Alpha 240P problem

[2tired2sleep]

About a month ago, I had been having several problems with my Alpha 240P boiler.
Initially, a relay on the main board was sticking and not allowing the boiler to fire up. I replaced the board and this resolved the problem.
However, a new problem has arisen. When the heating comes on in the morning, the boiler will not run for more than about 20 seconds. However, if I run the hot water for about 30 seconds, the heating starts and continues to run perfectly for the remainder of the day. I have replaced the complete diverter valve assembly hoping it would solve the problem but it persists.
Anybody have any suggestions?

 

[ChrisR]

When it goes off, is that ignition failure, overheat , or something else?

Could be that the pump is sometimes slow to start. It has to be enough for the flow switch to operate of course, but I suppose if it were slow, the boiler could be overheating. The heat extraction from the tap water is higher, which could be enough to keep the temp down until the pump decides to work properly.
That's about all I can think of!
The only parts which are distinct between CH and HW are the switches on the DV and the temp sensors.
You could check that the hw flow switch isn't sticking, and that its resistances are near zero and infinite in both states.
You could swap the temp sensors, as they're the same, to see what happens.

Don't remember hearing of this fault before so will have to contain my excitement until you tell us what it was!

 

[Agile]

I would be checking the gas valve settings and flame sensor position!

But you will need a CORGI to do that!

Tony

 

[2tired2sleep]

All seems OK now, it was the CH temperature sensor.
Thanks for the excellent responses which guided me to a successful outcome.
very much appreciated.

 

[Agile]

The sensor would not be the logical cause of what you first described!

Tony

 

[2tired2sleep]

Further examination of the failed sensor showed that the plastic moulding had a severe crack/moulding fault and was intermittently functioning.
In fact, it fell apart during examination and did reveal a moulding fissure.
Never mind, all is now well.