boiler problem

[Foofs]

i have a Baxi Combi 80 Eco boiler and every time i put on the centre heating the pressure go up past 3 bar but all the rad get hot,but when i turn the heating off the pressure on the boiler has now gone to 0 because all the water has gone out the pressure valve so i have to refill the boiler. Can anyone please tell me what is wrong is it the heat exchanger or the air vent or some thing else. Thank You

 

[corgiman]

expansion vessel is kaput

do a wiki or a search on it

 

[baxpoti]

Re-charge the pressure vessel to 1bar (13psi) when the system pressure is on zero. Re-pressurise the system to 1 bar. Re-try the c/h.