Potterton Netaheat Electronic 10-16

[Wolstan]

After working faultlessly for 23 years the following fault has developed:

Fan comes on
Sparks
Pilot light comes on
Main burner lights
Stops sparking
Burns as normal for 8 seconds or so

So far all as normal

After 8 seconds the main burner flames and the pilot light (as observed through the window) *slowly* die over about 3 seconds - they become progressivley thinner, lift off and waver, and then extinguish.
*Only then* click as the (main burner?) solenoid valve turns off

Go back to top - starts sparking again for about 13 seconds, with pilot attempting initially weakly to light and finally lighting strongly, and repeats indefinitely. There is no solenoid valve clicking while this is going on - I think the pilot gas supply is on all the time.

I deduce from the above that there is not enough throughput of air - but I have examined the flue/intake very carefully and it seems to be clear, and the boiler was cleaned only 6 months ago.

Does this diagnosis sound correct?

Could the fan be failing so that though apparently running normally it is only delivering a fraction of the intended pressure, which is neverthe less sufficient for the controller to think its working correctly?

 

[twgas]

sounds like a flue/fan problem, you need a RGI for it

 

[Andygasman2010]

Sounds like it's baffling out. On a positive-pressure boiler you need to be careful, the rear of the case can corrode on this one which is very serious and difficult to see. I'd get an RGI in on this fault.

 

[PEDANTICVINDICTIVEMAN]

Going by the age and nature of this boiler I would consider replacement.

 

[Boilerman2]

Are you sure the meter governor is not sticking, it sounds like you may be running out of gas each time the main burner lights get a RGI to carry out Static AND Working pressure tests on the inlet side of things

 

[Wolstan]

Thanks for all suggestions.

It turned out to be the flue. RGI dissembled it from the outside and unbelievably the supposedly professional CORGI installer had fudged it 23 years ago. To extend the length of the concentric pipes he had joined on pipes of the *same diameter* without a sleeve, by cutting slots round the ends to produce a 'frill', bending the frill inwards to reduce the diameter, and then ramming them into the original pipe. The frills naturally obstructed the internal area of the pipes and there were many gaps at the joins which would allow exhaust into the intake. Evidently for 23 years this proved just adequate, but the heat must eventually have caused the frill to bend further inwards at one point which had two effects (1) it obstructed the exhaust pipe by about 25% and (2) it provided a scoop to return that 25% of exhaust gasses back into the intake.

Would this be considered normal practice?

 

[Mr Smyth]

It has never been the correct way of doing things. But things were very different 23 yrs ago.

How long is the flue? I assume it is in excess of the MI's

Seeing as this cannot be rectified I hope that the engineer capped the supply and you are in the process of replacing it.

This is certainly not the kind of boiler to assume will be OK. Amazed it has taken 23 years to come to light.

 

[twgas]

Amazed it has taken 23 years to come to light.

Must have been a quality bodge

 

[Gasman77]

Personally i'd get the installer back

 

[twgas]

Personally i'd get the installer back

Good call