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Why is my air filter black?


Hey, don’t panic. It’s probably nothing serious. Black air filters have a number of causes that vary in severity and danger, but none of them are immediately dangerous. That said, a black air filter is not good. Let’s find out why it happens and what to do about it. There are two overarching causes for black air filters.

Your air filter is black because of soot.

Your air filter is black because of soot.

This sounds scary, but, it’s usually no big deal. The good news is that, if your filter is black with soot, it means it’s doing its job. Better to have soot on your air filter than in your lungs, right?

So, what’s putting soot out into your house? The answer is pretty obvious: fire!

There are a few sources of fire in your house that are fairly common that most people kind of forget are actually fire.

The first is candles. When you light a wick, especially an old one, soot gets deposited into the air in your house. The black smoke you see when you blow out a candle is almost all soot. It’s hot (you know, from the fire), so it rises until it gets sucked up by your AC intake vent.

This is where the importance of your air filters becomes very clear: a low-grade or old filter might let the soot through, which means that over time, it’ll find its way onto your clothes and in your lungs. Nasty.

A high quality, regularly replaced air filter keeps the soot trapped, so you’d never notice it until you change your air filter. Thanks, air filters!

Oh, yeah, the candles. Trim the wicks to a quarter inch every time before lighting them, and you’ll be good.

There’s another source of fire in a lot of homes that gets ignored – a gas water heater. Gas-powered water heaters use ignited gas (AKA, fire) to heat up the water that comes out of your sink and shower. It’s great, until it has problems.


To determine if your gas water heater is the source of the soot in your air filter, you’ll need to check the pilot light. It’s usually on the outside, easily visible. It’s a fire, you’ll find it.

The key to the pilot light is the way it burns. Natural gas is a fairly even-burning substance, so the pilot flame should be a nice, smooth blue flame. If your pilot is orange and wiggling around like an earthworm after a rain storm, it’s probably releasing soot into your air. Again, not a huge deal, but you’ll want to address it by calling your gas company and getting a technician out to look at it.

Just like with AC systems, fixing a gas water heater is not something you should try to do on your own. It’s a giant pressurized vessel with an exposed flame connected directly to a pipeline of extremely flammable gas. Just don’t.


Another potential cause of soot deposits on your air filter is a malfunctioning gas furnace. Check the burner flame. Is it yellow and wiggly? That’s a problem.

If your furnace is malfunctioning, turn it completely off and call a technician as soon as possible. Furnaces have a tendency to leak carbon monoxide when they break. Carbon monoxide can kill you pretty quickly.

Finally, if you have a gas fireplace, it’s probably spitting soot out by design. Gas fireplaces are engineered to burn that dirty, yellow, wiggly fire because it fits the hot-cocoa-and-a-good-book aesthetic better than a clean blue flame.

But what if it isn’t soot that’s making your filter black? The next possible cause is considerably more severe.

Your filter is black because of mold.


Ok, this is a bigger problem. But don’t panic. If you regularly check your air filters, and this is the first time they’re moldy, then it’s only been about 3 months since mold’s been present in your home. While this is certainly not good, it does mean that you’re probably safe from the long-term effects of mold exposure. You definitely need to have to taken care of, but don’t go to the ER just yet.

If you don’t check your filters every three months, start doing that now. Seriously, your lungs and allergy medication budget will thank you. If you have no idea how long the mold has been in your house, you might want to consult your doctor and make sure you don’t have any long-term damage. If you’re having symptoms like shortness of breath, fatigue, and headaches, you should definitely go see your doctor.

Mold only needs one thing to grow: moisture. You can usually tell that the blackness in your air filter is mold because it’ll be wet and slimy, rather than dry and powdery like soot. By the way, try not to touch the mold too much. It’s not good for you.

A moldy air filter is usually caused by a problem with the condensation drainage in your AC system. There can be many causes for this, but the main cause is condensation on your evaporator coil. This is usually dealt with by a drainage pipe to the outside of your house. If that pipe is broken or clogged, or if your AC system is having some sort of back pressure, that moisture can be transferred to your air filters.

This is a problem that will need to be solved by an HVAC professional. Don’t try to mess with your condensation drainage yourself. It’s buried deep in your walls, and if you’re not a licensed professional, you might end up doing more harm than good. You can also void your home warranty by doing this, which is going to make your wallet very sad in the long run.

How can I prevent black air filters?

Besides fixing the underlying issues that are causing your black filters, the absolute best thing you can do is change your air filters on schedule (at least every three months.) Letting your filters get clogged is not only bad for your health, but it can damage your AC system too.

The problem is that most people completely forget about their air filters. It’s like going to the dentist: you know you should do it, but who has the time? Also, it’s not fun at all, so why would we do it?

That’s a valid mindset. It’s also why Filter King offers quick, easy air filter delivery right to your door. Just hit a few buttons between responding to work emails, and your new filters will be on your doorstep within a few days.

You can also set up a recurring air filter subscription with Filter King. You never have to think about something as boring as an air filter again, because they’ll simply show up when you need to change them. That’s why Filter King is an industry leader.

Be sure to subscribe to Filter King’s blog to get more insights on air filters, air conditioning, and home improvement.