s plan overflowing

[splan1]

hi can you help i have relaced my danesmoor 15/19 with a danesmoor 20/25 and relocated boiler to a different location , i now have a problem with system overflowing thou the vent pipe after approx 5 mins of running could this be the result of to much air in the system .
please not the boiler was replaced with a biger output as the house has been enlarged. any ideas.

 

[dontknowitall]

Did you move the circulating pump or change any of the pipework significantly?

 

[SimonG]

Yes, you haven't done it right.

When does it do it, hot water only or heating? Where are the open vent and cold feed piped in? Was it S plan before? Have you moved the pump/s?