think lame is saying that any water left in dhw side will expand in the heatex from the ch side as its dualpass and this could cause probs unless a tap is opened, as op says hot tap is stuck open there will be no probs imho
I still think hes sprouting rubbish as its usually full of water at mains pressure so I cant see the problem the boiler has an overheat which will stop it boiling and the heat exchanger is designed to with stand the pressure of mains water.