i have a baxi 105e combi and i cant get hot water unless i turn the tap on 1st then turn the mains off and then back on to the boiler it fires up and works fine but as soon as you turn the tap off again it does the same no hot water the light for low presure or pump problem flasshes at this point and doing the above turning tap on etc is the only way it fires up ,i have changed the diaphram etc and checked the expansion vessel and recharged the system to just over 1 bar and it holds its preesure no problem has anybody got an idea what the problem is