Natural gas burns
cleaner than oil, and most gas furnaces present fewer operational
difficulties than oil furnaces do. The problems that affect gas furnaces usually
involve the furnace thermocouple, the pilot light, or some other partt of the
electrical system.
Gas furnace and gas heaters have control shutoffs to prevent gas leaks, but they
are not all 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 call the gas company or the fire
department right away Do not re-enter your home.
On some
gas furnaces and gas 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 or main duct on the upper housing of
the furnace.
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. Also see
thermocouple circuit.
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:
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.
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.
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.
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