If two controllers are wired in series, then both have to be “on” for the heater to be powered. if either controller is above the set temperature then it will go “off”. The tank will be maintained at the lowest set point.
If one of the controllers stays permanently ”on” regardless of the temperature reached or set, then the second one maintains the tank at its set point. Providing the highest set point is not too high, the tank is saved.
If one of the controllers stays permanently ”on” regardless of the temperature reached or set, then the second one maintains the tank at its set point. Providing the highest set point is not too high, the tank is saved.