the old one is a BI-METAL SPRING that unwinds as it gets warmer.
as it unwinds, the end moves away from it's contact, opening the switch.
As it cools, the wound BI-METAL spring winds until the tip makes contact, closing the switch. It requires absolutely no power to operate.
kinda like a light switch on the wall only in place of your finger, it uses a bi-metal coil to move the switch up or down... so weather it's 1V or 1000v it's just a switch and is only limited by the quality and composition of the contacts . Some times they even use a small capsule of mercury to make the contact as the coil winds or unwinds.