Vokera head scratcher

[GrahamM]

Called to an 18 month old Vision 30C earlier with poor hot water. Hot water luke warm at best showing a max of 35 deg on boiler display. If slow tap to little more than a trickle the temp gets up around 60 deg eventually. No issues with heating.
Boiler fitted to an old back boiler heating system and central heating water appears clean.
To rule out div valve I isolated flow/return. Gas rate, combustion etc is spot on.
Was thinking towards a blockage of some sort.
Boiler is in warranty and will be passed to Vokera anyway.
Any thought?

 

[Ch4 plumbing]

What's the flow rate through the tap your testing the temp at and what is the incoming cold main temp?

 

[GrahamM]

Restricted it down to 10 L/min flow and it is tepid water. Turn tap down to about 2 L/min and it is hot.

 

[thomas donovan]

could be debris in plate exchanger resulting in v. slow transfer of heat.
could be super fast forming scale in plate exchanger.
could be divertor valve not operating properly.
check ch flow pipe while hot water running if its really hot that would indicate a faulty divertor