A properly functioning thermocouple, when heated, should typically generate 15 to 35 millivolts (mV).
Understanding Thermocouple Millivolts
The millivolt output of a thermocouple is a crucial indicator of its performance, especially in gas appliances where it serves as a safety device. It generates a small electrical current when heated by the pilot flame, which then energizes a safety valve to keep the gas flowing.
- Optimal Range: A healthy thermocouple consistently produces between 15 and 35 mV when subjected to the heat of a pilot light or burner. This range ensures that enough voltage is supplied to hold the gas valve open, allowing the appliance to operate safely.
- Below Optimal: If the output falls below 15 mV, it indicates that the thermocouple is not generating enough power. This can lead to the gas valve closing, causing the pilot light to extinguish or preventing the main burner from igniting.
Why Millivolts Matter for Appliance Safety
The millivolt output is directly tied to the safety and reliability of appliances that use thermocouples, such as water heaters, furnaces, and gas fireplaces. A strong millivolt signal ensures that the gas supply remains open only when a flame is present, preventing hazardous gas leaks.
- Pilot and Ignition Problems: Low millivolt readings are a common cause of issues like:
- Pilot light not staying lit.
- Main burner failing to ignite.
- Intermittent operation of the appliance.
Troubleshooting and Replacement
Testing the millivolt output is a simple and effective way to diagnose if a thermocouple is the root cause of pilot or ignition problems.
| Millivolt Reading | Indication | Recommended Action |
|---|---|---|
| 15-35 mV | Functioning properly | No action needed for thermocouple |
| Below 15 mV | Insufficient voltage; thermocouple is failing | Replace the thermocouple |
| 0 mV | Complete failure or broken thermocouple | Replace the thermocouple |
If a thermocouple is found to be generating less than 15 millivolts when heated, it has likely degraded and must be replaced to restore proper and safe operation of the appliance. Replacing a faulty thermocouple is often a straightforward solution for resolving many common pilot and ignition issues.
