Hot Water in Expansion Tank

Hi, recently my system has run out of domestic hot water due to no water in the expansion tank. Having refilled the expansion tank to get the systems going again I have noticed that the water in the tank is VERY hot ............ almost boiling. Is this right ???

As long as the water in the tank doesn't start boiling and the airing cupboard becomes full of steam, that is fine. Make sure the header thank is full half way.