How a Gas Floor Furnace Works


Gas Flow

  • Current runs to the gas furnace's solenoid when the contacts of the thermostat in the house close. This current opens the diaphragm valve located in a combination control. Gas starts to flow through a supply line, running past a manual shutoff and through the manifold. From the manifold the gas travels through tubes in which it begins to mix with air.

Pilot Light Ignition

  • The mixture of gas and air then goes to a burner. This burner is ignited by the pilot light which creates heat that is turned into steam, hot water or warm air by a heat exchanger. The waste that is created travels up the furnace flue.

Pilot Light Adjustment

  • If the pilot light goes out, the thermocouple and another solenoid stop the flow of gas. The pilot light does not stay lit if it is too low for the thermocouple to be heated. The pilot light flame can be raised or lowered by removing the screw cap and turning the adjustment screw located on the combination control. Doing this will regulate the flow of gas inside the supply line.


  • A common cause of the pilot light not staying lit even after it has been adjusted is that the thermocouple is faulty. The thermocouple is simply an electric generator found inside a steel tube. Because the thermocouple is located so close to the pilot light flame, extended use often causes deterioration. In addition, a cooper lead that runs from the pilot assembly to the combination control can be defective over time.


  • The efficiency of a gas furnace depends in large part upon the proper combination of natural gas and air. Even adjusting the pilot light or replacing the thermocouple won't solve your heating problems with a furnace where the mixture is not right. An experienced furnace repairman is necessary to make sure the mixture is efficient.

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