pressure fine when you just need hot water, when you switch t
the

heating on pressure goes past 3 bar and out the overlow make optima

rob, June 2008

Sounds like the expansion vessel has lost it's precharge or has failed completely....you need the reduce the system pressure to zero on the gauge (usually by opening the safety vlave) and check the vessel pressure using a tyre pressure gauge....

Dave The Boiler Man, June 2008