Alpha Boiler fault

[PHILLIPD12]

Hello everyone
I have a Alpha CB24 boiler 11 years old which has a fault which my Plumber who is an Alpha Platinum Installer can not solve & I would be gratefull for any advice.
When the boiler starts due to a call for hot water or central heating it starts the pump & main burner & runs for about 1 minute then switches off as the red overheat fault led comes on,this goes out within seconds & the boiler reignites successfully. the boiler may then stay on until the room thermostat cuts in or the hot tap is turned off. Sometimes the fault cuts the boiler off again before it starts again within seconds.
I am concerned that the boiler will soon switch off & not come on again.
The following items have been replaced..................


Diverter valve, Expansion vessel,
Pressure relief valve, thermisters & the Plate to plate heat exchanger cleaned & there are no leaks anywhere.

 

[Hardy1]

Could be Overheat stat??, has your plumber checked that

 

[rm1]

if the red light comes on and then goes off its self it is going into open circuit check the primary thermistor is given the correct ohms readings when at a certain temperature refer to mi, also check there is no blockage or restriction which will cause it to over heat have you checked the pump?

 

[PHILLIPD12]

Thanks for your replies I will relay them to my Plumber.

 

[Alpha man]

Air pressure switch will be the problem.

 

[PHILLIPD12]

Thanks for that I will let my Plumber know.