At my cottage I have a new NG water heater. It's a standard vent model with standing pilot but has the enclosed base now required. My question is how does the gas valve operate with no external power source? It even has a flashing LED to indicate the pilot is lit.

I don't understand how the few mv generated by a thermocouple can open the gas valve, much less light an LED.

 

Although a thermocouple doesn't generate much voltage with a single junction, it can generate a fair amount of current. The current is enough to operate a sensitive electromagnet to open the gas valve. It is possible to use an electronic inverter circuit to generate a high enough voltage to operate an LED.

Alternately, a thermopile can be used, which has many thermocouples in series in order to generate a higher voltage.

Not sure which approach your particular WH uses.

 

It can be completely mechanical. A tubular probe extends inside the tank to sense the water temperature. The tube is filled with heat expanding liquid like a thermometer. But instead of having the liquid go up a glass tube extension with a numbered scale behind, the liquid pushes a diaphragm or lever that opens or closes the gas valve (or a spring could be used to push the valve the other way).

This technique is used in a variety of other applications, too, such as the zone valve for a heating system loop or the "thermostat" in a car engine that delays the flow of coolant to the radiator until after the engine reaches a certain temperature.

 

Thanks Paul. I didn't know (or had forgotten) that a thermopile was several junctions in series.
I've never peeked in the "chamber" to see what might be in the flame. It's rather difficult with these new enclosed burners.