On 16 Feb 2014, "harryagain" grunted:


"Lobster" wrote in message
. 236...
I've got a 100mm inline extractor fan in the ensuite; there's a vent
in the
ceiling and it ducts the air up into the roofspace (in corrugated
flexible ducting) and as this is a dormer-style house with a very low
roofline, it passes down under the tiles in 100mm rigid flat ducting
to a grille just below the guttering.

It's really very ineffective though, and while I'm scrabbling around
in the
roof space sorting out my downlighter wiring I thought I might try
fixing this too... I was considering replacing the 100mm fan with a
150mm model; however, realistically there's no easy way to upgrade
the length of fixed 100mm ducting (about 1-1.5m worth) which runs
under the roof tiles. I would replace the flexible stuff in the roof
space with 150mm ducting, probably rigid rather than the flexible
hose.

Question is, would that be worthwhile at all? Would the old 100mm
section, which I can't upgrade, just act as a bottleneck to eliminate
any benefit?

There are two aspects to fans, the volume they shift and the pressure
they develop.
(As with all bladed pumps, centrifugal and axial)
It sounds like your fan does not develop enough pressure.

There are special high pressure fans for long ducts, usually
centrifugal.

Yeah, I think the answer is maybe just to replace it with a 100mm
centrifugal model.

The plot thickens actually: the 150mm fan I was originally thinking of
going for was http://tinyurl.com/ma4xp3c (or
http://www.toolstation.com/shop/Elec...mm%20Part%20L%
20Inline
%20Shower%20Fan%20Kit%20with%20Timer/d190/sd280/p97314); however I
hadn't originally spotted that this doesn't coincide with the standard
ducting sizes which are 100mm or 125mm. Weird!


--
David