Thermocouples are used on residential gas furnaces equipped with standing pilots. A standing pilot is one which burns continuously, whether the main burner is on or off. A thermocouple standing pilot is only used with a 100% shutoff main gas valve. This type of gas valve shuts off gas to the pilot and will not open its main valve if there is insufficient pilot flame to ignite the main burners or if the thermocouple is defective.
The thermocouple (Figure SP -1 1-1) is a safety control device that senses the presence of the pilot flame by generating a small DC voltage (about 30 millivolts) when subjected to the heat of the pilot flame.
When the pilot is lit, the thermocouple develops the electrical signal that causes a small current to flow through the solenoid in the gas valve, energizing the solenoid. The energized solenoid holds the safety gas valve open, allowing gas to flow through the main gas valve to the pilot.
If the pilot flame goes out, the thermocouple cools and the voltage generated drops to zero. The current stops flowing in the solenoid, causing the safety valve to close, shutting off gas flow to the pilot and main gas valve. If the problem is suspected to be the thermocouple, visually check the pilot flame and the thermocouple for the conditions listed below before testing or replacing the thermocouple. Check for:
• Yellow pilot flame
• Pilot flame too small
• Pilot flame misdirected
• Pilot flame too low
• Pilot flame flickering
• Pilot flame floating
• Pilot flame lifting
• Thermocouple bent
• Thermocouple lead kinked
• Thermocouple insulator damaged
• Thermocouple end contact dirty
The thermocouple can be tested either under load or no-load conditions using a VOM/DMM. The procedure for both methods is provided in the detailed procedure given at the end of this section and is briefly described here.
To test the thermocouple under loaded conditions requires the use of a test adapter that allows voltage readings to be made while the thermocouple is connected to the gas valve. First, disconnect the thermocouple from the gas valve. Screw the thermocouple test adapter into the gas valve, then screw the thermocouple into the adapter (Figure SP-11-2). Connect a VOM/DMM set up on the lowest DC millivolt range to the adapter and thermocouple. Connect one meter lead to the adapter and the other lead to the thermocouple copper sheathing tube. Light the pilot by manually depressing and holding down the valve knob or reset button to keep gas flowing to the pilot during the test. After the pilot has been lit for five or more minutes, the reading on the VOM/DMM should be 9 millivolts or higher. This indicates that the thermocouple is good. If the reading is less than 9 millivolts, the thermocouple is defective and must be replaced.
No adapter is required to perform the thermocouple no-load test. First, the thermocouple is disconnected from the gas valve. Then the VOM/DMM is connected to the thermocouple. One meter lead is connected to the extreme end of the thermocouple lead and the other meter lead to the copper sheathing tube. Light the pilot by manually depressing and holding down the valve knob or reset button to keep gas flowing to the pilot during he test. After the pilot has been lit for five minutes, the reading on the VOM/DMM should be 18 millivolts or higher. This indicates that the thermocouple is probably good. If a reading of 18 millivolts or higher is not obtained, the thermocouple is defective and must be replaced.