WB 24CDi Burner Lockout

[Mathew Newton]

My Worcester Bosch 24CDi combi boiler keeps occasionally entering its 'burner lockout' state. The problem is so intermittent that resetting the boiler always brings it back to life and I have so far been unable to observe/determine whether the boiler is entering this state as a result of it failing to ignite, partial ignition, igniting but failing to detect the flame or the flame extinguishing when it shouldn't.

In the hope that it is either the spark electrodes or flame sensor that are to blame (i.e. not a more complicated/expensive cause like the control board, gas valve or gas supply etc) I purchased replacements whilst it was convenient to obtain them (and negligible cost too).

Can anyone tell from the following photos whether it looks like the spark electrodes and/or flame sensor are in need of replacement? I appreciate it may be hard to tell given that most of the time the boiler works fine and so if either is to blame it must be due to a borderline condition.

have verified the correct gaps (3-4mm spark electrodes and 5-6mm flame sensor) and the boiler is at least 5 (possibly 7 or more) years old now if that helps.

Unfortunately the replacement of the flame sensor requires the removal of the burner, i.e. disconnection from the gas valve, and I didn't feel competent that I could ensure (through testing) gas soundness of the reconstructed burner/valve union. That said, how would one test the soundness given it is on the low pressure side of the valve?

I replaced the spark electrodes anyway given I had the part and, whilst I may be imagining this, the new spark certainly seems more pronounced.

I am now keeping an eye on the boiler in the hope that it is either now fine or I will catch it in the act of entering the burner lockout state so I can determine at what stage it occurs and hence rule in/out potential factors.

Any thoughts/opinion would be most welcome.

Mathew

[geoff]

In message , Mathew Newton writes

Well, that's the crux. isn't it

Check the flame sense lead - physically give it a bit of a tug

[Mathew Newton]

I think I may be getting somewhere... but it's a place I don't like...

I managed to catch it in the act today - the stat called for heat, the pump/fan activated and the electrodes started sparking but nothing else. After 10 seconds or so the boiler gave up and entered burner lockout.

I am assuming the valve did not open, or is its mode of operation more complicated than this?

The service manual fault chart says that if resistance across the two sets of wires to the valve (main valve connections as opposed to regulator according to the book) is not

[Mathew Newton]

I forgot to mention; if you can this a 'test' I did confirm the gas supply was okay as the hob is working (I'm sure there's more to a supply than that though).

Mathew

[Mathew Newton]

Well... I've moved on now.. and I'm *definitely* in a place I don't like.

A few years back I replaced the control board following some problems with the overrun not working properly (pulsing fan). I forgot that I'd kept the old board so I've just fitted it and... occasional burner lockouts.

So, I'm assuming it's the gas valve so will get someone out to replace it. I'm not comfortable doing it myself, not least the setting pressures aspects.

Cheers,

Mathew

[geoff]

Have you measured the volts on the gas valve when it's sparking ?

When it's sparking the GV has to open

That would, IMO, be an obvious thing to do

volts to valve - valve problem

no volts to valve - pcb problem

[geoff]

Just a minute, as I read your post

new board - boiler doesn't fire

old board - boiler fires sometimes

Take one step back, and reassess your deductions

[Mathew Newton]

Sorry, perhaps I wasn't clear. Both boards fire most of the time...

I haven't checked the voltage at the valve - I simply can't get round the back of it to do so!

I've now booked a Worcester engineer for Friday - it's a flat rate fee of =A3185 inc. labour, VAT and upto 2 major parts (exc. heat exchanger). I figured that if it's either the PCB or valve then I'm reasonably happy with the price, particularly given they ought to be able to fix it and ensure everything is hunkydory. I'm a little worried about when he sees my new spark electrodes - do you think they'll kick up a fuss given I've been 'meddling'?

Thanks for you advice so far, it's just a shame I'm not managing to pinpoint the problem myself even with it.

Mathew

[geoff]

Ah - I thought it was the other way round

Do you need to ?

measure it on the connector on the pcb

This introduces loom wiring as an unknown, but on a relatively new boiler, it shouldn't be a problem

[Mathew Newton]

True. I'll give it a shot tomorrow.

There are six wires to the valve - 2 x blue for 'regulator', 2 x brown for 'main' and another 2 x violet for 'main'. Given my testing time is limited (intermittent fault, 10 second window) can you advise on which I ought to be concentrating on? It would help if I knew the principle of exactly how the valve works but Googling failed me.. (or rather I failed it!)

Given the service manual speaks of measuring the resistance of the two 'main' pairs (brown and violet) so I'm assuming these are two solenoids? (why two?) What does the regulator consist of?

Sorry for all these questions, I'm sure you must be doubting my competence but I just want to understand as much as possible to get the most of this situation.

Mathew

[geoff]

ITYF the two main valves are a two stage main valve - 240 volts

the "regulator" is what's known as a modureg, it's a 24v modulating valve which adjusts it's flow rate according to the voltage on the solenoid. It's purpose is to fine tune the output depending on the demand so that e.g. you get a constant temperature hot water irrespective of the flow rate through the tap

[Mathew Newton]

Thanks for that - very useful.

I gave this a shot this morning and found that, during (attempted) ignition the voltage across each of the main valve points on the PCB (MV1 and MV2) rose to 135v AC (I suppose it could've been on its way to 240v AC but the digital meter update refresh may not be quick enough) and this coincided with a small thud from the valve. During the 10 seconds of sparking the voltage remained steady at something like 15v AC. Does this sound right? It was the same for both valves points. At the end of the ignition period it gave up, the valve gave another small thud, the sparking stopped and it enter buner lockout.

Remember I am seeing the same issues with both boards, so I'm assuming it's definitely the gas valve. The small thuds I hear are worrying though - that to me suggests it is opening or is there more to the successful operation of the valve than that?

Mathew

[Stuart Noble]

When we had a WB engineer he replaced the faulty item plus a couple of other things that "might go", so it was probably a good deal compared to a local engineer

[Mathew Newton]

That's good to hear - I was wondering how it might work, not least given =A3185 doesn't go very far when it comes to many boiler parts. I suppose it may depend on the engineer but I'll certainly try to ease the process with tea and choccy biccies. Might even train the cats to shiver too.

My biggest concern is that the problem won't manifest itself on the day. I've taken a video of it happening so hopefully that might help should this happen.

Mathew

[Mathew Newton]

He came out today and firstly replaced the air pressure switch - not sure why. Anyway, that didn't sort it so he then put a new gas valve in. Job done... at least so far - I'll be absolutely gutted if I wake up tomorrow to a cold house!

Thanks for all your help Geoff - much appreciated.

Cheers,

Mathew

[Stuart Noble]

I'd be interested to know if you get any kind of written confirmation from WB about what's been replaced and what kind of warranty applies to the new bits. I was given no paperwork at all and, although I have the bank debit and the original bits as proof, it's not a very professional way of carrying on.

They replaced a leaking pump on mine in March and the same fault appears to be developing again. I somehow thought it might because the auto air vent (which apparently often leaks, but costs tuppence and is easily replaceable ) is right at the back of the pump. Nice one, WB.

[Derek Geldard]

We had a WB engineer in here this morning. He didn't leave a service report either. He said it was on his laptop and the office would be sending me a printed copy. We'll see.

We had also had an odd intermittent water leak on our 4 year old condensing boiler which came and went away. It came from a joint in the flue ! The engineer said whether it actually leaks or not depends much on patterns of usage inside / outside temperatures etc.

DG

[Stuart Noble]

All I got as a follow-up was an invitation to take out a yearly service agreement. I think their long term plan is to leave you no other option. If their stupid £120 pump is going to fail annually because of a design fault, they would seem to have one by the short and curlies.