Replacing a Water Heater Thermocouple or Flame Sensor
Understanding Thermocouples and Flame Sensors
A thermocouple or flame sensor is a device that plays a crucial role in controlling gas flow in gas water heaters. In older heaters with standing pilot lights, the device is typically called a thermocouple, while in newer units with electronic ignition, the same device is often referred to as a flame sensor. For simplicity, we’ll refer to both devices as thermocouples throughout this article.
Thermocouples are an integral part of the pilot burner assembly and connect to the gas control valve on the exterior of the water heater. They function by converting the heat from a burning flame into a small electrical current, which acts as a switch to regulate the flow of gas from the gas supply valve. In essence, when the thermocouple does not detect a flame, it prevents gas from reaching the burner, making it a key safety feature of your water heater system.
Common Issue: Faulty Thermocouple
A malfunctioning thermocouple is often the culprit behind a water heater pilot that fails to ignite or a burner that struggles to stay lit.
Types of Ignition Systems and Thermocouple Replacement
The type of ignition system in your water heater will determine the specific steps involved in replacing the thermocouple.
Standing Pilot Water Heaters:
- With a standing pilot system, the thermocouple is typically found attached to the burner assembly along with the pilot.
- To replace the thermocouple, disconnect the gas control valve connections and lift the burner assembly out of the water heater.
- Remove the old thermocouple by pulling and twisting it out of the bracket, insert the new thermocouple, and reconnect the burner assembly and gas control valve.
Electronic Ignition Water Heaters:
- Electronic ignition systems incorporate both a pilot and a thermocouple, as well as an electronic igniter.
- To replace the thermocouple in this case, remove the burner assembly manifold cover, disconnect the gas control valve connections, and pull the burner assembly out of the combustion chamber.
- Replace the old thermocouple by pulling and twisting it out of its mounting bracket and insert the new thermocouple. Reinstall the burner assembly and gas control valve connections.
Reconnecting the Gas Control Valve
After replacing the thermocouple, reconnect the pilot, burner, and thermocouple (and electronic igniter for electronic ignition units) to the gas control valve.
- Hand-tighten the mounting nut on the thermocouple no more than a quarter turn beyond hand-tight.
- Open the shutoff valve on the gas supply piping and turn the gas control valve to the ON position.
- For standing pilot units, relight the pilot according to the manufacturer’s instructions before setting the valve to ON.
Testing the Water Heater
- Apply a soap-and-water solution to the main supply tube and pilot supply tube at the gas control valve to check for gas leaks.
- Inspect for bubbles, which indicate a leak. Tighten all gas connections and retest until no leaks are detected.
Tips for Replacing a Water Heater Thermocouple
- Use caution when removing and installing the thermocouple to avoid damaging the connections or the thermocouple itself.
- Take the old thermocouple to a hardware store to ensure an exact replacement.
- Consider replacing the manifold cover plate gasket if it’s in poor condition.
- If you’re not comfortable working with gas appliances, it’s advisable to seek professional assistance.
