Vaillant Boiler keeps turning on and off - flashing timer -> solid radiator -> flashing radiator

[tencentral]

I have a Vaillant EcoTech Plus 624 boiler (conventional, not combi) that keeps turning itself on and off during the night (I can hear it), and has continued today. It cycles between:

• Flashing timer and solid radiator:
  

• Just solid radiator:
  
• Just flashing radiator:
  
• Back to flashing timer and solid radiator

I have had a Nest thermostat for the last year and this issue has only arisen last night. This morning I turned the Nest heat mode to Eco and the water to off (so the boiler should be off and not heating anything) - the boiler is still cycling between on and off. Any ideas what's wrong please? Any help would be much appreciated!

Our boiler repair company can't send somebody out for a few days due to everything that's going on, but I'd really like to know if it's worth asking somebody to come out during this lockdown.

 

[CBW]

Conventional system, I’d be looking towards a stuck motorised valve first.

 

[tencentral]

Conventional system, I’d be looking towards a stuck motorised valve first.

Thanks for your reply. The engineer replaced a piece (Danfoss Randall HPA2 2 port valve actuator) - is this what you were referring to? The issue still happens with the boiler coming on when the Nest is all off (eco temp, hot water off, even restarted and reset Nest!)