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How to Repair a Gas Furnace

Kalpana Kumari Sep 29, 2018
Let's learn about all the basics of a gas furnace, along with its working and how to solve the problems about same.
Let's see how to repair a gas furnace with all the basic instructions. No doubt this job is a bit tedious, but with a sound understanding of how this equipment works, its different parts and their functions, and the problems that tend to occur in different parts, troubleshooting the problems is easy.


Natural gas or propane is the fuel used for a gas furnace. The fuel is taken in from an outside source and burned to form heat. There are fans to blow the warm air through the ducts to distribute heat throughout the room.
In older furnaces, there used to be a standing-pilot ignition. The pilot had to be put out in the spring and lit in the winter. In newer furnaces, an electric spark is used to ignite the gas.

Common Problems

Your gas furnace is in a good condition if the flame is full, continuous, and doesn't sputter. It becomes problematic when it does not produce a sufficient heat. A problem can occur in any of the parts of this instrument, i.e., in the pilot, blower, or burner. Any malfunctioning part can impair the normal functioning of the furnace as a whole.
Some of the frequently occurring problems are a defect in the thermocouple, the pilot light repeatedly going out or no lighting at all, the pilot itself getting lit but failing to ignite the burner, and a sputtering or leaking furnace.
The last is the most common problem. Of course, the smell of the gas is enough to suggest that you have a leakage problem. If you suspect leakage of gas from the furnace, there is a way to confirm it. Prepare a solution of water (1 cup) and detergent (1 teaspoon).
Paint it on the gas supply pipeline, on the valves and connections. The soapy water will bubble at the point of leakage. If you find a leak, tighten the leaky connection with a pipe wrench, or seal the area with a sealant. If the problem prevails, call a professional.


If the problem with your gas furnace is major, you must consult a technician. In case you feel there is a small defect and that you can rectify it, then you can take care of the problem yourself. To troubleshoot gas furnace problems, you will have to learn to identify the problem in the first place.

Problem Identification

  • If you find no heat, check for the supply of electricity. You might find a burned fuse or a damaged circuit breaker.
  • If the heat produced is not enough, a dirt-blocked burner port is the most obvious reason. Clean the port and also adjust the shutter of the burner.
  • In case there is a problem with lighting the pilot, or if once lit it doesn't stay that way for too long, then the pilot orifice is not clean enough. You can clean the orifice with a toothpick. If the flame is not steady, adjust the pilot.
  • If the thermocouple is faulty, the ideal option is to replace it.
  • If the burner sputters on ignition or takes more time to ignite, there is definitely a blockage of the pilot orifice by dirt and dust. Set the pilot at a higher level and clean the orifice along with the burner ports.
  • If the flame is very yellow or uneven in appearance, the burner needs to be cleaned.
  • If you find the air too dry, the evaporator pad needs to be washed or replaced (if you have a humidifier). Also, test the humidistat. Adjust the float in order to raise the water level.
  • If there is uneven distribution of heat, i.e., if some rooms are too cool and some too warm, there is a problem with the distribution system.


Some parts of the gas furnace, like filters and fasteners, are interchangeable. These are easily available in hardware shops. Other parts, such as burners and controls, can be bought from an aftermarket supplier, manufacturer, or a heating equipment supplier. You'll need pliers, wrenches, multimeter, wire brush, and a screwdriver to repair a gas furnace.
1. Start the pilot by pressing and holding its control knob, and set it to the pilot position. Ignite the burner with a long match. Hold the control knob down till the flame burns brightly. Release pressure from the knob and set it to the ON state.
If you find the pilot going out when the control knob is released, hold the knob a little longer. If it happens again, check the thermocouple.
2. Light the pilot as described above. Use an open-ended wrench to unscrew the thermocouple fitting. Set a multimeter to the lowest voltage scale.
3. Clip one multimeter lead to the end of the thermocouple tube that is closest to the pilot and the other one to the fitting at the other end of the tube.
4. If the multimeter shows a non-zero reading, the thermocouple is working and only the tube is faulty and needs to be replaced. Otherwise, you need to replace the thermocouple too.
Steps to Replace the Thermocouple
1. Put off the electricity supply by releasing the control knob and turning the main gas off.
2. Take out the thermocouple from its mounting bracket.
3. Clean the combination control and install the new thermocouple. Tighten it and give it a one-quarter turn with the help of a wrench.
4. Place the thermocouple inside the pilot bracket. Be sure not to bend the tubing.
5. Put on the electric power to the furnace and rekindle the pilot.
By now, you must be feeling confident about repairing your troublesome gas furnace and save a few dollars that you would otherwise give to a technician.