Gas Furnace Repair

A furnace keeps your home toasty warm until a problem occurs. This Fix-It Guide on gas furnace repair tells how a gas furnace works, what often goes wrong, how to identify the problem with a gas furnace, and what parts and tools you will need to fix it. It then gives instructions for how to light the pilot, how to adjust the pilot, how to test and replace a thermocouple, how to adjust an adjustable burner air shutter, and how to clean removable burner tubes and ports. If you suspect problems in the forced-air distribution system or the comfort controls , check those guides as well. If you have another type of furnace, check the Electric Furnace Fix-It Guide, the Oil Furnace Fix-It Guide, The Steam Boiler Fix-It Guide, or the Hot Water Boiler Fix-It Guide.

How Does a Gas Furnace Work?

 Gas Furnace Repair

Make sure the flame is well-formed and blue.

Natural or propane gas from an outside source is piped to the furnace where it is burned to produce heat. Usually a fan-driven forced-air distribution system blows the warmed air through ducts that vent into the various rooms of the house. Older gas furnaces use a standing-pilot ignition. Maintenance involves turning off the pilot each spring and relighting it each fall. Newer, more efficient gas furnaces use an electric spark to light the gas as necessary.

What Can Go Wrong with a Gas Furnace?

Most gas furnaces are quite reliable. What are the symptoms of problems? The furnace may not produce heat or may not produce enough heat. The pilot light may go out repeatedly or refuse to light. The thermocouple may be faulty. The pilot may light but not ignite the burner. The furnace may be noisy. There are some maintenance and a few minor repairs that you can make. However, major service should be left to a trained technician.

Fix-It Tip

To minimize problems with your gas furnace, take time each month to check the air filter and clean or replace it if necessary. Once a year, clean the blower blades, lubricate the blower motor, and inspect the belt.

How Can I Identify the Gas Furnace Problem?

  • If there is no heat, check the electrical service panel for a burned fuse or tripped breaker. Re-light the pilot light (see below).
  • If there is not enough heat, adjust the burner air shutter (see below); and clean the burner ports (see below).
  • If the pilot light does not light or does not stay lit, clean the pilot orifice carefully with a toothpick, test the thermocouple and replace it if it is faulty (see below).
  • If the flame flickers, adjust the pilot (see below).
  • If there is an exploding sound when the burner ignites, adjust the pilot to a higher setting and clean the pilot orifice and the burner ports.
  • If the burner takes more than a few seconds to ignite, clean the pilot orifice and adjust the pilot light.
  • If the burner flame is uneven, clean the burner ports. If the burner flame is very yellow, clean the burner; open vents in the furnace room to provide more air; adjust the burner air shutter.
  • If the furnace makes a rumbling noise when the burners are off, clean the burner and adjust the burner air shutter.
  • If the air is too dry, wash or replace the evaporator pad if you have a humidifier; test the humidistat; and adjust the water-level float to raise the water level.
  • If some rooms are too cool and others too warm, the distribution system may require balancing. Refer to the Forced-Air Distribution Fix-It Guide .

Fix-It Tip

Be sure your filter is the right size for your furnace.

What Do I Need for Gas Furnace Repair?

Some replacement parts for gas furnaces are interchangeable (filters, fasteners) and available at your local hardware store. Others, such as burners and controls, must be purchased from the manufacturer or aftermarket supplier or through a heating equipment supplier listed in your local telephone book.

The primary tools you will need for fixing a gas furnace include these:

  • Screwdrivers
  • Wrenches
  • Pliers
  • Wire brush
  • Multimeter

What Are the Steps to Gas Furnace Repair?

To light the pilot on a standing-pilot (always on) ignition system, follow the lighting instructions located near the control. Otherwise, try these steps:

 Gas Furnace Repair

Press and hold the pilot control knob to start the pilot.

Light the gas furnace pilot:

  1. Set the control knob to the pilot position. Hold a long match under the pilot gas port.
  2. Press the control knob; the pilot should light. Hold the control knob down until the flame is burning brightly (about 30 seconds). Release pressure on the knob, and turn it to the on position.
  3. If the pilot goes out when you release the control knob, try relighting, holding the control knob down longer. If the pilot again goes out, check the thermocouple (below).

Adjust the gas furnace pilot:

  1. Remove any cap covering the pilot adjusting screw on a combination control.
  2. Turn the adjusting screw counterclockwise to increase the flame or clockwise to decrease it. It is correctly adjusted when the flame envelops the thermocouple bulb by 1/2 inch and appears dark blue with a small yellow tip.

Test and replace a gas furnace thermocouple:

  1. Hold the control knob to pilot and light the pilot as above.
  2. Unscrew the thermocouple fitting with an open-ended wrench.
  3. Set a multimeter to the DVC (lowest voltage) scale.
  4. Clip one multimeter lead to the end of the thermocouple tube nearest the pilot and the other lead to the fitting on the other end of the tube.
  5. If the multimeter shows a reading besides zero, the thermocouple is functioning. Replace the thermocouple tube.
  6. If there is no reading, you will need to clean or replace the thermocouple following steps 7 through 11.
  7. Release the control knob and shut off the main gas valve on the gas-supply pipe that leads into the burner. Shut off power to the burner at the electrical service panel.
  8. Remove the thermocouple from its mounting bracket.
  9. Wipe the combination control clean and install a new thermocouple, tightening it by hand, then give it a one-quarter turn with a wrench.
  10. Insert the thermocouple into the pilot bracket, being careful to not crimp the tubing.
  11. Turn on power to the furnace and relight the pilot (above).

Adjust an adjustable gas furnace burner air shutter:

  1. Set the thermostat to its highest setting to keep the burner running. Once the furnace has heated up, remove the burner access panel and loosen the locking screw.
  2. Open the shutter by turning it to the right until the blue base of the flame appears to lift slightly from the burner port surface. Then close the shutter until the flame re-seats itself on the surface.

Clean removable gas furnace burner tubes and ports:

  1. Shut off gas and power to the furnace.
  2. Unscrew or loosen and remove the tubes from the supporting bracket.
  3. Carefully clean the tubes with a brush or vacuum, making sure not to damage the burner ports.
  4. Use a stiff wire to clear any debris from clogged ports.

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