Common mechanical issues on gas furnaces typically involve the thermocouple and limit switch. The thermocouple is a gas furnace component located near the pilot light burner. It is a safety device that shuts off the gas in the event that the pilot light goes out or the electric igniter fails.
If the pilot light keeps going out, you may have a faulty thermocouple on your hands. It will need to be adjusted or replaced. To adjust the thermocouple, tighten the nut with a wrench. Take care not to apply too much pressure, just tighten it slightly. Then try lighting the pilot. If the pilot still won’t stay lit, replace the thermocouple with a new one of the same type.
The limit switch is for safety and you will find it just below the plenum on the unit. When the plenum gets too hot, the switch is triggered and shuts off the burner. It will also shut off the blower after the burner is off and the temperature drops below a set level. If your blower runs constantly or is sending out cold air, something has been set to ON – either the blower control or the thermostat. 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 to be adjusted.
Adjusting the switch is fairly simple. Remove the cover for the control and you’ll see a dial. One side is “Limit” the other is “Fan.” You will be working with the fan side; do not touch the limit side. 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 46-degrees Celsius and the lower one at about 32-degrees Celsius.
You may also need to make a burner adjustment from time to time. The flames on the gas burner should be strong and regular – no fluttering or sputtering. To adjust the flame height on the main burners, call a professional. To adjust the height of the pilot flame, turn the flame adjustment screw until the flame is from 3.5 to 5 centimeters high. The adjustment screw is located near the gas valve on the pilot assembly, if the control has this adjustment feature.
If you feel that you may have a gas leak around the furnace unit, create a mixture using 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 over tighten the connection. If the pipe connections or valves still leak, call a professional immediately.