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9 Dirty AC Filter Symptoms and What They Mean

An AC filter is something that’s easy to forget about: it’s not something you look at every day–even when you go into the furnace room, you don’t see the filter and get a look at how dirty it is, unless you’re there for that specific reason. But changing out your air filter is one of the most important things you can do for not only the life of your air conditioner, but your health and comfort as well. A poorly maintained air filter can be worse than no air filter at all (though we definitely don’t recommend going without one). 

So what are some of the problems that arise from having a dirty air conditioner filter, and what can you do about it? Here are just a few:

Short Cycling

A properly functioning air conditioner doesn’t run 24/7: it cycles on and off throughout the day. This is how it’s supposed to work, and it’s healthy for the lifespan of your air conditioner. But a dirty air filter can make those cycles happen more often than they’re supposed to. This can lead to a number of problems, not the least of which is the pollution that builds up from the constant turning on and off, but also it means wasted energy as the air conditioner takes more energy to start and stop than to simply run continuously (or stay off continuously). It also can lead to temperature changes in the house that aren’t what you want: too cold at times, too hot at others. 

Freezing Up

The purpose of an air filter is to allow good clean air to pass through the filter, in and out of your air conditioner. But a dirty air filter restricts the expulsion of cold air, causing the cold air to collect on the air filter and build up inside the air conditioner, lowering the internal temperature. If this happens often enough, and bad enough, ice starts to form on the air conditioner coils, and that leads to freezing up. When you have ice crystals forming inside your air conditioner, it’s often too late to save it. This lack of adequate airflow causes the condensation to freeze and the air conditioner can’t remove heat from the air (or in other words, it can’t cool your house), and soon your air conditioner will break down for good. And that’s a lot more expensive than just changing an air filter.

Uneven Cooling

When your filter is clogged, it doesn’t run at peak performance, and some parts of the filter allow more air to pass through than others. This leads to uneven cooling, because the restricted airflow is bad for your air conditioner’s cooling power. You’ll begin to notice that some parts of your house are cooler than others, and some are far too warm. You’ll wind up cranking up the air conditioner really high to keep the warm parts of the house cool, which will freeze other areas, and it all will have a strong impact on your power bill.

Allergen Buildup

Even if you’re not allergic to anything in particular–no hay fever, no pollen–you can still have an allergic reaction to a dirty air filter. When a dirty air filter is blowing air through it all kinds of microbes and allergens can get through–or worse: mold starts to grow on the dirty air filter. And everyone reacts poorly to mold and mildew. The bigger problem is that you can’t simply solve an allergen problem by changing your air filter: once the allergens have gotten into your air conditioner they settle in the ductwork, and when they’re in the ductwork, a replacement air filter is too late. At that point you’d need to hire someone to come and clean out your ducts–which can be quite an ordeal and an enormous expense. It’s better just to change the air filters while you can before it gets to this bad state.

General Wear and Tear

When you change an air filter in your air conditioner, it’s a lot like changing the oil in your car. Sure, your car might be running fine right now on the old oil, but sooner or later something bad is going to happen if you haven’t been doing routine maintenance. An air filter has a very specific purpose, and that is to keep the unit’s internal components clean. The dirtier that they get, the worse your air conditioner runs, and the longer it runs poorly, the closer it is to breaking down. So while it may seem like your air conditioner is running just fine with no noticeable problems, you may be waiting on a ticking time bomb that could cost you the replacement of the entire unit–and all because you didn’t want to change the air filter.

Reduced System Efficiency

It may seem like an unnecessary expense to replace your air filter on your air conditioner, but every time that your air conditioner has to shut down and recycle and restart, you’re paying extra money in utility bills. 

Unhealthy Air

Own a pet? Have asthma? Even if you’re not worried about allergens, a dirty air filter can wreak havoc on a weak immune system by spreading pet dander, dust, and other dangerous particles throughout the house. Want an illustration? Take a look at the dirtiest air filter you’ve pulled out of your air conditioner, and just think to yourself about all of that stuff going straight into your lungs–because that’s what’s happening when you’re not cleaning out your filter regularly. Things that you thought would be no problem are suddenly a problem, and if you have sensitive lungs (or live with a child or a senior who does) then you’re spreading those dangerous particles into their bodies.

Furnace Failure

We’ve been talking about air conditioners, but what about your furnace? Your air filter affects that just as well, and one of the worst things that can possibly happen to your furnace is a complete failure caused by a failure to change your air filter. According to the Diamond Certified Organization, a clogged furnace air filter is the number one cause of equipment failure. And a new furnace isn’t cheap.

Increased Energy Consumption

According to the Department of Energy, a clogged air filter can use 15% more energy than a clean one. That means higher pollution, a higher carbon footprint–and higher utility bills. The more clogged that the filter gets the worse it is for your energy consumption. This is because your air conditioner blower is trying really really hard to cool down your house, but the air it’s blowing just isn’t getting through the ducts. Nothing is being cooled, so your AC tries harder and harder, and pretty soon you end up with a massive bill–or a burned out air conditioner.