Some additional information :
As requested, I have attached a picture
The
water heater unit is a 1200W/220VAC unit, so the 4 amp rating is clearly too low to put in series with the
heater element.
on top of the
heating element, there's also an
engine coolant input/output, so it is expected that the tank can contain
water as hot as 90° in normal operation, so I don't think the contact thermostat is an
alarm mechanism (or it would trigger while underway under power)
The
heating element is controlled by a screw and can be set between 40°C and 80°C, so I don't think the contact thermostat is controlling the heating element in any way.
My guess is that it's just an indicator that can be connected to a LED so that I know that I have some hot water in the tank if I want to take a
shower ... but obviously I'm no expert ...
Thanks a lot to all of you for your helpful messages!