Hello First post so here goes (and its rather long). History My boiler was installed new at the end of 2005 (around about the time WB were retiring it from their range it now seems). I did a lot of research and it seemed to be one of if not the best manufacturers at the time. Till now, I've had no problems. So far I have looked extensively on this site and I believe that I have some sort of blockage in the hot water side of the system resulting in E9 tripping out. Detective work done so far includes...... 1) Changing the overheat stat switch screwed into front of the primary gas to water heat exchanger (I checked with a meter that this was the switch that was tripping out and it was a 2 minute job). 2) Checking the pump works. I borrowed a pulse generator from work (electronic engineer u c) and simulated the pulses into the motor. I believe from my stethoscope that it has 8 speeds (inc stop), and I plotted them out in excel and got a straight line with respect to pulse width, so I guess it is spinning at different speeds, but I can't tell exactly what they are. I also note that when it is connected back to the control PCB, it gets pulses that are in the range I used. Anyway, I'm going to assume it is good. 3) Checking spot temperatures around the primary heat exchanger with my point and shoot infra red temp sensor (incidentally I had to stick some black PVC tape to the points I wished to measure, as the shiny aluminium has poor emissivity so the sensor doesn't see the temperature properly), I can see the heat exchanger getting upto 95C in the region of the overheat switch and maybe higher when it E9s and trips off. 4) I have checked the primary temp sensor and am happy that it agrees with reality (resistance wise) 5) I have checked the DHW temp sensor and am happy with it also 6) I have checked the pulses from the flow turbine using a scope. All fine Just as a little anecdotal evidence.... I can get the system to stay on if I significantly drop the setting of the hot water temp knob. I also note that if I am running hot water and turn this knob up, the boiler responds and increases output (well the fan revs up and the pump pulses respond to demand higher pump speed). These two facts give me high confidence that the control PCB is OK. I will add also that I have noted a slow reduction in hot tap water temperature over the past 12 months or so in my bathroom sink tap (smaller flow tap). It has still been possible during that time to get sufficiently hot water from my bath tap, which has a higher flow capability. All the above leads me to think I have a blockage in the hot water circuit somewhere within the boiler. I have read extensively about problems with the DHW plate heat exchanger, but also of blocked pumps and even the main gas to water heat exchanger, which seems to be gold plated if you can believe the cost of many. Oh, and I live in a soft water area. Any opinions on my fault would be very useful before I plunge into getting repairs. What would also be really useful would be a full parts list. I would really like to find out the cost of parts before I start paying out in progressively expensive chunks to get parts replaced. Also, there are gaskets and O-rings on everything, so it would be useful to know these sizes, oh and to be sure I am looking at the right primary heat exchanger if I needed. WB don't list that in their 'short parts list', and there does seem to be many different ones with differing prices. Cheers.... Rob.