Image credit: Honeywell International, Inc.Image credit: Honeywell International, Inc.

Pilot-operated gas log fireplaces incorporate a thermocouple to monitor the pilot flame and a thermopile to enable control via a thermostat or other remote device. The thermocouple senses the gas pilot flame and prevents flow to the main burners when there is inadequate heat to ignite the burner. The thermopile powers a second circuit that operates the main valve. A brief understanding of how the thermocouple and thermopile work and what can go wrong will help troubleshoot a valve that is malfunctioning.

Thermocouple Operation

Thermocouples are passive sensors that generate an electrical current when the junction in an open circuit is exposed to a thermal gradient. The heat from the pilot flame is sensed by the thermocouple and the induced electrical charge actuates an electromagnet inside the gas valve that allows flow to the main burners.

A gas fireplace can fail for a number of reasons, but, typically, if the pilot light ignites and will not stay lit, there is an issue with the thermocouple. In some cases, loose connections or corrosion cause a thermocouple to fail, although it is much more common that the junction is covered with carbon soot and either needs to be cleaned or replaced.

Thermopile Operation

The thermopile operates similarly to a thermocouple generating a voltage when exposed to the pilot flame that ranges from 325 to 850 mV DC. When the main valve is activated, there is a drop in potential equal to approximately half of the full line voltage or a minimum of 110 mV.

If the pilot light stays on and the main burner fails to operate, there may be an issue with the thermopile. A digital multimeter can be used to check the voltage in the presence of a pilot flame as well as test for a drop in potential when the main valve is activated. This can be done by locating the red and white wires connected to the TP/TH and TP terminals on the main control valve.

If the thermopile is generating less than 325 mV, the sensor may need to be cleaned or replaced. A considerable drop in potential when the main valve is activated is another sign that the thermopile needs replacement. If there is no change in potential when the main valve is activated, then there is a fault or an interrupter may need to be reset. A very small drop in potential may be a sign that the main control valve needs replacement.

The Main Control Valve

A digital multimeter can also be used to test the main control valve. The main gas control valve is typically not serviceable and if its coil resistance or the associated safety magnet coil resistance fall outside of a specified range, the valve needs replacement.

To test the coil’s resistance, remove the thermocouple and thermopile contacts and set the multimeter to an Ohm’s (Ω) setting. Depending on the make and model, the resistance across the TP and TH contacts should fall within a specified range, 1.75 to 2.75Ω for SIT CONTROLS USA, Inc. SIT 820 Series Nova mV valve or 3.1 to 3.6Ω for Honeywell’s VS8420 Millivolt Gas Valve. The nominal resistance is typically disclosed on the manufacturer’s operators or troubleshooting guide for different makes or models.

The safety magnet is located on the back side of the valve and the valve may need to be removed to test its resistance. To test the magnet's resistance, place one lead on the soldered connection and the other to the ground. The resistance should measure between 0 and 0.2Ω for SIT’s SIT 820 Series Nova millivolt fireplace valve. Again, if values fall outside this range, the valve needs replacement.

It is strongly encouraged that a certified technician is called to maintenance a faulty gas fireplace control valve. The above illustration serves as a rough guide that can help one understand how the control valve works and how to troubleshoot if a gas fireplace fails to ignite. It is also worth noting that faulty wiring, wall switches, clogged pilot orifices or burner nozzles may cause similar issues.


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