Alpha CB28 not firing

[cmhicks]

Hi, I have an Alpha CB28 combi, which has gone wrong for the umpteenth time in its short life... When the boiler starts, everything seems normal (the fan starts, and the pump starts) but instead of the gas valve opening and the igniter spark, I get both indicator lights flashing together.

This happens exactly the same whether I have the boiler set to provide heating, or whether it is set for DHW only, but in the latter case the startup sequence only starts when I open a hot water tap, obviously. It stops in the same place, with the same pattern of indicator lights (both flashing together) in either case.

Looking at the troubleshooting guide, this seems to point the finger at the airflow pressure switch. However, I have checked this and it definitely works (ie the switch definitely opens and closes - measured by an electrical continuity test - depending on whether the fan is running).

Another suspect is the overheat thermostat. The manual is very unclear about whether this switch should be normally open, and close when an overheat event occurs, or whether it is normally closed and opens in the event of an overheat. I would have thought the latter would be safer, and the switch is in fact closed when cold. Is this right?

So, anybody have any pointers where I should look next.

Thanks in advance,

Chris
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[nickso]

its probably the pcb if its not the OH stat. ive seen this a few times, sometimes happens intermittantly , makes you think the OHS is faulty but put a new one on and it still doesnt work, its been the pcb everytime for me.

 

[Paul Barker]

don't think its oh stat as fault in that is led A flashing alone.

Not sure you have properly tested fan sense tubes or aps.

Someone corgi registered would clean fan blades lube motor bearings clean out sense tubes where they emerge in cowling (common to get a moths wing across them).

To see if the problem lies there wait for fan to start then to stop then to start again and at that moment bridge the aps leeds. If boiler goes on to ignition it is as I stated above do all the service points and then start ordering parts staring withcheapest ( aps).

Extremely rare to need a pcb for that boiler needed as much as a rocking horse **** shovel.

 

[nickso]

paul i agree its rare to need a pcb for them, but the 4 i have put on have all been for a similar fault, aps or ohs fault (sometimes both) shown by pcb's and its always been the pcb at fault once the components were tested.

i hope the guy takes your advice first though.

 

[Paul Barker]

Ok i will store that in the knowledge bank.

 

[cmhicks]

I thought the symptoms seemed electrical, so, being a professional electrical/electronic engineer, I had a good look at the PCB. I found cracked solder joints on two of the connectors - the one to the gas valve, and the one to the primary pressure switch. Once these connections were remade, the boiler works perfectly. The damage had probably been caused by heavy-handed removal and refitting of the wires to these connectors by various people in the past.

The boiler now works perfectly, but it is due for a service, so I'll get that done. I also feel duty-bound to point out the hazards of doing electrical work you are not competent to do, much the same as with gas.

Thanks for the advice I received here.

Christopher Hicks

 

[nickso]

good stuff, ive never found any dry joints on the boards ive changed.

edit, i meant to say....


ive never found any dry joints on the alpha boards ive changed.

 

[Paul Barker]

I have to agree op that maybe 15% of boards just need the dry joints sorted out another 25% need a relay the rest need a new board.

But it is the first time I have heard of dry joints on an Alpha board must have been very rough handling.