DIY Doctor UK DIY Forums

I have a Kingfisher MF boiler, conventional flue, HW and CH zone valves on a sealed system.
The boiler goes onto lockout condition every now and then. During that time pump still runs and reset button needs pressing before system will re-start. The problem appeared to be pretty random but now seems to be getting more frequent. According to the manual only the overheat thermostat or the TTB can put it into lockout.
So far I have replaced the PCB and the overheat thermostat but problem remains. Recently problem seems to be most frequent when boiler on for HW only when there are kettling noises after a few minutes of ignition then the lockout light goes on, sometimes it goes off for a while and then goes on permanently. I think the problem does not occur if CH and HW on together, but may be wrong. I thought maybe an airlock in the HW cylinder coil so I opened up the top pipe on the cylinder and let some water out but didn't hear any air and anyway did not fix the problem.
Any other ideas?

As you will have noticed in the manual the Kingfisher MF requires a by-pass to be installed. The by-pass wiil permit additional flow through the boiler when on HW only and help keep the boiler temperature from reaching trip condition. Worth while confirming one is installed.
The overheat stat you replaced might well have been just doing what it is designed for.

Check you do not have a blockage in the pipework where the cold feed from the F&E tank joins the primary circuit.

Cheers