Why Is My AC Filter Turning Black?

It’s one thing to check your air filter and find it grey and clogged with dust, dirt and the usual air contaminants.

It’s another to find it completely black, like this filter.

It doesn’t take a seasoned homeowner to know that an air filter that’s completely black isn’t normal.

Most likely your AC filter turning black is caused by a phenomenon called “Black Soot Deposition”.

Black Soot Deposition refers to the household problem marked by black stains on walls, carpet, furnishings and yes, air filters.

What causes these black stains?

Burning candles in your home. Or more specifically, burning scented candles in your home.

So how does a candle turn your air filter black?

The quick answer: When candle wax burns, it produces tiny, black soot particles that deposit themselves on surfaces such as walls, furniture and carpet. The more “impurities” (for example, scented oils) the candle wax contains, the more soot it produces.

The more technical answer: Candle wax (also called paraffin) is made from carbon-based materials. As the wax melts when it interacts with the candle flame not all of those carbon particles burn fully.

These partially combusted particles collect and attach themselves to surfaces to create “soot”. And scented candles often cause more soot because manufacturers typically use unsaturated scented oils that don’t properly combust.

Interesting note: The yellow part of a flame marks where carbon materials do not burn fully, while the blue part of the flame marks where carbon materials burn clean (i.e. a blue flame will not produce soot while a yellow flame will).

Candle soot can cause varying degrees of household damage, from black stains on carpets and walls to thousands of dollars worth to replace soot-contaminated ducts.

Obviously, the best way to prevent soot from damaging your home is to stop burning candles.

But we love a good scented candle just as much as anyone else.

So here are some tips to follow if you can’t bear to part with your candles:

• Choose candles that are hard to the touch at room temperature. Soft-to-the-touch candles are a good indication that there are a number of “impurities” in the wax that will produce more soot.
• Keep your wicks short. Get in the habit of trimming your candlewicks to ¼ inch. Remember that the amount of wick exposed increases as the candle burns so you’ll need to trim repeatedly.
• When possible, buy candles that were made and manufactured in the U.S. American-made candles are produced under strict guidelines that aim at reducing the amount of soot a candle produces. But foreign-made scented candles were likely manufactured under less strict guidelines and most likely contain more soot-producing materials.
• Steer clear of candles that have petroleum jelly or vegetable oil mixed into the wax. Higher percentages of either of these additives produce a large amount of soot.
• Keep your candle flame away from air movement. A candle flame disturbed by air movement produces up to 300% more soot. Avoid burning candles when the AC is blowing and try not to place them in areas where people repeatedly walk or under/near ceiling fans.
• Invest in an activated carbon filter. An activated carbon air filter purifies your home’s air with a 5-stage filtration system. The last layer contains activated carbon that traps gases, fumes, tobacco smoke, toxins and chemicals produced by candle burning.