Sime Friendly Format 80E: Burner Cuts Out, New PCB Needed?

[macman81]

I have one of the above combi boilers, installed in 1999. CH side works fine, but when using the shower the water goes cold after 2/3 minutes as the burner cuts out. It always reignites after about 30 seconds, is OK for a couple of minutes more and then goes cold again. The water pressure is not affected, it's just that there's no constant hot water.
Wife is not happy with hot/cold showers...
I recall there was a similar problem about 5 years ago and the PCB/Control Box was replaced (part no 6210203-Blue Box). Is it likely that this has failed again? If so, is there a known problem with these, as a new board every 5 years at £90 plus fitting is a bit steep...

 

[Agile]

It sounds more likely to me that its overheating as a result of a blocked secondary heat exchanger.

However a thermal survey would be needed to assertain that.

It could also be a gas related problem which would need a competent CORGI to fix.

Its unlikely to be the PCB but a new PCB every five years at £90 would be seen as heaven by those who own boilers like a Suprima or Ideal Icos whose PCBs typically fail every 2-3 years and cost a lot more !

Tony

 

[macman81]

Thanks for the advice, I've done a few more tests. The cycle is fairly consistently 40 seconds burner on and then 15 seconds off. It never fails to relight. The weird thing is that this only happens when using the shower. I've tried running the sink hot tap full bore and it burns as long as the tap is open. The sink is roughly 1 metre from the boiler, the shower on the same feed approx 2 metres further away.
Given this, does this rule out PCB/heat exchanger problem and point to something else to do with the water pressure to the shower maybe?
I thought maybe a blockage in the shower head, but it's clean and the flow does not reduce when the burner goes out.
PS: if it is the heat exchanger, can this be cleaned after removal, or does it have to be replaced?

 

[Agile]

If you turned the sink tap to the same flow rate as the shower I expect it will do the same.

It now seems less likely to be a blocked heat exchanger and more likely to be gas valve setting related or temperature control related.

In other words it could be several things and as some are gas related then I think that you need a competent CORGI engineer.

If you were able to do a thermal survey and measure the resistance of the sensors cold that that might indicate a closer area for the problem.

It seems you had the ignition circuit replaced. That might have been failing to detect the flame and turning the burner off after just 5 or 6 seconds.

Tony

 

[macman81]

Thanks. The 'burn time' is consistently 45 seconds or so, not 5 or 6. However since my original post the burner is failing to fire first time when hot water called for (bath/shower/sink, everything). Turning the tap off and on again usually gets it going and then it cycles off/on as originally described.
Strangely, the central heating side is fine, it comes on in response to the programmer and room stat without a problem.
Does this last fact indicate that it is not an ignition problem, but just confined to the DHW side of things?

 

[Agile]

Does this last fact indicate that it is not an ignition problem, but just confined to the DHW side of things?

Yes, of course which is why I started by saying that its not likely to be the ignition unit this time.

It would be better if you dont try to think but just did what I suggested of measuring the shower flow rate and then setting the basin tap to the same flow rate and I would expect the boiler will do the same as the shower.

But there could be other issues involved like a thermostatic shower valve combating the boiler controls.

You could also run the shower with the basin hot tap on at half flow!

Tony

 

[macman81]

Turned out to be the diverter valve. The rubber diaphragm was badly decayed. Once replaced, the problem has gone. Engineer advised that the driver PCB or the thermistor may also need replacing, as the burner is not modulating properly. But now at least when it cuts out on the overheat, it will always relight within a few seconds.
I'll change the thermistor first, and if still not modulating, change the driver PCB too.