Back Button

What Are the Causes of Soot on a Furnace Filter?

Josienita Borlongan

According to the Washington Post, you should replace high-efficiency filters every three months and older, standard furnace filters once a month. When black soot appears on your filter, it can be dangerous to your health.

A burning candle can cause soot deposits on furnace filters.

Finding the cause of the soot deposit will not only help you solve the problem, but prevent it from happening again.

Inadequate Combustion Air

Soot deposits can be due to inadequate combustion air, which can cause oil to burn inefficiently. When this happens, soot builds up in the combustion chamber and the exhaust stack of the furnace. When exhaust gas pulls back down the chimney, it can enter the draft control and the furnace room. Sealing your home too tightly can prevent proper air circulation. After the air leaves your house, nothing will replace it, causing the air inside to have a lower pressure than the air outside. Once the soot enters the furnace room, the air currents in the house will pick up the fine particles, which, in turn, drift into the room where they will enter the heating system return register. The soot finally ends up at the suction side of the air filter in the furnace room. To prevent soot build-up, make sure to ventilate your home, especially during the winter. Check appliances that produce combustion to make sure they produce clean air and have no backdraft.

Burning Candles

Using candles frequently can cause soot deposits on your furnace filter. The worst offenders are scented and aromatic candles, which can produce the most soot. The added heat and smoke from the burning candles can make the air warmer, causing it to rise. When the air cools down, the soot deposits in places, including the furnace filter. According to Bailey Engineering, burning one candle can be equivalent to burning 100 candles at once. If you must use candles in your home, make sure to use a hurricane lamp with partial cover to lower the smoke emitted.

Carbon Monoxide

Known as the invisible, odorless, silent killer, carbon monoxide can also cause soot deposits. When fuel does not burn completely, it produces carbon monoxide. Sometimes, the source of carbon monoxide is the furnace itself. Other possible sources are tailpipes of cars parked in an adjoining garage or a leaky gas fireplace. A carbon monoxide detector can help detect carbon monoxide in your home. If you have carbon monoxide in your home, you have problem far worse than just black soot on your furnace. Carbon monoxide can make people sick, and, in worst cases, causes death. Have an inspector check your furnace and other appliances that may be emitting carbon monoxide to prevent soot and save your life.


Mildew and mold can cause soot on your furnace. Mold appears when you do not have proper ventilation. Mold loves damp and humid locations. In most cases, you can prevent mold by spraying cleaning products to affected locations; however, when they spread and become thicker, they can be difficult to remove. In worst cases, the black soot can penetrate the walls and can make you sick. Keeping your home clean and well-ventilated can keep molds away.