Yet more Potterton woes

I posted some time ago concerning my Potterton, which had a new PCB earlier in the year, but is now locking out again.
Geoff suggested measuring the voltage at the solenoid valve, which I have now done, and it looks OK, going up to about 180V when the sparking starts. You can also feel it working, so I am pretty sure it's working.
If you reset it several times in a row, it will eventually light and will then work quite well, unless it is off for a while - ie., overnight.
I have taken the cover off and verified that it is sparking in the right place, and that the spark looks quite healthy.
What I now need to understand is how the ignition process works - I think that it lights a pilot jet first, and as long as there is a flame, it then turns on the whle burner. Is that correct? What senses the initial flame, and is there a 2nd stage to the solenoid valve, or is it a separate valve?
With the cover off, there is no audible indication of gas or any smell, so it seems that not enough is getting through. Could it be a blocked jet or something?
I have heard that the pressure switch can be faulty on these - would that have the same effect?
Any help would be appreciated...
Thanks. -- Colin Swan
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Colin Swan wrote:

If by the pressure switch you mean either the one for air flow proving or for water content you would not get to the pilot flame point if there were a fault.
Some modern boilers operate by starting at an ignition rate and then increasing to the full rate once the small flame has been detected.
A lot of boilers use a two stage process with two gas valves one for the pilot and the main one. The flame is detected by a wire in the flame which might also be the same wire and electrode that the spark voltage is sent along. A flame looks like a resistor+diode combination to the electronics.
HTH
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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I'm not sure whether you posted this before or after we talked on the phone (My NTL box has been dead for a couple of days), so I'll put some answers here anyway
Yes, the pcb senses on the resistive HT cable

See above

Can't answer that one at home without the manual

There are two pressure switches - the water pressure switch, if gone, the boiler wouldn't attempt to work, and the air pressure switch, if gone, your board wouldn't attempt to spark and open the valve, so I think you can rule them both out as possible faults

--
geoff

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SNIP
Thanks for the helpful chat on the phone Geoff, and the flowcharts. Unfortunately, the installer had left by the time I got back, so I wasn't able to speak to him. He did leave it working, but said that he hadn't fixed it, and we would need to get Potterton out to it. It did work for the rest of the day, but was dead in the morning.
I called Potterton, and had to pay up front (195) for a callout, but to my surprise, that includes callout fee, labour and all parts, except the heat exchanger.
The engineer is here now, and the first thing he did (which is quite telling) is to put a new PCB in, before he did any diagnosis at all. He then went through the checks before very quickly concluding, as I had, that the gas valve has gone. He's now fitting a new valve as I type, so we should hopefully have a reliable boiler once more...
I hope he doesn't take the old board away as I would like a spare :-)
In fact, I have been very impressed with Potterton service, and they guarantee the work for 12 months (they'll also come back for nothing in the first 28 days if you have a problem). I would still use CET for PCB issues though...
-- Colin Swan
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