Noisy in-line duct fan

22 Jan 2014
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United Kingdom
Hi all,

Not sure if I'm in the correct section or not but here goes anyway...

I have recently replaced and old and noisy "fan in a box" in the loft with one of these and have replaced the horrible looking vent with one of these

This extracts the air from the bathroom just above the shower bath via silver insulated ducting. Since fitting the in-line fan the sound of the air being extracted trough the vent is very loud and quite annoying when trying to enjoy a long hot bath.

I was wondering if I could quieten it down a bit or maybe I have installed it all incorrectly? The fan is midway between the bathroom vent and roof vent, approx 2 meter lengts of ducting to and from the fan. Should the fan be right at the beginning of the run at the bathroom vent, right at the end of the run at the roof vent or is in the middle ok?

I'm thinking of trying to refurbish the old "box fan" as this was much quieter or if anyone else had any ideas on what I else could do

Thanks a lot
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The middle is ideal; the noise is likely due to restricted air flow at the vent - could you not use a vent with more air space? Removing the vent cover would obviously tell you if this is the case in advance.
Thanks Grenage,

It's still quite loud with the center adjustable part totally removed
The fan is listed as being 39db, which isn't loud, so I'd guessing it's the airflow as you say. Is the flexible ducting pulled taut, and is rigid ducting an option?

Rigid ducting does apparently cut down on airflow noise more than you might think - but I wasn't able to use it with my installation.
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Thanks Grenage,

Yes its the actual airflow, not the fan it self. I'm using the existing ducting that was already there although I have moved the new fan away from the old fans position. The flexible ducting isn't taught, it's quite loose and in some places sags a little. I haven't taped it up to the rafters at all, it's just resting on the loft insulation etc. Should I go back and make the ducting as short as possible and make it as taught as I can?

Rigid is an option I guess but I'm trying to keep extra expense to a minimum
Rigid ducting is quite cheap, and I'd certainly be using it if I possible.

As for the flexible ducting, yes you definitely want it taut - the straighter the sides, the less resistance and noise. Ideally the fan is at the highest point, with the duct sloping down on both sides; that reduces condensation pooling. I'd make the ducting as short as is practical.

I know this is probably obvious, but you don't want it so taut that it is likely to get pulled off the grille or fan connections.
Thanks Grenage,

I guess I'll be in the loft for a while again tonight

Thanks for your help
Unfortunately it didn't make any difference what I did so I'm going to have to refit the old "box fan"

But I'm going to move it so it's not hanging from nails in the rafters as as this was where the noise was coming from, I'll hang it from fixings on the breeze block wall a few meters away
Ah, are you saying it's vibration noise, rather than air movement? You can always pad the box mounts with rubber; I actually used mini bungee cords to suspend my fan.
The old boxed fan was vibration noise yes due to it being hung by cord from nails in the rafters, it made quite a noise particularly in our bedroom and I could hear slight motor bearing noise through the vent as I think they are slightly worn. The new one is air movement noise, I think it moves a lot (probably too much) air and so is more noticeable than the old one. This too makes vibration noise when screwed to the rafters
240cm/h is quite a hefty fan. I suppose you'd have to either replace it with a quieter/adjustable fan, or get used to the noise.

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