Boiler cutting out every 20 seconds for hot water

[electrospark]

Hi all,

My combi boiler keeps cutting out every 20 to 30 seconds when the hot water is running and then refires and runs again for 20 seconds.the boiler is a Sabre HE (yes I know it's a crap unit). There are no fault lights showing!!
In the past the hot water and heating would cut out and the red fault light would come on but this was a simple fix cleaning out the pressure switch but this time it's different as the heating is working fine.

I'm a sparkie so electrics are no problem, but fault finding on boilers I could do with a little guidance.

I'm sure this is a simple fix of over heating and cutting out..


Thanks

 

[picasso]

Blocked plate heat exchanger on the primary side, nothing wrong with sabre boilers (rebadged vokera) .

 

[electrospark]

Blocked plate heat exchanger on the primary side, nothing wrong with sabre boilers (rebadged vokera) .

Thanks! What's the best way to go about unblocking it??

 

[AGAS]

not testing wires