Paul. If 7.5kW of heat is being produced in the unit and it's not coming out of the shower head, the only other route is to back up heat into the cold water supply, either as high-pressure steam bubbles or pressurised hot water. It's probably scaled-up between the element bottle and the outlet, causing restricted flow. Is this shower connected to a gravity cold water tank in the loft as the cold supply, the traditional bathroom feed in Britain?
I would not turn it on again till it has been dismantled and checked for blockages in the internal waterways.
Alan
