Keston C25 (open) problem not matched in diagnostics

[KesCon]

On heat demand, ignition sequence starts but fails in 3-5 seconds and then
restarts and repeats this cycle "for ever" - it does not stop and show diagnostic lights in an obvious "calm" state. It certainly does not approach the pre-purge stage.

The problem arose intermittently at first and the system recovered some days but is now always failing. This seems like a pcb-logic failure to me but could be sensor failure I suppose.

w.r.t. www.keston.co.uk downloads manuals cel25-m.pdf

If we get to 3 or 4 seconds before restart, it looks like the fan speed failure diagnostic, but could possibly be any of the last 3 on page 24 because of the short duration before the restart that is not allowed for in the diagnostic flowchart

It is not the fan (replaced that hoping for a quick fix) and we have (nearly!) 24V DC for it and diagnostics (page 26) points to the control PCB if the aborted lockout-red-led rate was 4 per 2 seconds and fan control PCB if 5 times per 2 seconds.

a) any suggestion as to what is causing the early and repeated restart

b) what is more likely to be the culprit,
fan PCB or control PCB ? (I'd guess control) ...or something else ?

Forum advice appreciated.
Thanks

 

[fitz1]

is there gas there.

 

[chrishutt]

You'd be better off speaking to Keston technical about a problem of that complexity. The boiler should go to lock-out after a few failed attempts at ignition. If that's not happening I would assume a PCB fault. Of course there may be a further fault causing the failed ignition, but I'd tackle the PCB fault first.

 

[corgiman]

Dont they have that overheat cut off which interrupts the live to the boiler, it goes if the combustion chamber overheats, sure that causes this fault

but you must look for why it did (if it did that is)

but could have saved meself sometyping by saying listern to chris and call their tech line, it is one of the best

 

[croydoncorgi]

If it's a few seconds, it's definitely not a gas / gas valve problem. The purge cycle lasts much longer than that.

Is the fan actually turning at all?

Have you checked the condensate trap overflow sensor (and checked for a real condensate overflow!) - I can't remember at what stage this gets into the control loop but it's a crude mechanism.... If it's fitted, try disconnecting it.

 

[Agile]

You need to keep calm and consider the problem logically!

There is a thermal fuse but that interrupts the supply to the whole boiler if I remember correctly so its not that.

The purge cycle always seems quite long, perhaps 6-10 seconds? In this case its not getting as far as turning on the gas or sparking so we have to consider a failure at an earlier stage.

My guess would be either fan speed failure or perhaps condensate trap contact.

Suppose the fan was pulling the condensate trap level onto the sensor and causing it to stop then?

I would suggest disconnecting the condensate trap sensor wire as that is easy to do. If that does not make it run then reluctantly I would start considering the PCBs. I would next look at the speed signal from the fan but that needs a scope display and in this case a replacement fan should have discounted that and finally the PCBs.

I cannot remember if a shorted flame detection electrode will stop it but its easy to disconnect that just in case. ( Nor can I remember if its a single spark/detection electrode either )

Most faults are simple when you have found them. Thats why women are so good at fault finding on boilers because they look at the simple things which are usually the cause whereas men dash at pcbs and fans first before the periferal devices have been properly checked.

Tony

 

[corgiman]

D'OH

Cheers tony I remember now if that thermal fuse goes nowt will happen

Well it is sunday

 

[croydoncorgi]

Cabinet overheat stops everything (I THINK!).
Flue overheat should lock it out but boiler will relight after reset (and then pop off again!).
If the fan turns (and is a replacement!) but nothing more happens, I'd be looking at the fan control PCB first.

 

[croydoncorgi]

Another question: does your model C25 have an air pressure switch? Later models don't. The absence of the APS makes reading the fault-finding flowchart in the manual very confusing!

With no APS, there's nothing (apparently) between fan-on and ignition attempt except purge - which takes 10 secs or so.

 

[ollski]

There are two fuses at the back of the rh side pcb, you need to disconnect it and pull it forward to check them though....have a look at those.

 

[Agile]

Very few premix boilers have any APS because that function is replaced by the fan speed feedback. Did the C25 really ever have one at first?

I dont remember the C25 but the larger Kestons have a gas inlet pressure switch which always seems odd as no gas will not cause any problem.

Its obvious but has the low system water pressure switch been checked?

All these little things I do at first without even thinking about it.

Tony

 

[croydoncorgi]

Did the C25 really ever have one at first?

Yes. Modification sequence was: APS, APS present but wired out by Keston engineers, APS absent but bracket for it underneath mains transformer still fitted, no bracket.

Also, the fault-finding still includes reference to low airflow sensed BEFORE ignition: with no APS I don't see how this is done! (bet it isn't!)

The odd thing about this fault is that the boiler is reportedly entering its startup sequence but NOT going to lockout. I've never seen this and don't know whether a fan control fault (not reading the tacho correctly) can hang it before it gets to lockout. It's nasty - it's probably a PCB fault - but which board?

 

[chrishutt]

...The odd thing about this fault is that the boiler is reportedly entering its startup sequence but NOT going to lockout...it's probably a PCB fault - but which board?

Which is the point the OP made to start with, which everyone (except me ) seems to have been overlooking. Lots of possible causes of ignition failure, but why should any of them stop it going to lock-out after 5 or whatever attempts?

 

[croydoncorgi]

My mistake: the two boards are 'control' and 'ignition' (not 'fan control' as reported). From the functional diagram, it looks like all the sequencing must be done on the control board. Can't be sure - never seen this fault. But all the ignition board failures I've had have caused lockout.

 

[ollski]

If it gets to fan running stage and no further and presuming the aps (if fitted) is ok (240v on red n/o) check for mains voltage on grey wire connection 3 control board pcb, if present check fuse on dbi board. I have had a few faulty boards on these which causes the fuse to blow.