I am just a little confused.
I have just carried out a test on a spare thermostat, lowering it into a kettle of water, bringing the water to the boil while observing the action.
Just before the water boiled, the main valve fully opened, but the sprung loaded circular plate on the other end did not move at all.
Does anyone know if this plate is designed to open with heat, or does it work on water flow ?...
Cheers