Potterton Netaheat 10/16 controller burnt?

[T i m]

Hi all,

I got a call tonight from an elderly neighbour we keep an eye on, saying her boiler hadn't come on (and her laptop won't come on either). This is the same one who needed a new CH pump, basic Potterton timer and thermal rad valves sorting that you advised me on previously (and are all now / still working fine). ;-)

I popped over and long-short it looks like there has been some burning on the controller PCB (she said she had heard a small 'bang' earlier but wasn't sure where it had come from).

The fuse had also blown.

So, two questions really. Is there anything running from that board that could start drawing more current than normal that could overload the PCB / relay / tracks like that (and if so what), or is it just one of those things that can happen on those boards please?

If it is just that the board has failed because of bad design (or dry joint / whatever) could someone with a reasonable eye, a soldering iron and some bright, tinned copper wire, repair the PCB and still find the rest of the board functional?

If I can find a replacement controller and she can afford / justify it (over getting a new boiler etc .. 'Warmfront', free / heavily subsidised / ripoff? ... as she is 70+ and registered disabled etc), what are the chances of that one going bang as well?

All I can see hanging off that board is the fan (been replaced twice now I think), the gas control valve, boiler stat and the air pressure switch so I'm not sure if any of those could go low resistance or short (happy to test resistance if anyone can give me a rough idea what they should be please).

All the best and thanks for your time etc.

Cheers, T i m

[Fredxxx]

It looks as if some corrosive liquid has gone across back of the PCB which has been conductive.

I assume mains voltages are on this PCB so creating heat and charring.

Unfortunately charred PCB is also rather conductive so I would regard the PCB is pretty terminal, or certainly the affected area.

You might find one on eBay if you're lucky, but best to make sure that whatever caused the charring in the first place doesn't happen again.

At a push you might be able to lift various components up and wire directly, and cut tracks that enter the charred area.

The ECO boiler replacement scheme is still running depending on her income. At least she will get some form of guarantee with it. Also a maintenance contract might cost less than savings in her gas bill. Having a non working boiler puts her at the top of the list.

[Graham.]

Excellent photography by the way.

What started it was a simple dry soldered joint on the second (counting from the bottom) of that 4 way terminal block. Looks like the fibreglass has actually caught fire, nasty. I very much doubt if anything external to the board "caused" it.

[T i m]

Firstly, thanks for your fast reply. ;-)

Whilst it's possible something has got on there, I didn't see any sign of anything in the surrounding area (above on the metalwork etc).

I believe so yes.

Ah, good point.

There are both new and s/h so that isn't such a problem (I've since checked). It was more if the charring was caused by a current overdraw then we would need to address that first. If it was an h/r joint then a new / working one should just carry on fine?

Understood.

So that would include any pension / benefits etc?

True.

Is that always the case? She has cavity wall insulation, double glazing and loft insulation. Even when the boiler was out for a few days the last time (timer fell to bits) the house seemed to stay pretty warm, even when quite cold outside. Coming back into this Victorian EOT reminded us how cold things can get! I'm thinking that if her existing boiler doesn't have to work very hard, even if less efficient than the offerings of today, how long would it take her to recoup the cost of a new boiler if she had to pay for one herself? If it's free then it's win-win of course, as long as it's as cheap to maintain long term).

That was my though but wondered how quick they might be able to respond, even if she was at the top etc?

Thanks again for your time.

Cheers, T i m

[Fredxxx]

OK - I was going by the suspicious vertical line on the back of the PCB.

Yes, I believe they have to be under £17k or so.

Well, what's her gas bill? Her current one, I believe, has a SEBUK rating of 68%, new condensing would be 90+%.

I'm afraid you will have to phone and find out. I guess at the time she phones the boiler won't be working, so what if you then repair it afterwards?

Pleasure.

[T i m]

Cheers! It's just the std camera on my Samsung Galaxy SII and is a bit of a compromise compared with even my little Fuji Z35 when it comes to close up work. That said, the SII is always on my hip so ....

I've just given it a quick clean up (some meths and a toothbrush) and you mat be able to see the seat of the problem a little clearer now:

I've highlighted the position of the relay so it looks now like it might have been an issue between one of the relay output pins and the track across to the connector you mentioned. It's not clear if it was a single point as it seems to be spread between the relay pin and the connector, so the track?

Yeah, the black smudge above the main damage washed off pretty quickly with the meths so I think it was just the smoke residue (burning resin)?

So, if it was started by a h/r joint (if it was just (say) the relay pin), would the damage have run sideways so much? Could the track itself have been underrated from new and has been heating up that area? I think the 3rd pin up on the lower connector block goes to the air pressure switch and (following all the tracks and links back across the board) is the live supply (via the fuse). I must admit that burnt track looked like it was at least as large as some of the main feed tracks?

Cheers, T i m

p.s. Oh, her 'dead laptop' turned out to be a broken wire in the very thin cloverleaf power lead into the PSU. After getting some strange / unpredictable test results (continuity / voltage) I cut the moulded plug off and fitted a std one a bit further down the lead and all was well. I'll get her a new / better cable asap. That earned me full access to the biscuit tin. ;-)

[T i m]

Ok, that's handy to know, thanks.

Good question, I'll see if I can find out.

So, ~20+% improvement on her gas bill. Nice if she gets the replacement system free and it doesn't cost her more that 20% of her current bill to maintain. ;-)

Understood.

Well, I think we will have to play that one by ear. If they offer her a free (or heavily discounted) solution and it's going to be done reasonably quickly then I'll not bother with her old system and we will sort out alternative heating for her over the interim (she has an immersion water heater etc). If it's going to cost her lots and / or take ages (weeks) then I might affect a temporary repair for her?

Thanks again, cheers, T i m

[Johny B Good]

That seems the most likely cause since the topside view doesn't show any burnt out components (they look like they've been victim to whatever else it was that caused the PCB to char).

So, in answer to the OP's implied question " is [it] just that the board has failed because of bad design" the answer is no. Also, to answer his first two questions, there's no evidence of blown _anything_ (other than the safety fuse) so it seems not to have suffered as a result of a peripheral fault and the "just one of those things that can happen on those boards" is answered by "Yes, if you count contamination by a conductive liquid as being "just one of those things"".

There are LNE markings top right corner of the topside picture so that seems to be a safe bet. Also, the fuse is in line with the connection marked L.

Whether the board is repaired or replaced, it looks like the OP will have to check for the source of the liquid ingress and repair the leak or rig up a plastic deflector to protect against condensation drips before putting the boiler back into service.

[Chris K]

I've had to reverse engineer this board owing to odd failures, it is straightforward to repair as it is mostly standard parts (only the trigger tube and transformer look difficult). Dry joints seem to be a problem though that does not look like yours has failed that way (it does look like something has dripped on it). You could flip the top off RLB and check the contacts for damage as well.

Also, the electrolytics dry out over the years causing intermittent failures and should be replaced.

I can send you the circuit diagram if you post an EMail contact.

The nominal efficiency of the boiler does not look too bad so replacing it will not save a great deal (it is nominally 69% though probably rather less by now). Remember the case seals are critical to safety as the internal pressure is positive, there are incidents posted on the HSE site.

HTH

Chris K

[Cursitor Doom]

On Thu, 27 Feb 2014 22:07:34 +0000, T i m wrote: [...]

It's a bit hard to see, but it looks like there's a wire filament bridging across and may have shorted out the board (to the above left from the charred area on the reverse shot of the board)?

[T i m]

Ok.

I looked for that today but couldn't see any way any liquid could have got to that location (not saying it didn't of course).

Hmmm, nor that's interesting. I've just gone to 'flip the top' off RLB using a little (2mm) screwdriver (gently, just to get the case moving etc), under the end away from the burnt bit and my fingers and I've just ripped the relay straight out of the board? ;-( Would that suggest that the relay pin in the burnt area could have also been 'questionable' (should I be able to rip a correctly soldered relay like that, out with a finger and thumb)?

Understood (I've done quite a few PC motherboard PSU caps and even our Swish electric curtain unit). ;-)

That would be great thanks. The email as shown here is valid.

And given her house seems to retain heat well (well insulated mid terrace etc).

Understood. I'm not going in there. ;-)

It may well indeed.

Cheers, T i m

[T i m]

Ah, yes, I can see what you mean on that 'pre cleaning' photo and I think it's just a bit of cloth or some such (well spotted though). ;-)

I have since mentioned that whilst checking the relay RLB I've managed with pretty well my finger and thumb and a little bit of effort to pull it (and the remaining 4 solder joints) right of the board. Looking closer at the burnt area it could be thought that the burning might have radiated around what is likely to be one of the two key relay pins (240V switching) and that if it's like those joints I've just pulled though, may well have been 'dry'?

[Mike Tomlinson]

En el artículo , T i m escribió:

No, but given the condition of the solder joints in your photos, I'm not surprised.

What was the state of the fuse? If it was a glass one, was the wire inside just parted or was it completely blackened inside? If blackened, there's something more fundamental wrong with the board.

I'd start by checking out the four diodes to the right of RLF, since the incoming mains passes through the fuse, RLF and those 4 diodes. If there's a short upwind of there, the PCB track may have vaporised in a vain attempt to save the fuse (cue old joke about expensive semiconductors sacrificing themselves to save the fuse...)

There's two relays visible in your photos - RLB and RLF. Maybe RLF is for the fan, but what is RLB for?

[Andy Burns]

Looks to have been worryingly easy, are those rings left on the relay terminals just solder blobs, or the plated-through vias? Are you planning to repair the board, replacing the burnt out track with a wire? I'd be de/re-soldering all the other joints in case ...

[meow2222]

Probably retaining the neighbours' heat!

Fires trash soldered joints, so one cant conclude anything there.

Dont overlook that that much overheating will almost surely have killed anything silicon that connected into the burn zone, so replace some diodes IIRC. It also might destroy other components, test or replace any burn zone ones where you can.

NT

[T i m]

Ok. ;-)

Vaporised I think Mike.

I have an update on that. I've now got a good second hand board so short (excuse the pun) of having any figures for the impedance / resistance for things like the fan (to know if the board was overloaded etc) and other than them showing short or very low resistance (we know the total load can't exceed 1A because of the fuse so (simply) 250 ohms total (fan + gas valve?)) I'm not sure if after such checks I shouldn't just stick the spare board on and see what happens? ;-(

I think RLF handles the gas valve, RLB for the fan.

Cheers, T i m

[T i m]

And why not. ;-)

I'm sure you are right but it seems strange that even away from the action (heat rises etc) the 3(!) relay pins just pulled out?

Ok. Only D6 is that close.

I might (OOI) but as mentioned elsewhere I already have a second hand replacement controller that looks like it's of a later deign with very much heavier tracks etc.

FWIW, I just dropped the old board on the carpet (it actually tumbled off the chair rather than fell as such) and when I picked it up the igniter TXFMR was hanging off!

Hmmmm ...

Cheers, T i m

[T i m]

Indeed.

I think they are just solder blobs Andy.

That was an initial though bus as I've now picked up a second hand board I'm not sure I can be bothered / it's a good idea.

Well, yes, as I mentioned elsewhere, after a minor tumble onto the carpet the igniter TXFMR was hanging off!

Cheers, T i m

[Dave W]

I too have worked out the part of the circuit shown in your good photos, by mirroring the back and scaling and superimposing it on the front.

The track from terminal T5 used to go to the top pin of RLB (normally open contact), and the track from T7 used to go to the common of RLB. Both these tracks are burnt out. D6 in series with R13 are just passing by.

T5 also goes to the boiler contacts via S1 or S2, the other one of which is fed from the fuse.

When RLB comes on it connects the Fan to the fuse via S1 & S2.

I think the most likely scenario is that the top RLB pin was arcing and burnt the board, making it conductive and escalating the fault.

Another possibility is that the 1 amp fuse had been replaced by a much higher value, plus the fan has gone short circuit, fusing the track.

The relay may well be OK as the contacts are rated at 10A. If it was me, I would probably cut away the burnt bits of the board, solder the relay back on all pins except the burnt one, and solder bits of tinned copper wire to replace the burnt tracks. It would be best to check the fan before switching on again.

[Chris K]

Looks well fried. Seems like there was a dry joint on the relay though that is the coil connection so it is hard to see how that got burnt as the currents are quite low (that is the feed to the gas valve(s)).

BTW I've posted the documents to you at your reply address.

Chris