I will need to put an extractor fan in the bathroom before I finish it.
The 'obvious' route will go through a stretch of (uninsulated)
roofspace for about 2-3m before going through a plastic slate vent.
This has me wondering if there could be a problem with condensation in
the ducting and, if so, how to deal with it.
I was thinking a downward slope all the way to the vent but this will
inevitably form a 'U-bend' at the junction. Will this cause a problem
in reality (ie I'm overthinking it) or should I plan for a less obvious
route, involving a lot of scaffolding?
I do not think it's worth worrying about - you tend to either slope the
pipe down to the vent outlet or down to the extract inlet.
What you do NOT want is a place water can pool and go stagnant and be a
breeding ground for germs.
Many places vent upwards to the roof - the amount of water that might
run back to the fan is miniscule and not really a problem.
I guess my thinking was that most extractors only take the
air/vapour/etc across the wall, so there's less opportunity for much to
condense. In this case, a couple of metres of cold pipe could be quite
an effective condenser. With an inevitable dip at the end of the pipe,
it could all collect there. I was thinking more of the potential for
leaks onto the ceiling below than anything else, as this sort of ducting
doesn't seem to seal perfectly.
Since Bill's experience is that it doesn't caus a problem, presumably
anything that collects evaporates eventually, one way or another.
I did same sort of thing in the old house, ran along in the loft joined
to a vertical bit of soil pipe to vent through the roof. Fan was inline
mounted in the loft. I arranged it so the duct exiting the fan dipped
down a bit before joining to the veritcal so any condensation didn't run
down into the fan.
I decides that if there was condensations in the pipe it wouldn't be
enough to worry about, and indeed there never was a problem with it
if you are worried you about it, you could do it the proper way and use
a condensation trap. Covering the duct with some insualtion would help
avoid it as well I guess
I had this very problem.
It was from the shower cubicle.
It depends on the weather. If it's cold outside, it's worse.
I put in a short vertical bit of pipe and then a downward slope to an outlet
in the gable end of the building so the condensate runs outside. A bit of
insulation on the vertical bit of pipe.
I used 110mm drain pipe.
The alternative is some sort of drain on the pipe to catch the condensate
(eg make a deliberate low point) and conduct it away to somewhere.
On Thursday, January 29, 2015 at 8:25:36 PM UTC, GMM wrote:
I know someone who gets condensation dripping from their fan ceiling vent,
so it can happen. When I temporarily fitted the fan to test things out, the
re was quite a lot of condensation in the pipe above the bathroom ceiling b
efore the inline fan, just from the air in the room - but at that time ther
e was no insulation around the duct. Anyway it looked as if that could prod
uce a few drips from the ceiling vent. So for peace of mind I put in a cond
I suspect in reality if the fan runs long enough it will dry out the pipe,
but if the fan was switched off when the exiting air is still damp there co
uld be an issue (spot the pun !?).
Look at the room size and air changes and make sure you get the right
fan for the job.
Vent Axia used to have a calculation/design brochure for fan and duct
sizing. I used this for both main bathroom and small downstairs toilet
and resulting extraction works well in both (toilet has long discharge
In en-suite shower there is only a through wall axial fan (in the
property when I bought it) which is cr@p. It is difficult to swap due
to tiling and tile availability, cable route etc so has been replaced 3
times with similar cheap but compatible model.
I still believe it's worth paying Xpelair level prices rather than going
for shed quality, if you want to do the job once and forget it for a
good number of years. The Xpelair fan I got for toilet IIRC cost about
£50 but is decent quality and came with 5 year guarantee.
BES supply plastic duct with elbows etc. suitable for air extraction.
This is lighter and more managable than 110mm drain piping and is
available in sizes which match fan outlets e. g. 100mm o/dia.
+1 on BES - they do have some very weird fittings that can be useful in
make very decent fans IME - I have a 100mm axial in line and it's been
It may or may not cause you problems. I have seen it cause problems several
However the simple solution - one that is approved for NHBC new builds is
It will cost you £5 to £6 per metre but is the the correct stuff to use
IMHO. eBay will probably sell per metre cut lengths.
Thanks Adam. Never noticed it before but now you mention it, it's quite
readily available by the look of things. By the time I do this job and
the long run for the vent from the downstairs bog I might even use up a
10m roll from Screwfix.
If the long run for the downstairs shower does not require insulated ducting
then I would suggest using rigid/smooth ducting if possible - either
rectangular or round - as this will give a better air flow. Just ensure that
there is a small fall in the ducting from the fan to the vent.
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