Worcester 28CDi burner lockout

[SteveZodiac]

I have recently got my boiler working again after resolving pressure problems caused by a slow leak (PRV I think) and a flat expansion vessel. It has worked fine for a few days (although the boiler pressure rises from 1.5 bar cold to just under 2.5 bar when hot which I think might be too much, and a tap has to be fully opened before I can get DHW which I guess is a faulty diverter valve diaphragm), but suddenly I now have a burner lockout problem.

When the CH is switched on the pump and fan run, the ignition electrodes spark and there are clicking noises from the gas valve but the burner will not light and the CH light starts flashing slowly indicating a burner lockout. At first I suspected the control board, so I removed it and there is some evidence of overheating around a couple of resistors. So I went out and bought a multimeter to test the resistance across pins 7 & 8 and 9 & 10 as instructed in the manual. If both are reading below 100 ohms then the manual says to replace the control board, but I got about 735 ohms from each set of pins and therefore in theory it's the gas valve that's faulty and not the board.

Is it more likely to be the gas valve or could the control board still be faulty even though the resistance test seems OK? I am happy to replace the control board, but don't want to touch the gas valve and only want to call an engineer as a last resort.

Any advice gratefully received.

 

[ollski]

Early gas valves on these are pretty prone to failure...more so than the boards imhe

 

[nickso]

test the gas valve solonoids from the multiway ribbon on the bottom of the pcb. then you will at least know if they are open circuit or not.

 

[tryitandsee]

Hi, had the same symptoms on a 26cdi.

WB technical recommended changing the pcb and flame recognition leads, worked ok for three days. Same symptoms again.
This time changed gas valve, watched engineer struggle to calibrate the new valve. (The gas valve has changed design from the old one taken out, be careful when ordering to get the new part number that will still be compatible!)
Worked ok for four days, but domestic hot water going from boiling hot to cold.
Back agin and replaced gas valve for another new one (supply shop luckily agreed the first one may have been faulty) Again engineer struggled to calibrate new valve, eventually ok.

It seems that the gas valve sticks intermittently causing us all into thinking it was the pcb. Also, just make sure the multi pin connections are fully engaged/connected bottom right.

Then sods law, the diaghragm then perished, but luckily he had one spare in the van.

 

[Gasguru]

Given the cost (and aggro) of a diverter and a possible gas valve or pcb a fixed price from Worcester might be a better option.

 

[tryitandsee]

Gasguru. On that occasion, (in my case) you would have been right.

 

[SteveZodiac]

Hi, thanks for all the advice.

tryitandsee, I reckon you're probably right about the gas valve sticking, as I've just remembered I have had a previous lockout which cleared OK. The boiler is 8 years old.

nickso, what exactly should I do? Test the resistance across pins 7&8 and 9&10 on the multiway plug and if both are zero then it's definitely the gas valve?

Another thing that crossed my mind was a drop in gas pressure supply at the meter (the supply pipe to the boiler is 22mm). Does this happen and could it cause the problem?

Gasguru, if I did call WB can I definitely combine the hot water and lockout problems and get both sorted for the same fixed fee?

One other thing: last Christmas (Boxing Day!) I had a catastrophic leak which turned out to be the heating return isolating valve. It wouldn't hold off or on, and nobody would touch it including WB because of poor access to the boiler (it's in a cupboard above the stairs). Luckily the boiler is at the highest point on the system so I drained the boiler and managed to cap the valve screw with a capping nut clamped on by a jubilee clip, which worked! Would this cause a WB engineer to refuse to do the diverter valve? Or do the PCB/gas valve for that matter?

Unless there's anything else simple it can be, I guess a fixed fee callout to WB is the best option as I'm hoping to keep the boiler going for another couple of years until I'm in a better financial position (in theory!) to have it replaced. As I say, it's 8 years old and all I've had replaced so far by an engineer is the d/v diaphragm a few years ago.

Thanks.

 

[SteveZodiac]

Hi, can you also tell me if you're better off buying parts yourself and then getting an engineer to fit them (unless it's WB of course)?

Thanks.

 

[tryitandsee]

Steve,
You might struggle to gets gas parts now as suppliers are wanting the CORGI number before they talk/dispatch goods. Even I, with my DIY hat on would not attempt the gas valve. More involved than I thought in calibrating etc. The part is expensive and if it turns out not to be that, then WB would take tat on the chin. It's not worth the risk.

I'm afraid it's bite the bullet time. Get WB out and yes they will want to replace things that are non standard. Be nice, have the kettle on, prepare the access ready for engineer and generally make their life more bearable.......you might get lucky and they replace the lot for the fixed fee.

 

[nickso]

nickso, what exactly should I do? Test the resistance across pins 7&8 and 9&10 on the multiway plug and if both are zero then it's definitely the gas valve?

not sure of the pin numbers but you can trace it back from the gas valve wiring (blue and purple??)

if either are open circuit then thats the problem, the fault finding in the manual is pretty good on these so refer to that if you have it.

 

[SteveZodiac]

Thought you might like to know I had WB out and he replaced the gas valve, diverter valve, prv and heat exchanger for £245 so thanks for the advice.

He also recommended a Magnaclean unit. Any thoughts? Any reason I shouldn't fit one myself? (I've done some research and it looks straightforward enough).

 

[tryitandsee]

That's a fantastic result for all the parts he replaced.

 

[superdupergasman]

Magnaclean, great product, follow the destructions, its fairly straight forward!