Extractor fan duct noise

Have finally managed to replace our old, ceiling mounted extractor fans.  They were noisy and completely ineffective, and the flexible duct in the loft filled with water.  Now replaced with lovely new inline extractor fans with rigid ducting.  Fans now actually work at clearing the water vapour in the bathrooms.

But… they are quite noisy still. Here’s the thing though, it’s not the fans themselves.  The fan are really quiet, and even in the loft you can hardly hear them.  What’s noisy, I think, is the air moving through the duct. A kind of whooshing noise all the time.

I don’t want to block the ducts to reduce airflow as that would seem to defeat the point.  Has anyone any experience of an inline duct silencer?  Or any other ideas?  It’s really frustrating after all the work put it…

Is it a long run? I don’t hear a whoosing sound with mine but it’s a short and near vertical run. Where is it venting out (wall, roof?) and is that generating the noise rather than the ducting itself?

I found with our extractor there were two sources of the noise – the vibration through the joists where it had been hard-screwed in, and the noise of air moving through the flexible duct pipe.

I have solved the vibration issue by using conical isolating feet on the bottom of the fan & some 3-D printed feet that prevent it from moving – so there is no hard contact between the fan & the joists now. That made a massive difference.
Are the fans hard-screwed to the joists? You could see if there is any noise-reduction to be had there by sitting the fan on a piece of soft foam temporarily to isolate it? I was surprised at how quiet our fan was while in the loft, but how much mechanical vibration was getting transmitted into the joist which seemed to present itself, in the bathroom.

There is still a whooshing noise from the duct itself. I think the only way to solve this is to swap it for rigid ducting. I have considered doing this, but the path to the outlet in the soffit is not very easy to access, & it doesn’t bother me enough to get on with it.
Maybe you could swap out the flexible hoses for the rigid stuff – I imagine the concertina’d nature of the flexible ducting adds to quite a lot of noise, and also reduces performance a bit.

Hmm. Thanks for your thoughts.  They are hard fixed to the joists, so I could try some isolation feet.  However, it definitely sounds like most of the noise is airflow.

It is rigid ducts.  I think this might make it worse than flexible?   Flexible can absorb sound whereas it’s a long straight, slightly inclined run of rigid (about 2m to the fan and 2m beyond the fan).  It runs to the gable wall.  I feel like this might set up resonance inside the duct making the noise, hence wondering about silencer…

Ah, sorry. I missed your comment about rigid ducting.

A silencer might be worth a try – relatively cheap & simple to install.
I guess it’s complete runs of ducting, so hard to ‘break into’ and try a few different things without cutting the pipe?

You could also try attaching sticky-backed closed cell foam to the outside of the ducting, to dampen any vibration of the pipe itself, rather than a resonance within the pipe?
You can get various thicknesses of closed cell foam pretty cheap from ebay & that would be quite a simple thing to try out.

Where does the ducting sit? Is it in free air between the fan unit & the inlet/outlet points or does it sit on joists? If the latter, you might need to put some absorbent material at contact points to reduce vibration transmission?

Is the ducting already insulated to stop condensation?

If the noise is coming from the rigid ducting it will be caused by a combination of air speed and resonance i.e. the air speed is causing the rigid duct to resonate or the motor is creating resonance in the joists.

My first attempt would be to reduce the fan speed – that will slow the air down and should reduce both sources of the noise.

All other devices, including silencers, are just doing the same thing (slowing the air speed) via a different mechanism. They may reduce the air noise in the duct but they can’t reduce the motor resonance.

By slowing down the fan you can also save some money on the electric bill!

Is the whistle coming from any joints? Could either be air being forced through the joints whistling of just holes let the sound out. If the sound is noisier near a joint try taping it to seal it (5cm insulation tape should do). Otherwise lagging the pipe to deaden the sound may help (will also reduce condensation potential in the pipe).

Diameter of ducting makes a difference. As does shape. As does routing.

Diameter – as big as possible.

Shape – round is better than square

Routing – as smooth as possible with as few kinks and bends.

We had problems with narrow letter box ducting on top of a cupboard to make it less visible. Changed it to large diameter round ducting and made a massive difference. Ultimately you are channelling air at speed down a narrowing and into a pipe using a motor and can. It won’t be silent but there are ways of making it quieter.

Thanks all.  Sounds like a trip to the loft to check things out a bit more closely… to answer your questions:

• It’s round, rigid ducting, straight run from vent to fan (About 3m)
• The ducting runs under the loft insulation, which should damp a bit of vibration
• The joints are duct taped together.  But not too hard to separate to install a silencer.
• The motor has 3 speeds – it’s currently on the lowest speed which has reduced the sound quite a lot.

My hunch is that it’s the air resonating in the duct, like an organ pipe, rather than the pipe itself.  That combined with, as you say @solarider – air entering a narrow opening causing flow noise.

I just wanted some thoughts before I bite the bullet and get a silencer.  They look like £30-£50, so more than just a try-it-and-see kind of thing:

https://www.amazon.co.uk/Blauberg-UK-Solid-Circular-Silencer/dp/B06XWCZ7TD

If you still have the old ducting, you could try a section of flexible ducting at the end closest to the bathroom ceiling vent. I.e. bathroom ceiling vent into flexible pipe then rigid pipe. Similar to a silencer, you’d be adding a section where the sides of the pipe are more absorbent/diffractive closest to where the sound is emitted.

But I did the same as you and went from a ceiling fan with flexible duct to inline fan with rigid duct. Was going to switch in a section of flexible duct because of the sound. However we just got used to it. It was much quieter than the in-room fan it replaced.

Just in case anyone was desparately hanging on to this, or more likely searches and finds it…the answer was inline duct silencer.  Made a MASSIVE difference.  Can barely hear the fan running now.  Amazing!

I am having a similar issue having “upgraded” to a rigid duct run. In addition, I can hear the fan blades hitting droplets of moisture that build up in the fan body of the inline fan – where the fan in actually mounted the diameter increases, allowing water to pool. It only takes a mm of water to make contact with the fan blade and make an irregular ticking sound.

The fan is the same unit as before – Manrose MF100T – and I never previously heard the droplet/fan noise. Perhaps the flexi duct didn’t transmit the sound, or the water was condensing out on the inner surface of the duct before reaching the fan.

The fan is extracting well. At night, a torch shows a huge plume of water vapour coming out into the garden, and the bathroom clears well. The only other change is that I have added a Humidistat to trigger the fan, so the fan does not come on immediately when the light is switched on.

This might mean that the duct stays colder until steamy air (triggered by the Humidistat) is drawn through the duct, contributing to the in-fan condensation issues.

 They are hard fixed to the joists

Yeah do this and it will sound like a Chinook is landing on your roof. I have tried a couple of DIY hacks to solve this, last time I needed to raise the fan because of new loft insulation going in, I screwed two batons to the rafters, screwed several hooks in the batons to support several elasticated cords stretched between them. The fan is on a rectangle of plywood sat on top of the mesh of cords and attached with clips. Works very well no vibration noise whatsoever.

As for the air sucking noise… the design of the vent has a bearing on this and also the capacity of the fan to move air.. ours is pretty noticeable but I like it in an ASMR kind of way