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Why Is My Furnace Filter Black? A Phoenix Tech Answers

March 18, 2021

If you go to change your furnace filter and notice that it’s black, soot buildup (also called black soot deposition) could be the reason why.

Soot deposition can be caused by several different things inside your home, including:

  1. Mold growth
  2. Candle soot
  3. Faulty gas appliances
  4. Gas fireplace

Indoor soot buildup is more than just annoying and difficult to clean. It can also be dangerous to your health. According to one 2017 study published in the academic journal Frontiers In Immunology, excessive soot exposure can lead to respiratory diseases, cardiovascular problems, cancer and even premature death.

To help you get rid of all that harmful soot buildup in your home, we’ll walk through some of the most common causes of soot buildup and their respective solutions.

Want a Phoenix-area pro to take a look at your furnace? We provide upfront and honest pricing for all our services, including furnace repairs. Plus, we offer a 100% satisfaction guarantee on all our work.

Cause #1: Mold growth

If an excessive buildup of dirt accumulates on the filter or inside the furnace, a black, sooty mold can develop.

Your AC’s evaporator coil creates the prime condition for mold growth: dirt, dust and moisture. As hot air is blown over the evaporator coil, moisture from that air condenses onto the coils and is captured there. If mold develops, it can spread to the filter that is shared by the furnace and AC system.

Central heating systems that are not cleaned regularly are more likely to grow mold. If the furnace filter becomes moldy, simply removing it won’t solve the problem. You’ll need a professional cleaning to completely rid your HVAC system of the mold.

To prevent future problems, regular maintenance and cleaning is the key to keeping mold from growing inside your HVAC equipment.

Cause #2: Candle soot

Many scented candles are made with unsaturated oils, which don’t burn clean. This means they produce a soot residue when they’re burned. Although most of that soot ends up on your walls, floors and in your lungs, some of it will also end up on your furnace’s filter.

So, how do you know if your candles are producing soot?

If a candle’s flame is primarily orange or yellow instead of blue, it’s probably producing soot residue.

The easiest way to prevent indoor soot buildup from scented candles is to stop burning them inside your home. If you just can’t part with your candles, there are a few things you can do to reduce black soot deposition in your home:

  • Don’t buy candles that contain vegetable oil or petroleum jelly. (Check the ingredients list for details.)
  • Only buy candles that are hard to the touch at room temperature.
  • Strategically place candles away from vents and fans.
  • Trim your candle wicks to ¼ inch to prevent them from producing excess smoke and soot.

Cause #3: Faulty gas appliances

If your gas appliances are not functioning properly, they may produce soot as a byproduct.

If you have a gas furnace, it may be producing soot if the pilot light flame is burning orange or yellow instead of blue. This may be caused by:

  • A cracked heat exchanger: This can allow soot to enter your home’s air supply.
  • Poor ventilation: Improper ventilation can trap soot and circulate it through your home instead of sending it outside.
  • Clogged burners: The burners are the part of the furnace where gas mixes with the air. If they are clogged or misaligned, they can produce soot.
  • Clogged gas valves: Excessive debris or dirt can clog a furnace’s gas valves, which open and close to enable the flow of gas to the pilot light and burners. Soot deposits can form as a result.

If you have a gas water heater and the burner flame is yellow or orange, you likely have a problem that may be caused by one of the following issues:

  • Clogged burners: Much like with a gas furnace, soot deposition can occur if the burners inside your gas water heater get clogged or excessively dirty.
  • Clogged flue pipe: The flue pipe allows exhaust to vent outside. If this part of your water heater gets clogged or is obstructed, it can cause soot buildup.

If the flame in your gas furnace or gas water heater is burning orange or yellow instead of blue, an experienced HVAC technician can inspect your appliances, determine the source of the problem and provide effective solutions.

Cause #4: Gas fireplace

Gas fireplaces spread smoke and soot into the air, which can get into the system and end up on your furnace filter.

Although your gas fireplace is designed to produce yellow flames with minimal soot production, it will inevitably produce some soot. If you don’t take certain precautions or maintain regular maintenance on your fireplace, the buildup may become harmful to your health.

Some of the most common causes of soot buildup from a gas fireplace are:

  • Improperly positioned ceramic fire logs: Sometimes, you might shift ceramic fire logs out of position to better suit your visual preferences. Your fireplace might look better this way, but re-positioning the logs can interfere with the flame path. This can lead to soot formation on the logs, glass fireplace doors and elsewhere in your home.
  • Clogged ports on the gas burner: Clogged ports can produce an interrupted or unbalanced burn which may result in soot buildup on the logs and doors of the fireplace.
  • Improper airflow: The proper mixture of gas and air produces pretty yellow flames in a gas fireplace and reduces soot buildup. However, if the adjustable air shutters and vents on the fireplace get clogged or blocked, the flame will lack oxygen and produce excessive amounts of soot.
  • Old age: Over time, gas fireplace burner sets wear out. As a result, they may begin to release gas differently or in varying amounts. This can change the way the fire burns and make it burn less “clean” which will produce more soot.

A few ways you can reduce soot buildup caused by a gas fireplace include:

  • Making sure your logs aren’t in the direct path of the flames
  • Removing any old or damaged logs
  • Replacing your air filter and furnace filter regularly
  • Hiring a professional to inspect your gas fireplace for leaks, obstructions, etc. annually

Need a professional opinion? The Pros at Patrick Riley are here to provide answers

Suddenly discovering that your furnace filter is black can be alarming. It’s especially concerning considering the health risks associated with black soot deposition.

If you need help determining the cause of your indoor soot buildup, the experienced techs at Patrick Riley can provide solutions, repairs and peace of mind. We offer flexible scheduling and upfront flat-rate pricing, so you won’t have to deal with any unfortunate surprises when you receive the bill.

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By Ty Lindsay

Ty Lindsay is the Director of Field Operations at Patrick Riley | Isley’s and a 15-year veteran of the plumbing and HVAC trades. In 2010, Ty earned his Journeyman’s plumbing license. He became a Master Plumber five years later and earned his Journeyman HVAC technician’s license that same year. Ty’s breadth of knowledge in plumbing and HVAC includes both residential and commercial work. He’s been a loyal member of the Patrick Riley | Isley’s team since 2016.