Potterton Puma 80 yes it's still going! Just about. Please Help!

[SHooppie]

Hi everyone

1994 Potterton Puma 80 with permanent pilot. A trusty workhorse, but showing signs of age!

Extension due next year so hoping keep it going until then when it'll be all change.

The pilot light has lately been going out quite a lot and I'm trying to get a bit of an understanding myself before calling someone out. The flow switch, which incorporates a diaphragm which activates a micro switch to fire the boiler seems to be working (I can see the spring moving back and forth and the red light on the PCB activates during hot water demand).

Questions:

a) first thoughts are a faulty thermocouple, but I can relight the boiler and the pilot will stay lit for multiple hours before it goes out (often over night). Does the fact that I can relight the boiler pilot mean the thermocouple is fine? Or can carbon deposits cause random failings of thermocouple/pilot?

b) the diverter has a wax assembly within it. I'm pretty confident this is faulty as when heating DHW the radiators slightly heat up! So a new diverter seems required, but my question is, would a slightly faulty diverter valve cause pilot to extinguish? The pilot seems to be going out at some arbitrary time, not upon demand of heating or DHW.... So unsure if it's the culprit.

c) possible fan assembly not working efficiently? Causing cut out?

Thoughts welcome!

Thank you one and all for your help!

Stuart

[aeromech3]

Not an heating Engineer but I used to have an old Potts, mostly the thermocouple burnt through but then a complete failure. The other instance was caused by a draft (outside cupboard door vented). Draughty!
Not sure if this diagram helps from the manual from the Boiler Manual section.

[SHooppie]

Thank you for your reply aeromech3 and the manual excerpt. I have wondered about draft but it was occuring so often that I ruled that out. Could be the pilot injector.... Maybe it's a bit blocked. Something is glowing red as I look through the small window but I think it may be part of the electrode and not the thermocouple. I will take a better look. Besides, I agree with you, that the thermocouple probably fails completely in most instances.

[gas4you]

Could be minute deposits in the pilot injector causing flame to be small and not cover the thermocouple fully all the time.

I don't think the wax capsule is available ant more.

[SHooppie]

Thanks Dave. I was starting to convince myself it was the wax capsule. The DHW was getting very hot and I wonder whether that's because there was relatively little water flow (due to some being sent around the CH loop) but still getting all that heat.

It worked fine for a few days. But is now playing up again. New clue, my better half said she heard a click (a switch) and the flame was out. She hadn't used the water beforehand, so I'm now thinking it's not the diverter/was capsule after all.

Dave, I reckon you're onto something. Just surprised it lights up at all and lasts for hours, but it seems to just be shutting off so got to be: pilot (carbon deposits/faulty thermocouple), perhaps fan assembly, or random pcb error.