Combi boiler intermittent fault: Vaillant EcoFIt pure 830 does not respond when HW demanded

[michael-c-s]

Can anyone advise please. Since installation (Jan 2020) there has been an occasional/intermittent problem with this otherwise brilliant boiler: no HW water comes, even when the tap demanding it is the one closest to the boiler. Leave it half an hour, open any HW tap and there is instant response, ie boiler fires up and full normal flow. Related problem: in mid shower, if you turn the HW flow down or mix any CW, it tends to stop delivering HW, and then you can give up that shower as no more HW will come.
Different people have suggested: sticky diverter valve; faulty/stuck impeller. Anyone care to suggest if it's one of these, or something else?
I do hate intermittent faults.....

 

[CBW]

Does the boiler do anything when it’s faulty, eg does it fire up but no hot water is delivered?

 

[michael-c-s]

When it's faulty it does nothing. When it's working it fires instantly.

 

[Johntheo5]

DHW flow switch faulty? Since hot water supply ceases when the flow is reduced, these operate at a flow rate of - 2LPM.

 

[Ltnman]

The boiler will be under warranty if you had it serviced every year so call out Vaillent.

 

[michael-c-s]

Many thanks to all. Calling Vaillant now.