Gas furnaces heat by burning fuel, which then heats either air or water that is then circulated throughout the house, heating it.  Because gas burns cleaner than oil, the yearly service is therefore less challenging.  With most problems being as simple as relighting the pilot light, or replacing the igniter, which is high-resistance metal that lights the furnace without the use of a pilot light.

While homeowners can easily maintain their gas furnaces, any repairs or installation of gas lines should be left to a certified professional.  When working on your gas furnace, always shut off the gas first.  To do this, follow the gas line from the furnace back to the red-handled valve.  This is the manual shutoff valve.  Turn the handle to the off position, which is usually perpendicular to the pipe.  You will need to wait approximately 5 minutes before you perform any work.  

Burners can have dirt build up around them, which will cause a yellow flame or delayed ignition.  By simply cleaning the burners once a year you can avoid having to place a service call.  To do this, you will need a vacuum and your owner’s manual.  Because every furnace is different, the following directions are general.

Begin by tracing the gas line to the burners and carefully examine the fittings and brackets.  You want to take careful notes of how the burners are aligned and how they are held in place, so once you have then out, you can reinstall them correctly.

If your pilot light fails to stay lit when you release the control knob, you will need to relight the pilot light and then examine the flame’s color.  If it is mostly yellow, the flame is not getting enough oxygen and possible not enough heat to trigger the thermocouple.  However, if the fame is mostly blue, then the problem may be with the thermocouple. 

With a gas furnace, also check the most obvious problem first, such as making sure that the pilot is lit.  Next, clean the nozzle.  If this doesn’t fix the problem, then check the thermocouple to see if it needs to be cleaned or replaced.

To clean the pilot, first turn off the gas and disconnect the pilot line from the bracket housing the burner.  After the line is disconnected, remove and clean the nozzle.  You can gently blow through the nozzle to remove debris, or replace it with a new nozzle.   

To clean or replace the thermocouple, remove the nut at the end nearest the pilot, and then carefully pull the thermocouple out of the bracket.  You want to carefully and gently clean the end of the thermocouple as ash and other debris can build up on it, thus preventing it to fire the furnace correctly.  You can clean it with very fine-grit sandpaper.  Once you have cleaned it, replace the thermocouple and then retest the flame.  If the flame fails to light or is the wrong color, replace the thermocouple.

If you have to replace the thermocouple, turn off the pilot and cut off the gas at the manual shutoff value.  Turn off the power at the circuit breaker or fuse box.  You have to allow the thermocouple to cool down before loosening the nuts with a wrench and removing the thermocouple.  Once you have removed the thermocouple, take it with you to a dealer for a new one, then install the new thermocouple and relight the pilot.

If you have a newer furnace, then chances are you have an igniter and not a pilot light.  To check the igniter, watch it as someone turns up the thermostat.  If the igniter does not glow through the observation window, then it may need to be replaced.

You can use a multimeter to check your igniter.  Unplug the wired leading into the igniter, and then place a lead on each of the igniter wires leading back into the furnace.  Next, set the multimeter to read AC volts, then turn the thermostat up high enough to start the furnace.  If the reading on the meter is approximately 120 volts, then the igniter is faulty.  If there is no power, then the control box could be faulty.  Do not attempt to repair of replace this yourself—call a service person. 

To replace the igniter, loosen the screw in the bracket, which connects it to the furnace.  You will need to then replace it with an igniter that is of the same model and make.
When handling the igniter, you want to hold it by the porcelain or the bracket.  If you touch the surface, this can shorten the life of the igniter.  Reattach the igniter wires, and screw it in place.  Next, turn up the thermostat to test it before you reattach the cover.

WARNING: If you discover a gas leak, get out of the house immediately.
Remove all family, friends, and pets from the house.  Go to a neighbor’s home and call the fire department and your gas company’s emergency number.  DO NOT CALL FROM THE HOUSE WHERE THE GAS LEAK IS!  GET OUT OF THE HOUSE.  DO NOT TURN ON OR OFF ANY ELECTRICAL DEVICES, POWER TOOLS.  SPARKS CAN CAUSE AN EXPLOSION; THIS ALSO INCLUDES SPARKS FROM STATIC ELECTRICITY.  Do not return to the home until the fire department and/or gas company instruct you to do so.