Fit new Potterton Suprima PCB - doesn't work - please help!

We have a Potterton Suprima 100L which appeared to have developed the well-publicised PCB fault.

I ordered a new PCB and have fitted it. When I power it up however, it has a continuous red LED. The fault diagnosis stuff tells me that it is a wiring fault and that either the PCB is faulty or the pump's Live and Neutral are reversed. As the board is brand new and I have done nothing to harm it, I swapped the pump's Live and Neutral and that blew the 3A fuse in the spur!

I therefore replaced the fuse and swapped the pump connection back.

Desperately hoping that it wasn't a faulty PCB, I tried the diagnostics instead (hold reset button in for 10 secs). That gives me a grenn LED while initialising and then ten red LED flashes - roughly 1 per second - repeated with a break. The fault finding tells me it might be the thermistor or the PCB and to test the thermistor resistance. That's reading ~865kOhms which seems to be about right. I have also tested it at the PCB end in the multiway connector - same reading.

Any suggestions please? I don't want to have to throw away GBP150's worth of PCB and try again!

For those of you with the reset problem, swapping the PCB is fiddly but not too difficult. The instructions are very clear and you just need about an hour's peace and quiet, a Philips screwdriver and a small flat head screwdriver to swap the mains cable feed - that's it. It is a self-contained unit. Beware though that if your boiler's serial number is earlier than AHHV833 0001 then you have to replace the fan as well which is Part No. 5000860.

All help most welcome!

I have literally changed dozens of these PCBs and the red light will light up at the slightest excuse. Does the boiler have a flow switch?.... Is the pump powered off the boiler ? It isnt heating a Gledhill unit or thermal store is it??
Usually its caused by a basic wiring fault , thermisters rarely fault unless broken, likewise the overheat stat( very brittle plastic).
Unwire the terminal strip except the L, N & E of the permanent power supply, the pcb should reset and sit in standby (orange), if it does you have a problem with the wiring to the pump ( if wired from boiler) or switched live or simply misplaced the wiring into the terminal strip . Presuming the boiler worked of sorts before its more than likely a misplaced wire or method of wiring which the new style pcb simply disagrees with. Good luck

If the pump is directly wired off the PCB then reversing the L&N won't make any difference.

However, if it blows the fuse when reversed then there is a commoned neutral elsewhere in the wiring after the pump. Shorting out the PCB is not a good idea and they are non returnable, so you had better hope yours is OK.....

You haven't said why you changed the PCB in the first place or why you were sure it would solve the problem. This is relevant.

Many thanks for the feedback - much appreciated.

To try and answer the questions:

1) I am not an expert but I assume there is no flow switch fitted because there appears to be no wiring that fits the description in the fitting instructions for flow switches.
2) I believe that the pump is switched off the boiler because the main electrical feed into the boiler is a five-way - twin and earth and two blacks each with a loose red sleeve on them. The two blacks were connected on the original to the pump live and neutral which is why I assume it is fed off the boiler. It has recently been replaced ( 2 months ago) and is a new unit.
3) It is not heating a Gledhill unit or any thermal store.
4) I am changing the PCB because of the well-publicised PCB fault which was diagnosed by the guys who service the boiler annually - we had been reduced to resetting the boiler at least once a day - and who wanted £325 for the job. As compared to that, £150 for a board seemed reasonable!

With the exception of the lockout problem, the system was working normally before I replaced the board.

Kristoflampie, I disconnected the two black-with-loose red sleeve cables from the main terminal strip and then powered the unit on. The LED went straight to red. I then held the reset button in for ten seconds (as per Fault Finding instructions) and still got the red flashing once per second for ten seconds.

If I disconnect the multi-way connectors at the top of the PCB one at a time to try to isolate the fault (assuming it is not with the PCB itself) will that work? For example, can I disconnect the thermistor and test and replace, and then the limit stat and test replace etc or will the apparent absence of a component give me the same 'wiring fault' symptoms that I'm seeing at the moment?

The new PCB comes with a label 'Fix Label to Inside Controls Cover Door' which reads: 'If the Boiler Fails to operate, check the colour and number of flashes on the indicator light.'
'Flashing Red once/sec: Lockout - Press Reset Button for 5 seconds.'
'Continuous Red: System Wiring Fault - Correct Wiring Fault. Either the pump live is not wired directly from the boiler to the pump or Live and Neutral are reversed'.
This last point made me swap the pump connections and blow the fuse. It also made me double-check that my main incoming Live and Neutral cables are in the right place in the terminal block and they are and always were.

You said that the overheat stat is made of brittle plastic. I had quite a job to remove the spade connectors from it. While I have no reason (no nasty cracks or noises) to suppose that I damaged it, is it possible that I did so and am unaware of it and is there any way I can test that?

All input welcome - thanks in advance!

you should have a "switched live" coming into the boiler (connected to the orange wire in mv - usually). are you sure you've got everything exactly as you ought to.

It is almost impossible to diagnose your wiring from your description - twin and earth and two blacks with red sleeves - !

A twin and earth I can understand. Where do the two black cables come from? You can't get a cable with just two black cores in it.

My guess is that the twin and earth went to LNE. The two blacks, sleeved red, went to the 'switched live, boiler on' and pump live respectively. It would be VERY unusual for a black to be sleeved red AND connected to a neutral.

There may have been a link between the pump N and boiler N.

I think this may be a classic case of why parts distributors do not take returned PCBs.......

I was once given a reel of 5 core flex - it had 3 browns, blue and green/yellow, have also seen them with more than 1 black - although thats a good point about red sleeve on neutral

Yes, I've used that too. If you are lucky the blacks have a number tracer on them.

But the OP says he has a twin and earth, plus two blacks. That is why I ruled out the multicore scenario.

fair comment, but you cant rely on what some people say as being correct terminology,accurate (with best intensions) or a downright lie!

OK - thank you all for your input and sorry to create confusion.

The SINGLE cable is a five way multicore cable. As manufactured, it is twin and earth plus two black cables in it. Clearly the twin and earth are the bog standard supply to the boiler. The other two have two very loose short red sleeves that slip up and down them between the end of the cable and the main white sheath that encases all five singles.

When I removed the old PCB, the two blacks were connected to the pump terminals on the main terminal block - to the Pump Live and - I thought - to Pump Neutral so that's what I reconnected when I put in the new unit.

On reflection, your suggestion simond seems sensible and that I mis remembered where the second black with red sleeve went and it should have gone instead to Switched Live.

I have now had a chance to look at another installation done by the same crew at the same time as the one in question and that is exactly how it is wired.

So I have just now tried connecting the unit up that way but no luck. Is it possible that connecting the two blacks, which presumably are the electrical feed to the pump, to the pump live and neutral would have damaged the board and so now the new board is u/s ? I can see that not connecting it correctly might have caused the system as a whole to misfunction but connecting the pump to Live and Neutral doesn't seem to me to be asking too much of it?

As ever - all contributions most gratefully received and thanks for your input to date.

The SINGLE cable is a five way multicore cable. As manufactured, it is twin and earth plus two black cables in it. Clearly the twin and earth are the bog standard supply to the boiler.

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twin and earth is the usually used term for flat cable - two insulated cores (red and black or brown and blue) and a seperate, bare, earth core. Your's would be 5core flex. or at least thats how I interpret the wording.

And you need to make sure the right black is connected to the right bit at both ends.

In a word, yes.

You have shorted out the PCB output connections and therefore there is a very good chance you have fried it with an instant burst of a few hundred amps (or more depending on cable resistances).

I haven't looked over the newest Suprima Siemens PCB for internal fuses (we can't afford to wire then up wrongly!) but this could be worth doing before you bin it.

Some jobs are best left to the professionalls. pay for 2 pcb's and do it yourself or 1 pcb and an engineer to do it properly.

best left to the professionalls.

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Oh, the irony!

sebh007 said:

When I removed the old PCB, the two blacks were connected to the pump terminals on the main terminal block - to the Pump Live and - I thought - to Pump Neutral so that's what I reconnected when I put in the new unit.

On reflection, your suggestion simond seems sensible and that I mis remembered where the second black with red sleeve went and it should have gone instead to Switched Live.

Click to expand...

Wow, what risks you are prepared to take with a new £150 part!

Before I wired anything to a new board, I would check the external connections to ensure the correct component was at the other end of the cable and there were no connections to earth or anything else!

An independent cannot afford to blow up new parts so we have to take 110% care with everything we do.

Its nine years since I damaged a new PCB, luckily not a very expensive one, just because I had failed to measure the gas valve solenoid resistance. Never again!

Tony

Tony