies. They are partially right, but the problem isn’t with the air conditioner. The problem is a clogged filter that doesn’t capture any of the allergens. So when the AC is on, those allergens are being blown through the ductwork back into the home.

When your air filter is dirty, it becomes less effective at blocking dirt, dust, mildew, and other allergens..

Given the money you've already invested in your home, not to mention the priority you place on your health, failing to change your air filters regularly doesn't make sense. It only takes a few minutes, doesn't cost a ton of money, and your family will thank you for it.

Clogs May Cause the Furnace to Shut Down

A clogged air filter means reduced air flow, which in turn causes the fan motor to work harder. If the fan motor burns out from being overworked, it could result in overheating and eventually system failure.

One dirty furnace filter, which can cost as little as $10, can cause the shutdown of a $7,000 furnace.

If your air filter gets too congested during the summer cooling season, it may cause a lack of airflow to the evaporator or cooling coils. This diminished airflow causes condensation that is normally produced during the cooling process to freeze.

Frost buildup on coils or fans makes it more difficult for your AC unit to remove heat from the air and cool your home. It may even result in the ultimate headache and financial disaster: a total breakdown.

Air Ducts Will Get Dirty

It’s easy to tell if your ducts are dirty. This will be obvious at the supply registers, where the dirty air is entering rooms. If they are dirty, it’s a certainty that the duct is dirty.

Read more: Air Duct Cleaning in Columbus, Ohio: All of Your Questions Answered

Unless your ducts have been cleaned recently there’s a good chance they need it now.

Dust, mold, mildew, and other allergens are going to accumulate in your ducts.

According to the EPA, the levels of indoor air pollutants are often 2 to 5 times higher than outdoor levels, and in some cases, those levels can exceed 100 times that of outdoor levels of the same pollutants. In other words, sometimes the air inside can be far more harmful than the air outside.

A blocked filter continuously recirculates those particles back into the home, causing you and your family any number of health-related symptoms, including headaches, respiratory problems, and chronic allergies.

Your Indoor Air Will Be Less Comfortable

Even if you can’t tell from your gas or electricity bill that your HVAC system is struggling, you will certainly be able to tell by the feel of your home.

Cold days will seem even colder, and the furnace will run and run, driving up your energy bill while providing inadequate heat.

Summer will feel worse. Not only will your air conditioner run longer and still not manage to keep the indoor temperature at a comfortable level, but it also won’t dehumidify the air as well. That’s part of its job.

And with higher humidity, warm air will feel even warmer. Meteorologists call it a heat index: temperature plus humidity. Damp heat makes for a more uncomfortable environment, both inside your home and out.

Your central air conditioning depends on the continual recirculation of air. When that airflow is impeded by a dirty furnace filter, it means your system has to struggle much harder to cool your house.

With reduced airflow, there's also a chance you'll experience inconsistent room temperatures (hot and cold spots) throughout your home.

Dirty filters restrict the flow of cold air, which can cause it to build up inside the air conditioner. The final result could be the formation of ice on the coils. That will cause the AC to operate at a lower efficiency, which means it does not cool your house as well as it should.

How to Change Your Furnace Filter

This is one of the few HVAC repairs/maintenance tasks that you can do yourself without jeopardizing your warranty.

Your filter should be right next to your furnace, and it may have a door at the front. In some instances, it sits above or below the furnace.

Be sure to look for the arrows printed on the filters' sides. They show you which way the filters are supposed to be installed. Yes, it does matter. The arrows must face toward the furnace and away from the return duct.

If you install your filter backward, air will have a harder time flowing through the filter and your blower will have to work harder to make up for the loss of airflow. This will lead to higher energy bills and possibly damage your furnace or air conditioner.

If you’re not comfortable doing this yourself, an HVAC technician will be more than happy to assist.

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How Often Should You Change Your Filter?

The rule of thumb is that air filters should be replaced every ninety days. (The most expensive filters can last up to six months and even a year before the manufacturer suggests changing or washing them.) If the indoor air quality is especially poor - especially if you have multiple indoor pets or smokers - you may need to change the filter more frequently, such as once a month.

The best routine you can follow is to check on the filter once a month. If it’s filthy, or especially if it’s clogged, take the time to replace it then and there.

Read more: Furnace Filter Facts: All Your Questions Answered

Why not let us make it easy? Click on the graphic below to start your journey to a healthy home where equipment can function as it should.

Buy Your Filter

If you have a thermostat with a timer, it can remind you when it’s time to change your filter. It doesn’t actually know how dirty the filter is; it’s just a clock. But it does provide a good reminder.

You can also sign up for one of our maintenance agreements, which guarantees an HVAC technician will inspect your system twice a year for a tune-up.

We want to make sure that we have not only the customer’s comfort concerns in mind, but if we’re about to install a good system properly, there is one problem. We can do our job and can provide A-1 service during routine maintenance. But the customer has to replace the filter when it’s recommended.

Importance of Changing Your Air Filters When Necessary

When we say a system should last 15-20 years, there’s an assumption that homeowners are going to change their filters when they need to.

If you don’t change your filter, it doesn’t matter how good the installation is, your HVAC system is not going to do what it’s supposed to do.

The U.S. population, on average, spends as much as 90% of its time indoors.

Doesn’t it make sense to spend that time in an environment that’s not detrimental to your health, where the air is clean, at a proper humidity level, and comfortable?

If you’re ready to reach out to an HVAC company about filters or any other heating or cooling issue, we’re ready to help. We look forward to having a conversation with you.

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