Boiler Overheating On Shut-off

[Pot-Man]

I have recently drained down the system to replace all TRV's. I have only installed new TRV's where there were old ones, no extra ones have been added.
Now, when the heating has been running for a while, 30 mins or so, the boiler is overheating and activating the over temperature cut out. This always happens just after the normal boiler stat has activated to switch off the main burner and flue fan. The water can then be heard to boil and shortly afterwards the over temp button pops up.
This is happening even when I set the boiler stat to the lowest setting. I have tested the boiler stat and it is OK. The pump is also functioning correctly and continues to run whilst the burner is off to draw heat from the exchanger.
The system has been thoroughly bled and performs well in all other aspects. There is an air separator in the system which I suppose could be blocked. Could this be causing the problem?
I have tried changing pump speeds and bleeding the boiler but the problem persists.
The boiler is a Glow Worm Space Saver KFB 50.
Any ideas would be welcome.

Thanks

 

[wilhelm]

do a search for "air lock" or "blocked"
thatll keep ya busy till monday.

 

[Angi]

I am now having the exact same problem as you had with the same boiler! Did you manage to fix the problem or find out why it keeps shutting off??

 

[ricarbo]

potman, it sounds like the flow stops when the trv's shut, so the boiler can't get rid of the heat. Is there a bypass, and can you open it up a bit? You should be able to tell if there's a flow through it by the temperature of the pipes each side of it.
I don't think the air separator is implicated.

 

[HarrogateGas]

The pump over run stat may be faulty..........does the pump continue to run when the boiler shuts off?

 

[kevplumb]

if it does it's been running since last march