Thermocouple on a standing pilot or constant flame

The thermocouple lead is probably a coax cable. The outer shell is automatically grounded when installed. The thermocouple itself is a short where two dissimilar wires are welded to each other. It is only when that junction is heated that there will be a deviation seen from what you can expect from a short circuit.

Bill

There are several possibilities, including that the wire is coated in clear varnish and the sensor is a thermopile:

-- Sue

Are you certain that its a copper wire? It might be a capillary tube, sealed at the flame end. Heat it and the internal pressure increases, which pushes on a diaphragm in the gas valve.

I doubt that it is a thermocouple.(unless the boiler is new and has an electronic controller) Otherwise you'd lose your pilot light on every power failure (elec). It is probably a thermal switch, (A capillary with a small "bulb" on either end and filled with liquid. Apparently the liquid may be mercury, but I tend to doubt that is common. One of the bulbs gets heated by the pilot flame. The other end is specially shaped (bent) will change its shape somewhat as the pressure of the liquid increases. (Apparenttly since there is a little more surface area on the outside of the bend than on the inside it tends to flatten the bend a little. That can open or close the switch contact - if they arre properly placed.

-- See W a G for all you ever wanted to know about engineering .

There are millivolt solenoids that run off the output of a thermocouple. No external power source needed.

It seems that the OP has traced the system out sufficiently to rule out the capillary type.

I spent a couple of hours yesterday getting my furnace to run again. It turned out there was indeed a thermocouple which energized a solenoid which kept a spring loaded switch from opening. The solenoid only had to hold the armature which was manually held while lighting the pilot. This switch was in series with the thermostat and gas valve energized by a 28V transformer. The intent was to not allow the gas valve to open if the pilot went out and the thermocouple released the armature which opened the switch.

To my relief it turned out the series switch had dirty contacts and was not conducting even when not held open! Cleaning the contacts was much easier than replacing the thermocouple or gas valve. I suspect that if I'd called a serviceman, he'd say I need a complete gas valve and thermocouple! Nobody repairs anything anymore; It's just R&R, (remove and replace).

It is common to have a gas valve that includes a safety valve that is held open by a solenoid operated by a milivolt thermocouple. You push a button to manually open the safety valve until the thermocouple heats from a pilot flame, then the solenoid holds the safety valve open. The main burner valve probably can't open while the reset button is pushed. Water heaters may use them. Also a safety valve in a furnace gas control valve - there is also an operating valve that is run by the thermostat and 24VAC. Probably only used in standing pilot applications.