Just got a new shower installed. The water is cold. Unless I turn off the cold water feed. Is this safe?

Even then its only just warm enough on full. The boiler (ocean alfa 240x)fires up with out problem. When i run the taps it produes the hottest heat at 3/4 turn, runs tepid on full.

Any ideas? plumber has mentioned a diaphram/sensor?
cheers

jim balls, February 2009