Forced Air Gas Furnace

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A gas forced-air heating system has a pilot light that ignites a burner inside the combustion chamber, creating heat that is then transferred to the furnace’s heat exchanger, a metal chamber around which air flows and is then heated. That heated air is then forced into the hot-air plenum and into the rooms through ducts. Gas fumes and carbon monoxide are vented through a flue in the roof.

The forced air gas furnace provides the option for an attached air conditioning unit and humidifier, part of a complete central air HVAC (Heating, Ventilating, and Air Conditioning) system.

Gas Furnace Diagram


This type of furnace produces heat through the combustion of natural gas in the furnace's burner. The heat produced from this process then passes through a heat exchanger. Air from your home's return air ducts is blown over the heat exchanger, thus warming the air. The furnace's blower then blows the warmed air into the ductwork, which carries and disperses the warmed air throughout the home.

Thermal Coupler

The thermal coupler is primarily a safety device that allows the pilot light to stay light, however, in the event the pilot light does go out, the thermal coupler prevents gas from leaking into the air. The thermal coupler shuts off the gas supply to the pilot of the pilot goes out. The heat from the pilot light keeps the valve in the thermal coupler open. If the pilot goes out, the metal cools and closes the valve. When a thermal coupler fails it tends to close the valve even when the pilot is burning and the metal warm. A thermal coupler is easy to replace and you will likely have to replace one on a regular basis.

How to Install a Gas Furnace Thermal Coupler

If the pilot light on a gas furnace will not stay lit the problem may be with a part called the thermal coupler. The thermal coupler is screwed into the gas control valve and has a thin wire which sets in the flame of the pilot. If there is no pilot light burning the thermal coupler tells the gas valve to shut off the gas supply.

  1. Locate the shut off valve on the gas line which goes to the furnace and shut it off.
  2. Locate the thermal coupler on the furnace. Look at where the pilot light is and trace the wire that sets in the flame of the pilot back to the gas control valve.
  3. Unscrew the nut which attaches the thermal coupler to the gas control valve. Remove the old thermal coupler and take it to the hardware store to make sure the right size replacement thermal coupler is purchased.
  4. Tighten the nut by hand which holds the thermal coupler onto the gas control valve. Use an adjustable wrench to tighten the nut the rest of the way.
  5. Place the wire end of the thermal coupler in the pilot the same way as the old thermal coupler. Usually only about the top 1/3 of the thermal coupler wire should be heated in the pilot light.
  6. Turn on the gas supply and re-light the pilot.


The Heat Exchanger

The heat exchanger on a furnace is the metal that separates the fire from the air stream. The heat exchanger was designed to keep the toxic fumes produces from burning gas separated from the clean heated air that is blown in your home. The inside of the heat exchanger allows the toxic flue gases produced from the burners to exhaust out through the furnace flue. The outside of the heat exchanger is where the cold air passes over, becomes warmed, and is blown throughout the ductwork of the home. The heat exchanger is the only wall separating the toxic flue gases from the supply air.

If a crack develops in the heat exchanger, there is a potential for carbon monoxide gas to leak over to the supply side and be blown throughout your rooms. Carbon monoxide gas is odorless. The human senses will not detect its presence. When the fan or blower is running it puts the outside area of the combustion chamber (heat exchanger) under positive pressure so there is no way the flue products could ever come out. Another problem with having a crack on one of these furnaces is that the flames will roll out because the draft air is leaking through the crack.

Inspecting the Heat Exchanger

Visually examine the heat exchanger with the aid of a flashlight. Many furnaces will have hair line cracks in the metal and sometimes welds will pop and leave a tiny hole. Small holes may not pose a problem. Look for a significant breach in the heat exchanger. When the furnace comes in an flames roll through the cracks then it is a significant problem.

In your visual inspection consider.

  1. Where is the crack?
  2. How big is the crack?
  3. How much air is leaking in out out of the crack?
  4. Is the crack actually in the combustion area or in the sheet metal of the frame?
  5. How will this crack affect the operation of the furnace?
  6. How can this crack cause combustion products to leak into the living space?
  7. Can you see light (blower removed) from the air side of the furnace?
  8. How does the crack compare to the burner openings and the flue opening on each end of the heat exchanger?

WARNING! Many experts recommend that you never rely on visual inspection alone to determine the safety of a furnace heat exchanger. carbon monoxide - CO - is a dangerous gas that can result in health problems and death. You don't want CO building up in your home due to a flaw in your furnace heat exchanger.

Ways to Find or Suspect a Leak in a Furnace Heat Exchanger

  • Visual inspection with flashlight and mirror.
  • Look at the heat exchanger through the Supply air plenum.
  • Inspect at the furnace burner: for evidence of a heat exchanger crack or leak by watching for a change in flame pattern or color.

Recalls and Furnace Models Known to be Bad

  • Lennox Pulse Furnace Safety Problems, Recall, Inspection, Advice
  • Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
  • Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
  • Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]