new worcester 550I combi boler?

When the hot tap is opened fully the temp of the water increases until the boiler trips out .Worcester service engineer attende twice and could not sort the problem .On his first visit said it was our cold water stop cock damage.Had plumber check it out 20 l/m and 2.3 bar water pressue .Next visit he blamed the plumbers instalation of the central heating and DHW .He also pointed out that we would have problems in the summer when the ch was off that we would have no DHW .Had a convential system for 40 years with out any problems This new Combi boier system is a complete disaster .