The water doesn't boil at 108C because the coolant system is under pressure. The higher the pressure the higher the temperature can be without the water boiling. The expansion tank water will be at the same pressure as the rest of the coolant system, but it will be cooler because it does not form part of the circulation system, so I am not surprised it wasn't boiling.And ? i don't inderstand.
I opened the water tank and the water was not boiling.
There was even pressure on the water tank cap.