Natural gas and propane burn cleaner than fuel oil, and most gas furnaces present fewer operational difficulties than oil burners do. In fact, the problems that affect gas furnaces typically involve the furnace’s thermocouple, the pilot light, or some component of the electrical system.
Gas furnaces and heaters have control shutoffs to prevent gas leaks, but they are not fail-safe. If you smell gas in your house, do not turn any lights on or off, and do not try to shut off the gas leading to the furnace. Get out of the house, leaving the door open, and immediately call the gas company or the fire department to report a leak. Do not re-enter your home.On some gas furnaces and heaters, a plug-type door covers the pilot light assembly. To gain access to the pilot burner, pull the door out of the furnace housing. On other units, remove the panel that covers the pilot and gas burners.
The pilot light controls, reset buttons, gas valves, and thermocouple are usually contained in an assembly at the front of the furnace. The furnace limit switch is located on the plenum (main chamber) or main duct junction on the upper housing of the furnace.
What You’ll Need
You’ll want to have these tools on hand to check the pilot light:
* Matches
* Fine wire
The Pilot Light
The pilot light on a gas furnace can go out because of drafts. To relight the pilot, follow the manufacturer’s instructions exactly; they are usually fastened to the furnace. If instructions for relighting the pilot are not provided, follow this general procedure:
Step 1: Find pilot light assembly. It typically has a gas valve with on, off, and pilot settings.
Step 2: Turn valve to off and wait three minutes.
Step 3: Switch valve to pilot setting. Hold lighted match to pilot opening while you push reset button on pilot control panel. Keep this button depressed until pilot flame burns brightly, then set valve to on position.
Step 4: If pilot flame won’t stay lit, opening may be clogged. Turn gas valve off, and clean opening with piece of fine wire. If it won’t stay lit after several attempts, you may have faulty thermocouple. If pilot flame still won’t stay lit, call professional service person.
Some furnaces have an electrical system to ignite the gas; in these systems there is no pilot light. Instead, an electric element heats up and ignites the burners. If this electric ignition system malfunctions, call a professional service person.
Replacing the Thermocouple
The thermocouple is a gas furnace component located near the pilot light burner. It is a safety device that shuts off the gas if the pilot light goes out or the electric igniter fails.
If the pilot light won’t stay lit, the thermocouple may be faulty and should be adjusted or replaced. To adjust the thermocouple, you must tighten the thermocouple nut with a wrench. Take care not to apply too much pressure to the nut — just tighten it slightly. Then try lighting the pilot. If the pilot won’t stay lit, replace the thermocouple with a new one of the same type. Here’s how to replace a thermocouple:
What You’ll Need
You’ll want to have these tools on hand to replace a thermocouple:
* Wrench
* Replacement thermocouple
Step 1: Unscrew copper lead and connection nut inside threaded connection to gas line. Under mounting bracket at thermocouple tube, unscrew bracket nut that holds tube in place.
Step 2: Insert new thermocouple into hole in bracket. Be sure steel tube is up and copper lead is down. Under bracket, screw bracket nut over tube. Push connection nut to threaded connection where copper lead connects to gas line. Make sure connection is clean and dry.
Step 3: Tightly screw nut into place, but do not over-tighten. Both bracket nut and connection nut should be only a little tighter than if hand-tightened.
The thermocouple is installed next to the pilot light on a gas furnace.
© 2006 Publications International, Ltd.
The thermocouple is installed next to the pilot light. A bracket holds
it in place, steel tube up and copper lead down.
Limit Switch
The limit switch is a safety control switch located on the furnace just below the plenum. If the plenum gets too hot, the limit switch shuts off the burner. It also shuts off the blower when the temperature drops to a certain level after the burner has shut off. If the blower runs continuously, either the blower control on the thermostat has been set to the ON position or the limit control switch needs adjustment. Check the thermostat first. If the blower control has been set to ON, change it to AUTO; if the blower control is already on AUTO, the limit switch needs adjusting.
Gas Furnace
Care Guide
There are a myriad of problems that can cause your gas furnace to malfunction. Click here for instructions on how to deal with specific issues.
To adjust the switch, remove the control’s cover. Under it is a toothed dial with one side marked LIMIT; don’t touch this side. The other side of the control is marked FAN. There are two pointers on the fan side; the blower goes on at the upper pointer setting and turns off at the lower pointer setting. The pointers should be set about 25 degrees apart. Set the upper pointer at about 115 degrees Fahrenheit and the lower one at about 90 degrees Fahrenheit.
Burner Adjustment
The flames on the gas burner should be full and steady, with no sputtering and no trace of yellow. To adjust the flame height on the main burners, call a professional service person. To adjust the height of the pilot flame, turn the flame adjustment screw until the flame is from 11/2 to 2 inches high. The adjustment screw is located near the gas valve on the pilot assembly, if the control has this adjustment feature.
Gas Leaks
If you suspect leaks around the furnace unit, stir up a mixture of liquid detergent and water. Paint this mixture on the gas supply line along its connections and valves; the soapy water will bubble at any point where there’s a leak. If you find a leak, try tightening the leaking connection with a pipe wrench, but be careful not to overtighten the connection. If the pipe connections or valves still leak, call a professional service person.
An oil furnace has an entire new set of the problems from the ones we’ve just conquered in gas furnaces.
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