overpressurised combi boiler, plumber says new boiler is needed, is it ?

All depends, on the face of it, it would seem unlikely that you would need a new boiler.

Leaving 'the taps open' (I'm assuming you mean the filling valves) would merely cause the safety valve to discharge the excess pressure in the boiler & system to the outside.

If the boiler was on during this time some damage may have been done to the boiler...but the fact that you can still get hot water would indicate that it is unlikely to need much more than a spare part or two.

Very difficult to be precise without seeing the boiler..I'm afraid.

jondeau
January 2011