Alpha cb28 boiler problem

[loyd30]

Hi all
i have a problem with my boiler alpha cb28.hot water runs fine most of the time but does occasionally run cold. The central heating after it has been on for about 1 hr it shuts down with a solid red light & yellow light flashing although the fan is still running & stays like this for about 1/4 hr then starts itself up again & the red light goes off untill the next time, which could be 1/2hr or sometimes its ok for hrs, i have drained & refilled the system,the pump is working & clean & all rads get hot. we do live in a hard water area & boiler is about 13yr old but is serviced anually.it has had a diverter valve & flow switch fitted few years ago any advice appriciated.
regards loyd

 

[ianmcd]

either your pump is sticking or the primary flow switch is

 

[loyd30]

hi & thanks do you mean the low water pressure switch or the dhw flow switch as dhw flow switch has been replaced before

 

[ianmcd]

low water pressure switch ,it probably just needs cleaning, but your pump could be sticking, both would give you that fault