glow worm 60FF boiling

[jooles]

Our glow worm gas boiler, when it has been running for an hour or so, every so often makes an enormous rattling noise in the pipes above the boiler that sounds like boiling. I think it is also spurting water out of the header tank overflow. Is this cavitation and what can we do about it?
Thanks

 

[Puddle]

It could be,if only just started happenng
First thing to try is turning boiler thermostat down as probably out of calibration,if works replace thermostate
Also if system dirty could be a restriction build up just before pump impellor,usually in pump casing or isolation valve in which case we could be into a system clean

 

[jooles]

Many thanks Puddle - will give it a go

 

[jooles]

Puddle - thanks for your advice: turning the thermostat down has done the trick. So how do we work out whether it is the thermostat that needs replacing or dirty in the pump impellor? Is replacing the thermostat expensive?