Black HVAC Filter – What Is It And Is It Harmful

HVAC filters turn black because of the presence of mold, clogged air filters, or soot from fires, gas water heaters and candles. Mineral build-up, or leaking carbon monoxide gas and poor air flow can also make an HVAC filter black, but are less common problems.

More than 90 percent of us use an HVAC or AC system in our home to improve our air quality, and to cool or heat our space. But if our HVAC filter has turned black, we need to stop using it until we’ve identified and fixed the problem.

This guide will take you through all the possible reasons why your HVAC filters are black, and also in-depth steps on how to fix each of the problems.

What the black stuff in your HVAC filter really is

A black HVAC filter can be caused by a few different factors, although a filter issue is usually to blame:

1. Mold

An HVAC filter can be black because of mold.

HVAC units can create a moist environment, perfect for mold to grow on the filter. Condensation from running the AC component can also cause a damp evaporator coil, which then passes the moisture onto the air filter.

If left unchecked, the moisture can mix with dirt and dust and eventually produce black mold, which migrates to your HVAC filter, turning it black.

Mold can grow in your ducts as well, especially in areas of your home that are constantly damp and even spread throughout your home.

Types of mold that grow in HVAC filters include:

  • Mucor – Mucor is an allergenic mold that grows around HVAC systems and is white or gray in color. Because of the dampness from condensation, it usually grows in thick spots and, if not addressed, will turn your HVAC filter black.
  • Stachybotrys – Stachybotrys is a type of mold that’s often found in air ducts and vents. It’s easy to spot, but it’s dangerous since it spreads quickly throughout your home, turning your HVAC black and can lead to allergic reactions.
  • Chaetomium –Chaetomium is a mold with a cotton-like feel that grows in air ducts and water-damaged areas and gradually changes color. If not addressed, this mold quickly spreads to HVAC filters, turning them black as they spread.
  • Acremonium – Acremonium is a form of mold that can grow in an HVAC filter and is white and powdery but can also affect your HVAC filter. It is toxin-producing and allergenic. This mold can appear black when combined with the regular dust and dirt that accumulates on an HVAC filter.

How to tell if your HVAC filter is black due to mold:

  • Turn off the entire HVAC unit.
  • Access the filter and remove it.
  • Examine the color of the filter. If it’s black after the wet summer season, it’s likely mold because dampness is what leads to mold growth.
  • It could also be mold if you detect black dust surrounding your air conditioning vents.
  • Does it smell? A musty smell also indicates mold.
  • If you’re not sure, contact an HVAC technician to examine if it’s mold.

Other signs of mold presence in the HVAC filter include:

  • Frequent and severe allergic reactions. A stuffy nose, red and itchy eyes, coughing, and wheezing are all symptoms of mold exposure.
  • Mold allergies can induce fever and shortness of breath in people who are allergic to it, so if anyone who is allergic to mold in your home starts experiencing these symptoms, check your filter (and ducts).

Solutions to a moldy black HVAC filter:

  • If you notice mold on your air filter, it’s important to remove it and replace it as soon as possible. Don’t clean the filter. Mold and its spores are almost impossible to remove from the weave of HVAC filters, so it’s always best to replace them.
  • Find the source of the mold and get your HVAC system inspected if need be, to make sure mold is not forming inside the air handler.
  • Make sure there’s no damp space around your HVAC system, as this encourages mold growth.
  • Maintaining and cleaning your HVAC system regularly will help keep mold out of your system and ensure your air filter doesn’t turn black.

How to prevent mold growth in your HVAC system.

  • Use an air filter with a MERV 13 rating to keep mold spores out.
  • Clean and replace your HVAC filters as recommended by the manufacturer.
  • Consider installing an air scrubber that will capture and eliminate impurities like mold and its spores.
  • Schedule a regular maintenance plan.

Plentiful Air has detailed information on adding an air purifier to your HVAC available here.

2. Dirt and debris

Dirt and debris can cause your HVAC filter to turn black by clogging it up, which is especially common in dusty environments (so most houses!).

High levels of dust, dirt, and debris can change the color of an air filter to black pretty quickly, meaning that it’s nearly totally clogged and won’t work effectively. Because this can also damage the HVAC system, it’s important to keep dust inside the unit to a minimum.

How to know your HVAC filter has dirt and debris:

  • Turn off the HVAC system.
  • Access the filter.
  • Are there changes in the color of the filter, but no growths or clumps> If yes, it’s probably dirty.
  • Are there traces of dust and debris on it, around it and in the filter housing area.

Other signs of dirt and debris on a clogged filter include:

  • Experiencing less effective cooling from the HVAC system.
  • Reduced air flow.
  • Try a white sheet test by putting a white sheet close to the vents. If the sheet gets dirty in 30 minutes, then the filter is dirty.
  • Poor indoor air quality.
  • Higher electricity bills.
  • Buildup on blower fans and ductwork.
  • Dust near air vents.

Solutions to a dirty filter:

  • Cleaning the HVAC filter more often.
  • Changing the filters every 2 to 3 months.
  • Consider investing in an air purifier to reduce the overall dust in your home (optional, but not necessary).
  • Scheduling a HVAC maintenance plan and strictly following it.

3. Soot

Soot is a strong possibility for why your HVAC filter is black. Soot can turn your air filters black surprisingly quickly.

There are a few ways HVAC filters catch soot and turn black:

  • Candles
  • Fires
  • Gas water heaters


Candles produce black soot as a by product. Excess candle soot can reduce the efficiency of your air filter in the long run—particularly scented ones. Many scented candles are created with unsaturated oils, which give off soot when burned. 

Both colored and soft candles produce soot.


Fires, from both inside the fireplace and outside from wild fires create ash, soot and smoke. The build-up of soot and ash leads to poor air quality inside our homes. The soot turns the HVAC filter black, while the ash accumulates in the housing area or outside the vents as the particles aren’t the right size to be captured by the HVAC.

Maintenance, cleaning, and conducting a system and air quality check each year can help prevent this issue.

Gas water heaters

Even gas water heaters can produce black soot. If you have a gas water heater, check to see if the flame is blue, indicating a clean burn. The flame is likely creating soot if it is flickering, yellow, or orange in color.

How to tell if your HVAC filter is black due to soot:

  • The only way to know if your HVAC filter is black due to soot is if you use gas water heaters or candles, among other things that can produce soot.
  • If any of your gas appliances are producing a yellow or flame.
  • Recent wild fires in the area, or use of fire inside the home

Solutions to black HVAC filters due to soot:

  • Have a technician inspect your gas water heater’s burners to see if they are clogged or unclean, as this can lead to soot buildup.
  • Examine the position of the gas water heater to see if there is a lack of oxygen in the vicinity; this could throw off the air-to-gas ratio required for a clean flame.
  • Examine your gas water heater’s flue pipe, which vents exhaust outside. Your water heater will not be able to get rid of the exhaust if this pipe is clogged or blocked, which can lead to soot buildup.
  • Trim candle wicks to a one-quarter inch before lighting the candle each time to prevent soot from making your AC filters black.
  • Avoid using candles made from petroleum jelly or vegetable oil but rather use natural candles i.e., soy or beeswax candles.
  • Consider using hard candles and not soft ones.
  • Purchase quality candles
  • Use vases and hurricanes that are open on both ends
  • To prevent soot production, avoid excessive airflow around the candle.

4. Carbon monoxide

Another reason your HVAC filter is black is carbon monoxide. Carbon monoxide is a colorless, odorless gas produced by incomplete combustion. It’s well known to also produce soot buildup on the HVAC filters.

A leaking gas fireplace and damage inside the furnace are some of the most common producers of carbon monoxide. As well as turning HVAC filters black, carbon monoxide is harmful to your health and can even be fatal in high concentrations.

How to know if there is Carbon Monoxide:

As carbon monoxide has no odor or color, you can’t notice it easily unless you have a carbon monoxide detector.

Note: Because carbon monoxide gas is so deadly and it may be too late to detect it, don’t rely solely on physical symptoms, instead install a Carbon Monoxide detector to guide you.

Solutions to Carbon Monoxide:

  • Install carbon monoxide sensors in strategic locations throughout your home to warn you of gas leaks.
  • Call a HVAC company to figure out what’s causing the problem and advise you on how to fix it.
  • Every 2 to 3 months, clean and replace your furnace filter.
  • If the source of the leak is the heating system, get the repairs done as soon as possible.

5. Low air flow

A HVAC filter can be black because of low air flow. Your venting system can become too blocked or even leak at times.

These leaks can cause improper furnace combustion, which can lead to a black deposit on your air filter.

Solutions to improper air flow:

  • Schedule a regular maintenance routine and follow it.
  • Replace and clean filters when needed and as recommended by your manufacturer.

6. Mineral build-up

Mineral build-up can result in an HVAC filter turning black, although it can also lead to white deposits as well.

If you come live in an area with hard water (which is most of the U.S), minerals like calcium and magnesium solidify into solid particles, which can clog up in your HVAC system, mixing with dirt and dust to make it black.

This mineral buildup can also prevent heating or cooling from being released into your home properly, and won’t be able to control the temperature.

The heating and cooling unit can also become damaged if the filter isn’t cleaned or replaced quickly.

How to tell your HVAC filter is black due to mineral build-up:

  • The black build-up on the filter is hardened

Solutions to mineral build-up:

  • Take a home water test to determine if your water is hard or soft, or check with your local community water system.
  • Change and clean the filters more often.
  • Schedule a regular maintenance routine.

Frequently Asked Questions

Why is my HVAC filter wet?

An HVAC filter is wet because of water dripping directly on to it, a clogged condensate line, a frozen evaporator coil due to restricted air flow, or a broken/cracked drain pan. Other causes can include an incorrectly installed filter, a faulty condenser, or a low refrigerant level.

If you come across a wet air filter, remove it and dry the surrounding area with a towel or absorbent cloth. Next, inspect the condensate drain lines for any signs of cracks or loose fittings.

If you see dirt, debris, or algae growth while checking the condensate drain lines, carefully scrub it away with a mild detergent and a sponge or a soft bristle brush, and finally, replace it with a new filter as soon as possible.

Why is my HVAC filter so dirty?

An HVAC filter can be excessively dirty because of leaking air ducts, it’s hotter or colder outdoors than usual, or if you are using a 1-inch pleated air filter and have a high MERV rating on your filter.


There are various reasons behind a black HVAC filter, as mentioned in the post. However, you will not be able to know the exact reason why your HVAC filter is black if you do not examine the filter closely. Fortunately, if you have a good understanding of all the possible reasons behind a black filter, you can look at each reason as you eliminate them on the list until you find out the exact reason for a black HVAC filter.

Ultimately, if you clean and wash your HVAC filters on a regular basis, replace them as needed, and stick to a regular HVAC maintenance schedule, your HVAC filter should be able to function appropriately and not turn black.

Although, if your HVAC filter is black for any other cause, such as leaking carbon monoxide gas, mineral build-up, or soot from burning candles, simply follow each of the step-by-step treatments listed above to solve it, as well as the preventative steps, to ensure that it does not happen again in the future.

Theresa Orr

Theresa Orr holds a PhD in Earth Science and specializes in determining past climates from rocks using geochemistry. Her passion for clean water, soil and air drives her to provide easy to understand information for everyone to read.

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