To service a gas furnace and valve, turn off the power and gas, examine the filter and blower, clean the burners, and run safety trials. The required supplies include gloves, safety goggles, a flashlight, a cloth, a thin wire brush, water, a stopwatch and a carbon monoxide meter.
Inspect the furnace, ducts and vents for damage, leaks and other problems. Turn off the furnace, disconnect the electricity, and turn off the gas. Open the service panel, and expose the filter. Replace the filter if it is clogged, aiming the arrow printed on its side in the same direction as the furnace airflow.
Remove the burners, and clean them with water. Use a thin wire brush to clean the pilot light and heat sensor. Dry and replace the burners, close the service panel, and reconnect the electrical supply. Open the gas valve, and press the "ignite" button to start the furnace.
Locate the furnace gas dials, and use a stopwatch to determine how long it takes the needle on the smallest dial to rotate 360 degrees. Consult the furnace owner's manual to compare the usage rate with the manufacturer's recommendations. Use a carbon monoxide meter to evaluate the gases in the flue. Refer to the furnace manual for acceptable oxygen and carbon monoxide levels.