In article ,
writes:

What I wonder about condensing boilers is whether the low temprerature plume
can spread legionella. Can water sit anywhere at a suitable temperature to
allow growth of the bug. What precautions need to be taken to prevent this.
Has a risk assessment been done?

My Keston, which ran at around 45C flow through most of the winter,
did manage to grow something a bit jellyfish like in the bottom of
the heat exchanger (which would have been slightly cooler than 45C).
This then blocked the condensate drain causing the heat exchanger
to start filling up until it was making quite a gurgling noise and
failed to light. I didn't conclusively prove if the jellyfish like
thing was animal, vegetable, or mineral and it was broken into
small pieces by the time I got it out.

--
Andrew Gabriel