Just in case somebody else has one, it is typically placed externally on the tank and monitors for overheat situations (your hot air furnace has the same style unit), yours was set up to break on temp rise, so it breaks the connection when you go into overheat, the snap discs on the hot air furnaces that many of to have are set up to make on temp rise, this is why, if your rad is placed to close to the snap disc and you have water flowing through the rad all the time, your blower will come on without a call for heat! I have had the ones you use fail in the cold weather and had to bypass them to get the boiler to come back on, it is done by disconnecting the power supply then pulling the wires off the terminals and tying them both together, Please do not leave them jumped out as it is a safety switch and should only be jumped out in an emergency only.