Fire

• A number of problems in the furnace system cause fires to develop beyond what is necessary to produce heat. This is mainly caused by excess fuel gathering in the combustion chamber between heating cycles, according to the Newbury Fire Department. Since the fuel is gathering during the heating process instead of while the device is off, a continuous fire burns instead of exploding in a puff back. This fire produces thick black smoke that pours out of the chimney or vents.
  
  

Puff Back

• An explosion known as puff back blows soot and black smoke through the warm air vents throughout the home, according to InspectApedia. When extra oil or gas builds up in the combustion chamber due to a valve leak, they eventually ignite in a large explosion. The smoke and soot is accompanied by a loud boom or rumbling noise in the furnace. If this occurs more than once, the combustion chamber and ventilation ducts are being seriously damaged.
  
  

Blockages and Leaks

• There are a number of important points on a furnace that must stay sealed to keep the air in your home clean and safe. If cracks or warping develop around the burner mounting plate, smoke will leak into your air supply, notes Michigan State University Cooperative Extension. Blocks in the vents for the furnace also cause black smoke to blow out of the furnace instead of traveling out of the house. Animal and bird nests are a common cause of this problem.
  
  

Heat Exchanger

• Damage to the heat exchanger allows black smoke to enter the combustion chamber or leak into the air ducts. A cracked heat exchanger requires an expensive replacement, but other parts of the heat exchanger could go bad without ruining the entire part, according to "Mother Earth News." The gaskets that sealing both parts of the exchanger warp over time. Replacing these gaskets is inexpensive compared to the entire heat exchanger, so check that the gaskets aren't damaged before paying for the full replacement.