Vokera 20 80 rs

[sibar]

I have a 10 year old boiler that seems to make a noise sometimes on the CH side. It seems to happen when the heating is set up pasted midway on the boiler thermostat. I thought it might be a faulty pump so I removed and checked the pump also I ran the pump in my hand to see if there was vibration. I also fitted a new thermostat with probe.
When CH asks for a high flame, the high flame starts but then after about 3 or 4 secs the noise starts and it goes to no flame. When I run the CH on a lower thermostat setting, the low flame starts up and it seems to be able to cope with it.
My next thought would be a blocked HE, am I barking up the right tree or just barking?
I don't want to buy a new HE only to find there is nothing wrong with the one I already have.

 

[bengasman]

No point buying a new hex ( if that is the problem ) as the rest of the system would still be full of crap and thus you'd be back to start before long

 

[DP]

Service, service, service and service is all that needs to be said. Boiler almost certainly had not had a service for a while (anyway, the ones that I come across). Burner should drop down to low flame.

What is the temperature and are the radiators hot? Does the flame downsize during hot water?

Good boiler that c5ap does not effect like a modern boiler.