Vokera Compact 25he Cuts out on heating?

[Kewat66]

Hi All,
i have vokera compact 25 that was not working at all. Had a good look at it and it ran with the cover off but not on. Diagnosed it as a Venturi. Replaced this and thought problem solved. Now the boiler works great on DHW but lights for about 10 seconds, then stops for 3 minutes, then lights & so on.
the pump seems to be working as I have checked this with magnetic wheel so it doesn't look like the pump.
temp remains the same on flow and return though?
vokera technical said there is a blockage on the system but boiler looks fine? From cold the boiler runs for about 3-5 mins then goes back into this cycle?
Surely if there was a blockage it wouldn't run from cold for a few minutes? Can the pump look as if it is running but actually not be?
any ideas on what I am missing?

thanks
kewat66

 

[quality]

if it works on hot water but cuts out on heating that rules the pump out

 

[stevetheplumber]

check the min settings could be it runs in hw as its full flame most of the time but heating cuts out when flame modulates down