Furnace Operation

• Oil or gas-fired furnaces heat your house by warming water or other fluid in a boiler. The fluid in the boiler is then pumped out through pipes and radiators to the rest of the house. As the warm fluid passes through the radiators, it gives off heat, warming the room. The cool fluid then passes back to the boiler, where it is heated again and repeats the cycle.
  
  

Flue Connections

• As the gas or oil inside the heater heats the boiler, it creates gases, including carbon dioxide. These gases can be dangerous if they're allowed to accumulate in a small area, so the resulting gases are vented out through a furnace flue. The flue is a cylindrical pipe, approximately 3 or 4 inches in diameter, that runs to a chimney or into a flue ventilation system, where the gases are dispersed into the atmosphere through the roof.
  
  

Flue Pipe Disconnection

• Most furnaces, unless they are very high-tech models with built-in leak detection, will turn on if the flue pipe is disconnected. However, this is a dangerous way to operate your furnace, and the appliance should never be operated in this manner. A disconnected flue pipe will allow the carbon dioxide and other gases created during furnace operation to seep out into the surrounding area. The fumes can quickly fill a room such as a basement, creating a deadly environment.
  
  

Flue Pipe Repair

• Shut down the furnace immediately if you notice a flue pipe disconnection. Turn off the gas, if it is a gas-fired burner. Find the disconnection in the flue pipe. Take a careful look at the pipe surfaces. This will help you determine the cause of the disconnection. If the pipe has rusted through, replace the flue section, and run a liner through the inside of the flue to protect it against condensation. If a strap or connection has severed, replace and reconnect the sections of flue pipe.