Why Is My Furnace Filter Black?

December 15, 2016

What causes black on a furnace filter? Why is my furnace filter so dirty? Why does my furnace filter get dirty so fast? We commonly answer these questions during our furnace maintenance visits in customers’ homes…and we’re glad they ask! The air filter is an important component of your home’s heating system. The initial reason filters were used in air-forced systems was to prevent large pieces of dust from getting into and settling on the HVAC equipment. Today, tightly woven filters capture even tinier particles and contribute to cleaner indoor air.

According to the National Fire Protection Association, a clogged filter blocks airflow, reduces system efficiency, and potentially causes premature breakdowns. You should clean or replace the filter every 30 – 90 days or according to the manufacturer’s recommendations to avoid these problems.

Reasons Your Furnace Filter Is Black

You may examine your furnace filter and see that it has turned completely black. Under normal conditions, your filter should last at least a month and gradually become clogged with grey-colored dust and debris. It’s not normal, or healthy, for a furnace filter to turn completely black.

If this happens, one of the following possibilities could be the cause:

  • Candle soot: The blackened filter could have excess soot on it from burning candles. The amount of soot collected on the filter is only a small percentage of the amount of soot actually accumulating in the air, settling on the walls and floor, and entering your lungs. If you’ve been burning candles lately, discontinue this practice and see if a new filter retains its lighter color.
  • Fireplace soot: Similarly to burning candles, an improperly vented fireplace can leave soot deposits on the furnace filter. Consider having your fireplace checked to ensure it is venting properly.
  • Carbon monoxide: This odorless, invisible gas is the byproduct of incomplete combustion and can cause soot deposits on your furnace filter. Common sources of carbon monoxide include the furnace itself, vehicle exhaust in an attached garage, and a leaky gas fireplace. Make sure your carbon monoxide detectors are working and have your combustion appliances inspected immediately because, if this is the source of soot on your furnace filter, you could suffer serious health problems if you ignore it.
  • Mold growth: When you run the air conditioner in the summer, the evaporator coil becomes damp with condensation. This can transfer to the dirt and dust collected on the furnace. If left wet long enough, black, sooty mold can begin to grow and spread to the furnace filter. To prevent this, have the furnace cleaned in the spring before you start running the air conditioner all summer long.

Schedule Furnace Maintenance & Repair Services in Your Cincinnati Area Home Today

We’ve been the Cincinnati area’s heating and cooling contractor since 1986. Families and businesses have trusted Rick’s Heating & Cooling with all their heating and cooling needs for over two decades! We serve Warren, Butler, Hamilton, Clermont, and Clinton counties in Ohio, to name a few.

We proudly provide the following furnace services:

To learn more about our furnace services or to schedule service in your home, contact Rick’s Heating & Cooling online or give us a call at 513-899-6005 today!