what can it be?

Heating boilers have a water pump associated with them to move the heated water around the system - if the pump is not operating the water in the heat exchanger just gets hotter and hotter until it really does boil - this is not a good situation. There is often an overheat safety thermostat to prevent an explosion - I am not familiar with your boiler - sounds more like a steam engine.

Good luck...

Peccavi
December 2009