v. noisy cylinder and over hot radiators

[PeteB]

Yesterday, with HW & CH on, hot water cylinder suddenly making a huge racket with banging, hissing and vibrations and generally sounding quite scary! Radiators were also extremely hot. Went to switch CH & HW off at timeswitch but boiler and boiler burner kept running -ended up switching boiler off altogether. Have recently had radiator off in one room to re-decorate, now replaced. Banging has happened once before and thought it was just air in system - checked header tank and ball valve was stuck so tank empty. Released valve, tank filled and system has been okay for a couple of weeks. What could be causing this overheating (if that is what it is?). Air in the system or faulty boiler thermostat or something else? Any advice gratefully received - it's pretty cold and my wife is due to give birth any time now!

 

[oilman]

It sounds like lack of water, have you checked the header tank?

 

[PeteB]

Header tank is full so not sure it is that unless there is some sort of blockage (sludged up??).
Strangest thing is that once the boiler fires up, it seems very reluctant to switch off, even if the HW & CH is switched off at the controller and the boiler thermostat turned to off. Last night, had to switch off at grey gas valve to stop it going nuclear again! This is why I was thinking boiler thermostat is on the blink but I can't understand why it does not switch off from the timeswitch control?
Anyway, got a plumber coming round today to have a look so hoping he may be able to shed some light on the problem. Thanks for the response.

 

[PeteB]

Further to last message, plumber has diagnosed failed gas valve which is either not opening or if it does, not shutting off. This is why burners keep going after all else is switched off. As boiler is about 20 yrs old, have opted for a new one. Hopefully, a trouble free heating system will result...!