Open vented system - Boiler turns on and then loses heating pressure?

[MattStorey]

Hi guys,

We have an open vented heating system and a Worcester Bosch greenstar boiler. Recently there is little to no heat getting to all radiatiors.

It seems that when we turn on the nest heating the boiler starts whirring as expected as then dies down quite fast.

I cleaned the magna filter earlier and checked the pump and motorised valves are working by the immersion heater tank.

Does anyone have any thoughts? I'm quite ignorant in the ways of solving these issues but am trying to learn.
Attached a video of the whirring dying down towards the end.

We do have microbore pipes so if suspect is probably sludge/air block.

Thanks,
Matt

 

[Deleted member 120897]

More likely to be pump or motorised valve failure

 

[MattStorey]

More likely to be pump or motorised valve failure

Thanks for your response, they're dreyton valves and have tried checking the operation and they are turning the valves 45 degrees as expected on turning the heating on and off. The pump was replaced just last year but I will check that again