From the flueing question in General Plumbing Forum.

[AWheating]

Re: Outside flue hole not filled in by plumber

If you plaster the inside and put rubber seal on external what are the actual odds of a poc issue??

another concern would be water ingress, if the rubber seal didnt make a fully water tights seal you could get damp problems etc, more of a problem now boiler flues rise upwards .

 

[Ermintrude]

Re: Outside flue hole not filled in by plumber

another concern would be water ingress, if the rubber seal didnt make a fully water tights seal you could get damp problems etc, more of a problem now boiler flues rise upwards .

Kudos good point !

 

[Dotty]

Thank god that worked!