new he boiler fitted onto boilermate 2 system

hi i hope some one can help me. i am a gas safe registered fitted and have just removed a potterton suprima boiler and fitted a open vent 16kw he viessmann boiler to the existing pipe work and yes it is not working viessmann say there is air in the boiler and that is why it is going off after 30 seconds but have properly vented the boiler and the boilermate have flushed all the pipe work even fitted both new pumps. my real question is can u fit a modern boiler with flow sensors etc to a boilermate system ?. we got it all to work when we shorted out the flow sensor on the boiler and put it into installer mode ( flat out, top temp ) but it still kept shutting down on overheat but as soon as u put the boiler on to normal setting and the flow sensor back together nothing works at all. its as if there is not enough flow to fire the boiler. do u think an extra pump on the flow from the boiler would be the answer ?

How did you clean the system?
Why or what reason was the new boiler fitted?
What have you done wiring wise?
Mike

hi mike thanks for your reply sorry but i was out all night, the old boiler yes it was a suprima was cutting out all the time we thought it was a pcb fault and the customer had already had fitted 2 so he opted for a new boiler but to be honest this new boiler is doing just the same as the old one did. we flushed all the pipe work out with mains pressure sure nothing is blocked anymore the cold feed was and so was the expansion i think there is just not enough flow where the pipes go into the boilermate mate they are not that big look like 18mm not 22mm.
the wiring is the same as before ie live neutral and earth to power the new boiler and switched live from the boiler mate to fire the boiler which is all working correctly u are to far away for a call out but are willing to pay for your time via a phone call

thanks Ian

So is the boiler pump working?
Is it still wired via the BM II ?
The flow and return from the boiler to a BM II are either 22mm or 28 mm for the bigger units.
Have you removed the connections and checked for a restriction at the unit connections / pipe work?
I an in Norwich.
Mike

Is the boiler pump being powered via the boiler?
Or the BM II ?
Mike