In article ,
"IMM" writes:

Extending externally is fine. If the gasses cool and condense even further
then they will just flow back into the boiler. No problem.

IIRC, the Keston allows a max of only 4m of flue draining back
into the boiler. Anything longer than this and you have to add
additional condensate drain-off points. It shows you how to make
them in the boiler instructions.

--
Andrew Gabriel