Alpha 240p problem

[andygmac]

My 10 year old alpha 240p has developed a strange problem, after drawing hot water, or after ch has been on, boiler shuts down as it's ment to but after a few seconds fires back up again and heats up radiators.
During this time no lights are illuminated (except power light). I can only stop this if I move the selector to water only, then back again.
I have recently replaced the diverter valve, flushed the system, and cleaned the dhw exchanger.
Has anyone got any ideas?
Cheers Andy.

 

[Agile]

I expect the problem is associated with the diverter valve repair you did!

See if the diverter valve pin rapidly retracts when the tap is shut off.

If it does then see if the microswitch is not switching off by pushing the lever backwards.


At face value though it would seem to be a phantom demand for CH !

Tony

 

[gas4you]

Have you replaced the microswitches in the correct places and not got them the wrong way round?

 

[andygmac]

The diverter valve pin at the rear (primary flow) takes a while to retract, when the ch goes off, but retracts instantly when hw is turned off. Then it seems for no logical reason, pump starts and primary flow swich activates, then boiler fires up??????

Micro switches both in correct positions.

Diverter valve is relitivly new (8 months old), I took it out yesterday and it is spotless inside and all diaphrams are intact.

 

[Agile]

I was interested in the diverter valve pin by which I mean the one activated by DHW demand!

Tony

 

[andygmac]

Sorry Tony, yes the DHW pin works perfectly, and instantly retracts when DHW demand is stoped. This pin does not move (nor should it) when the boiler starts its phantom goings on.
Micro switch does disengage.
Thanks Andy

 

[andygmac]

May have found the answer. Just got home from work and CH was on (dont know for how long but house was hot!!). I drew some hot water and boiler shut down when I turned off tap. both DHW and Primary flow switches turned off. Then 30 seconds /1 min later pump starts primary flow activates boiler fires up. I pulled the wires off the CH temp sensor and boiler stops Primary flow switches off, if I re-connect temp sensor primary flow activates and boiler fires up.
Could a faulty sensor cause this to happen? If the boiler thinks water is too hot does it pump it arround to dissipate the heat?
For the cost of it I shall probably replace both CH and DHW sensors anyway!
Cheers Andy

 

[Agile]

I am not sure if your boiler has frost protection as it was not so common when that model was designed.

Lets assume the boiler was advanced for its time! If the sensor resistance is too high then it thinks the boiler may freeze and brings on the heating to warm it up to about 10°C.

Disconnecting the sensor is a red herring because the boiler assumes there is a fault and to protect the boiler turns it off.

The sensors should be 13k cold but measure the resistance of both and compare them.

To check if this is the cause, you could leave the boiler turned off and put a 13k resistor in place of the sensor and see if the boiler still fires up.

Tony

 

[andygmac]

Thanks Tony.
Yes this boiler does have a frost stat built into the main pcb, I will try the test you advised.
Another problem this morning, when drawing hot water, the boiler fires up for about 30 seconds (ish) then goes out for about 10 seconds, fires up 30 ish seconds goes out etc. this continues for about 4 or 5 times then it works fine giving piping hot water continiously.
Sorry I failed to mention in my first post that this problem was why I flushed the DHW exchanger (talk about not giving all the info ) but that side of the boiler worked fine after the flushing, (until this morning) so I thought (dangerous!) that I had fixed that prob .
Thanks for replies.
Will post when fixed
Andy