Potterton kingfisher MF lockout problem

[vikla]

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?

 

[kevplumb]

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.

why not the ttb stat instead of the pcb

 

[XKESTONMAN]

if it helps,is there a 22mm gate valve on cylinder return,if so open it more,
it may have collected debri & blocked.
if S PLAN should be a bypass,maybe sticking.
take pump head out it may have debri inside impelleror pump worn.
or the micro switch on zone valve sticking.

 

[mrgas]

TTB?

 

[rojaca]

flow and return temperatures shut off on blr stat?
flow and return temps when up to temp and zone valves shut off? Does temp continue to rise on pump overun?

 

[vikla]

Trying to reply to the posts so far.

TTB is the sensor on the flue to detect blockages and downdraughts.

No gate valves on the HW return. Only gate valve is on a short overrun bypass. I have played about with this. I leave it open by one turn.

The zone valves all seem to work OK. I have not looked at the pump other than to check it turns OK. However when the boiler runs I have no problem with CH or HW, both come up to temperature.

 

[XKESTONMAN]

need r.g.i out to check,service boiler,ventilation,spillage test ect,for peice of mind.