Glowworm Swiftflow75.Boiler goes out when CH been on a while

[wsrus]

Hi, I've had this problem with my Glow worm swift flow 75 combi.(installed 1997) for a few months now.
What is happening is that the boiler works fine when the CH is in the "off" position, but when the CH's been on for a while and the temperature has risen in the house the boiler turns itself off (pilot light out). It will relight ok and work as before absolutely fine. If it's a really cold day it works fine with the CH on, but as soon as the ambient temp. goes up....off goes the boiler.
I've given the burners a clean out and changed the thermocouple.
There is a large towel heater radiator which is always on to act as a heat sink thingy.
I've read the fault finding section in the instructions and it sort of points to the overheat cutoff, but before I take the plunge a change that, I thought I'd ask grater minds than mine for their opinion. Could it be the gas burner not being controlled by the internal thermostat (if it has all that stuff in it)? CH pump not pumping fast enough? Gas valve sticking? Flu partially blocked? Air in the system?
Any help you can pass on to me will be fantastic.
Thanks.

 

[cross thread]

try turning the c/h temp down on the pcb potentiometer

 

[Agile]

I've given the burners a clean out and changed the thermocouple.

Thanks.

We dont give DIY gas advice on an open forum.

Turning down the Ch will probably solve the prolem but if not then let us know. However some of the possible causes would be gas related.

Tony

 

[Beerlover]

My advice :- 'Instant cover' repair with Glowworm/Homeserve; could be stats, pump, pcb, main heat exchanger or just Ntcs - or all of the above.

Your boiler is 13 years old - spend £300 and be patient while they rebuild her, OR take the old dog for one last walk, to the barn and shoot the F***ker.

 

[mickyg]

ch ntc thermistor faulty. connected to 22mm pipe lhs of boiler.