Too much preassure in Bosch boiler?

I have the problem with my central heating displaying 3 bar preasure, when I turn a tap on in the house (hot or cold) the preasure drops, the filling loop is disconnected. Also when I turn the stop cock off the preasure drops to zero if I turn a tap on. I'm guessing theres a leak between the radiators and the hot water system. Could this be the heat exchanger?