::Thermopile is the little bi-metal sensor that is in the pilot flame that has a thin “tube” running back to the gas valve. If there is a lit pilot, and it is functioning proper, it expands and allows the gas valve to open when the thermostat or other main gas valve control (switch) is calling for “heat”. Older technology and is used where there is a standing pilot to insure the main gas valve is NOT opened with out there being a source of ignition. For me it has been cheap and easy to replace when I have had an ignition issue with the old H2O heater at M.L.
Bladepuller