Unknown Fault ??

[problem]

Halstead quattro gold combi boiler sealed system - A progressively worse problem......

Cold water pressure set at 1 bar, after the boiler reaches 1.25 bar it starts to bang loudly and the gas cuts out immediately and the boiler does nothing for about 45 seconds - 1min it then starts up again and goes thru the same cycle again and again. Consequently it never reches above 1.25 bar as it keeps cutting off the gas, it used to modulate the gas and reach 2 bar +

Can anyone help in identifying the fault ? Thanks

Also, just before it cuts out the pressure gauge jumps wildly to approx. 2.0 bar and then drops back immediately to 1.25 bar as it switches the gas off.

 

[oilman]

does the pump run?

 

[problem]

pump runs - i can feel it vibrate + hear when listen closely - how do i check if it is efficient/ powerful enough ?

Thanks