In article ,
"John" writes:

"Owain" wrote in message
.. .

Isn't it water vapour, rather than steam? I think it's quite pretty,
especially on a frosty day.

Steam is actually invisible (and highly dangerous as a result).

Yes. The plume is actually a fog -- tiny condensed water droplets
suspended in air. Steam is invisible, but often forms a fog at the
boundary where it mixes with colder air. A condensing boiler should
not be emitting any steam -- the flue gases aren't hot enough (even
when it's not operating in condensing mode).

Glow worm have recognised the plume as being a problem and make a kit to
extend the discharge upwards and away from the flue wall exit point.

I don't think - although I could be wrong - the flue gets dangerously hot.
AFAIK it's acceptable to have the flue somewhere someone walks past
provided there's a wire cage over it to prevent direct contact.

The cage is a throwback to flue terminals which got hot enough to brand you.
A cage was to stop this happening so as a result you got a criss-cross wire
pattern burnt into your flesh as a resultg
The requirement is still included in the GSIUR of course

Likewise, required clearances below plastic drain pipes made
me smile, when the flue itself is a plastic drain pipe;-)
I had to go through the rather pointless task of moving the
rainwater pipework when I installed mine.

--
Andrew Gabriel