According to M.Joshi :

Chris Lewis Wrote:

Probably condensation of water vapor in the exhaust. This is probably
entirely normal - especially so if the unit is high efficiency
and the flue is plastic pipe.

Call the installer and ask.

I thought that the condensed water vapour was supposed to drain via the
two drainage pipes from the boiler and not the flue?

Much of it does, however, they don't extract _all_ of the water
vapor, and the exhaust is continues to cool as it goes thru the pipe
and hits outside yet colder air. Meaning condensation continues
to occur right to the flue outlet.

Basically, whereever the exhaust is colder than the dew point
w.r.t. humidity, condensation will occur. The drainage lines are
for those portions of the exhaust path _inside_ the equipment,
the plastic flue acts as drainage for the portion of the exhaust
path outside of the equipment (path from the equipment to outside).
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.