Delayed pump overrun.

[sym]

Any ideas why there would be a delay in my pump overrun. the delay is about 5 seconds and causes residual heat in the boiler to boil the water which results in loud knocking.

 

[gasmarc]

a bit more information would be helpful like make of boiler etc

 

[sym]

It's an old Glow Worm 40BR Mk 2. Would there be a 'stat to control the overrun? It seams that once the water boils the pump cuts in.