Trace a thermoelectric safety circuit, distinguish the two generators and interpret voltage checks with and without a load.
Heat produces the signal
A thermocouple has junctions made from different metals. When the sensing end is heated by the pilot and the other junction remains cooler, the temperature difference produces a small direct voltage. The output is measured in millivolts (mV). It is generated by the thermocouple itself, rather than supplied by the mains.
The flame must heat the sensor as the appliance instructions specify. The correct pilot shape and sensor position matter, but a visible pilot alone does not prove that the complete safety circuit works.
Two generators, two jobs
Some standing-pilot appliances have both a thermocouple and a thermopile. They look towards the same flame but serve separate electrical circuits:
- The thermocouple supplies the safety magnet. Its holding current allows the flame-supervision valve to remain open after the lighting control is released.
- The thermopile contains several thermocouples in series. Their voltages add, providing enough output for a suitable main gas-valve operator. The room thermostat can switch this demand circuit.
This arrangement is used by the SIT 820 NOVA mV. Its gas control can operate from pilot-generated electricity without an external mains supply. That does not make every appliance using it independent of electricity: other equipment, such as a fan or pump, may still need mains power.
Do not substitute a thermopile for a thermocouple, or vice versa, just because both are heated by a flame. Follow the exact appliance diagram and approved parts list.
Follow the complete holding circuit
A safety cut-out can be connected in series with the thermoelectric magnet. When that contact opens, the circuit is broken, the magnet loses its holding current and the gas shut-off releases. The thermocouple may still be hot; the open cut-out prevents its current reaching the magnet.
A poor connection can cause a different problem. Oxidation or looseness adds resistance to a circuit with only a small available voltage. The resulting current may be too low to hold the magnet. Inspect and test the specified circuit rather than replacing the sensor solely because the flame goes out on release.
The screw-in connection from a thermocouple lead to its safety magnet is an electrical contact. It is not a pilot-gas union. Keep the contact clean, use the correct fitting and follow its tightening instruction. Sealing material on the contact can interfere with the circuit. The separate gas connections still require their prescribed tightness checks.
Never leave a safety cut-out bridged to keep an appliance running. Diagnose the reason for its operation and restore the complete protection before return to use.
Unloaded and loaded voltage
An open-circuit measurement is taken without the normal load connected. A connected-circuit measurement checks the output while the operating circuit draws current. These are different tests, with different acceptance limits.
For the SIT 820 NOVA mV, the stated supply requirements are:
- Open circuit: at least 370 mV.
- Circuit connected: at least 145 mV.
For example, 520 mV without the load meets the first requirement. If that output falls to 120 mV with the circuit connected, it fails the second. The good unloaded value cannot cancel the failed loaded check.
Those two readings do not by themselves identify a defective thermopile, valve or connection. Follow the appliance procedure, including the required pilot conditions and test connections, to establish the cause. Use the figures for the actual control; there is no single millivolt pass value for every flame-supervision system.