Baxi Barcelona tries to start even though no call for heat..

System boiler, about 7 years old.

Just started playing up. It shuts itself down a few seconds after the burner LED illuminates, yet solid green LED remains on, indicating mains power. A forum member has advised this could be a flame-sensing issue.

I noticed a new issue tonight: the boiler is trying to start even though there is no call for heat - I have set both the digital thermostat and the programmable timer to OFF. The green LED is remaining on too - I thought this should only be illuminated if the roomstat/timer were calling for heat. As I sit here typing I can hear the boiler trying to restart every three minutes.

Roomstat is honeywell cm907. Timer is horstmann channelplus h27xl (I just leave the CH channel to ON). However, tonight I have turned both of these to OFF yet the boiler still has power and is trying to light...

Any advice appreciated so if by a miracle I can persuade a professional to come to the wilds of Surrey I can tell them what to check first.

Just tried a different tack which seems to work:

Set roomstat and timer to ON and really cranked up the roomstat temperature target. Boiler lit and stayed lit, no problems. On that basis it seems the boiler isn't happy to run if the ambient temp in the hall is already close to the roomstat target temp, but will happily light and run if the target temp is a fair bit higher.

So that would seem to rule out a flame-sensing problem? The problem seems to be electrical - PCB?

Mulling this some more.

Blaming the Barcelona is a logical assumption, since it's known to be a weak model.

However, the problem does seem electrical and I wonder if the roomstat or timer are at fault? The solid green light on the Barcelona indicates that either the roomstat or timer are calling for heat when they shouldn't be, because I've set them to OFF. I'm wondering if the boiler is shutting itself down when it fires because a sensor is saying it doesn't need to run.

Any specific weaknesses known with the CM907 or H27XL?

Your symptoms are more like a motorized valve problem than a boiler prob

Thanks. I've since been learning a bit about motorized valves.

In the upstairs airing cupboard there are two obvious two-port valves. Are there likely to be any more lurking around the house (boiler is downstairs)?

There is a silver honeywell valve on the pipework just before the "primary flow" inlet into the Megaflo. There is also a blue horstmann valve on a thicker pipe. Both have the levers set to Auto.

Ummm...so what can I check? I assume one controls the HW and one the CH? The boiler behaved perfectly when HW was demanded earlier tonight, so I'm guessing the potential issue may lie with the valve controlling the CH (blue one?). It's been mild tonight so no call for CH yet. When CH is demanded should I listen for a noise from the valve? Thanks.

Presumably a valve can be considered to be working if the pipework downstream of it gets hot? I'll check for this tomorrow.

This CH problem is definitely caused by a faulty 2-port zone valve and I've ordered a new head for the Horstmann Z228.

Is this sequence feasible:
Microswitch in the head is faulty and is telling boiler to fire even though no call for heat from stat. Zone valve remains closed.

Boiler fires. Zone valve is closed so flow switch tells boiler to shut down for 3 minute anti-cycle.

To cure the above I lock the lever in the override position. This opens the valve, permitting flow, so the boiler is now happy to run.

To satisfy my curiosity I would appreciate knowing if this is a possibility.