Noisy Potterton boiler with suspected ventalation issue

[Coldaitken]

We have a 14 year old boiler a Potterton Ultra 2 DV. About 18 months ago it started to get very noisy re-firing all the time and making a banging noise. After much investigation this was fixed by replacing the pressure switch. The noise recently started again and our excellent plumber is at his wits end as to what to try. He has also spoken at length with the manufacturer for their ideas.
The boiler works fine with the cover off suggesting a ventilation problem and thus a easy fix. The pressure switch has been changed so has the fan and the circuit board. The flu has been checked and re-checked and smoke cannisters used to check airflow. Finally the O ring has been changed. The symptons are continuous re-firing without the flame quite going out (about once every 1-2 seconds) with accompanying banging sounds. Actually is working OK but obviously not healthy and very noisy.
We are currently looking at a new boiler but if anyone can give us inspiration on what else to try that would be great. Thanks Cold Aitken

 

[bigbow]

Ok the problem needs to be indentified as wether it is ignition or temperature related. Fire the boiler and using multimeters ensure that the air pressure swith is switching over. Usually 3 wires one is common always voltage, then it will either power the off side or the on side. (usually open or usually closed) Once the boiler has proved the air pressure switch it will power the gas valve to open at ignition rate and spark. If the spark stops and opens to full the boiler will light. if not it could be the gas valve dropping out or the ignition electodes burning through, or the pcb. Also worth checking that the fan venturis are clear. The venturi's is where the 2 pressure tubes join the fan.

 

[twgas]

has the pilot been cleaned, when main burner lights it may be pulling the pilot flame away from the electrode and thus restarting the ignition sequence

 

[gavinda]

filthy heat exchnger would give this fault.