Worth installing 150mm fan with only 100mm ducting?

[Lobster]

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?

Thanks

[bert]

In message , Lobster writes

Monrose do a high powered 100mm fan.

[gremlin_95]

Have you considered a mixed flow fan? They can deliver a good flow rate under high resistance. I have a 100mm Vent Axia one which has about a metre of flexible ductwork on each side and it vents out through a dedicated roof vent tile. It works extremely well, the roof tile creates a lot of resistance due to its design but the fan is still able to deliver a decent flow rate.

[PeterC]

Rigid would, in itself, help, as corrugations cause a lot of drag.

[DerbyBorn]

PeterC wrote in news: [e-mail]:

Corrugations also allow a build up of fluff.

What is meant by "mixed flow"?

[gremlin_95]

"In a mixed flow fan the air flows in both axial and radial direction relative to the shaft. Mixed flow fans develops higher pressures than axial fans."

Read more:

[Capitol]

Is the fan the right way round? Yes, I know it's a stupid question!

[harryagain]

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.

[Brian Gaff]

Assuming you can join them in a sloping kind of way, it will work, is all i can say, as I've seen this sort ogf thing done via a reducer. I guess it depends on how much 'oomph' the fan can deliver! Brian

[Chris J Dixon]

After recommendation here, I bought one of these:

Admittedly not cheap, but I was amazed by how quiet it was, I had to feel the air to be sure it was running. The airflow is far better than the (failed) cheap one it replaced. It can push-fit into the pipework, and is readily removable for cleaning.

Chris

[Lobster]

On 16 Feb 2014, "harryagain" grunted:

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

(or

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!

[sm_jamieson]

+1 Excellent fans, very quiet. That is until you control them with a triac controller, but that is a matter for the other post ! Simon.