Squirrel C 1 series wall-hung boilers can still work normally when the thermostat is closed.

It depends on whether the thermostat you install is used for temperature measurement or control. If it is used for temperature measurement, it can still work when the power is off. If it's for control, it won't work. This control is usually controlled by testing the relay output function of the thermostat. If you cut off the power supply, the main circuit will be disconnected.