MVHR drone

[Mattg4321]

I’ve finally got my system up and running and slightly disappointed with the slight droning noise I’m getting from the supply terminals. It’s by no means horrendous, but quite a few people on here say they can’t hear it even with their ear to it. 
 

Perhaps I’m (and also my wife!) sensitive to it, or perhaps it’s the unit I’ve chosen/the silencer I’ve fitted. Would be grateful for your thoughts. 
 

I have the Vent Axia sentinel kinetic fh 

 

With this silencer between both the supply and extract manifolds

 

https://bpcventilation.com/products/quiet-vent-semi-flexible-silencer-range?variant=53736188707142
 

These manifolds

 

https://bpcventilation.com/products/quiet-vent-10point-re?variant=53736193753414&country=GB&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gad_source=1&gad_campaignid=22309731029&gbraid=0AAAAADpzxaWZxxdpXsjssay4R_gcCEWm-&gclid=Cj0KCQjwrojHBhDdARIsAJdEJ_fCPWKRHSNb-UR7UjrVugdzT9dmzVPeRhvT1YwSjs0a52VTXHXolz8aAivpEALw_wcB

 

A double run of 75mm semi rigid ducting to the bedroom. The terminal is just past the foot of the bed and to the side by about 300mm. 
 

Unit is set to 15% overnight and still audible. Although to be honest not a great deal of difference between 15% and 40% boost. The drone stays similar. The air noise becomes audible as you get above 30-35%

 

Edit: To clarify the noise is from all supply terminals. The extract terminals don’t really suffer the problem, or it’s much reduced - which probably makes sense to me. 
 

Thanks 

Edited October 5 by Mattg4321

[JohnMo]

How is the MVHR unit actually mounted and where. Is the MVHR mechanical linked to the manifold by anything other than the silencers? Are they both hard fixed to the same wall for example?

 

It could be vibration noise.

 

Air movement noise generally isn't a droning noise, assuming you and I call a droning noise the same thing.

 

Air house more Ssssssss.

[Mattg4321]

To add to this, just been weighing up my options, other than accepting that’s as good as it gets with this unit…

 

1. Change silencer for something else. Will be awkward because of space, which is a big part of the reason I went flexi to start with. 
 

2. Fit those foam disc baffles where you poke sections out. Problem with this is where? Fitting in the terminal doesn’t seem like it would help much to me - only a gut feeling. 
 

3. Do something with the manifold box, which seems like it could be part of the reason for the noise, or at least an option for a solution. A square box with flat hard sides might be amplifying any sound. 
 

I could pull this out without too much trouble and use the 150mm entry hole for access to line the sides with eggbox acoustic foam? I wonder how much difference this would make as the 150mm inlet from the MVHR is still directly in line with the outputs. I’m not an expert, but guess the sound ‘bounces around’ rather than travelling in a straight line, past the eggbox foam?!!

 

 

[Mattg4321]

5 minutes ago, JohnMo said:

How is the MVHR unit actually mounted and where. Is the MVHR mechanical linked to the manifold by anything other than the silencers? Are they both hard fixed to the same wall for example?

 

It could be vibration noise.

 

Air movement noise generally isn't a droning noise, assuming you and I call a droning noise the same thing.

 

Air house more Ssssssss.

MVHR mounted to ply on stud wall and solid ducting up into the loft via a 90 fitting. Then there’s the flexi silencer and only then is the manifold a few joists away, so it’s almost certainly not going to be vibration I’d say. 
 

I think it’s fan noise. Possibly being amplified by the manifold/ducting? I’m no expert on acoustics though. 
 

I think we’re on the same page re droning noise. The old bathroom extractors were way noisier from an air noise point of view. But any drone wasn’t really noticeable. These were in line units fitted in loft. 

Edited October 5 by Mattg4321

[JohnMo]

3 minutes ago, Mattg4321 said:

2. Fit those foam disc baffles where you poke sections out. Problem with this is where? Fitting in the terminal doesn’t seem like it would help much to me - only a gut feeling

They are just flow setters - so doubt they will help just unbalance current settings.

 

Your plenum manifold is the same as mine so may not be an issue.

[JohnMo]

4 minutes ago, Mattg4321 said:

solid ducting

It could be the solid duct as it goes through the ceiling, the ceiling structure allows it to bridge the flexible element from a vibration perspective. The vibration then gets amplified in the hollow manifold.

 

Put a couple of heavy books on the manifolds if the drone stops that is the issue. The fix may be as easy as some heavy sound deadening on the outside if the manifold - similar to that inside a car door to make it more solid.

[Mattg4321]

9 minutes ago, JohnMo said:

It could be the solid duct as it goes through the ceiling, the ceiling structure allows it to bridge the flexible element from a vibration perspective. The vibration then gets amplified in the hollow manifold.

 

Put a couple of heavy books on the manifolds if the drone stops that is the issue. The fix may be as easy as some heavy sound deadening on the outside if the manifold - similar to that inside a car door to make it more solid.

Thanks, I’ll definitely give that a go. My gut feeling is that it’s fan noise/noise from the unit being transmitted through the ducting though. I don’t think the ducting even touches the plasterboard as it passes through. It’s then just foamed in. 

Edited October 5 by Mattg4321

[Mattg4321]

What silencer do you have @JohnMo?

[JohnMo]

3 hours ago, Mattg4321 said:

What silencer do you have @JohnMo?

I have a similar silencer, but it's 2m long. But all other ducts attached to MVHR unit are foam.

[MPH243]

Do you have solid metal ducting between mvhr and silencer or foam like this 

https://www.epicair.co.uk/products/Ubbink-aerfoam-150mm-2m-pipe-insulated-duct?utm_source=google&utm_medium=cpc&utm_campaign=20379623923&gad_source=1&gad_campaignid=20375261885&gbraid=0AAAAAD8qV0kJRL43zBaZRBZ16PVsqJvqB&gclid=Cj0KCQjwrojHBhDdARIsAJdEJ_cB4QtbWmQTL0iscIrgurD10fWtrsWbSvCQmMconP2pdEfZATfGfoAaAn-MEALw_wcB

 

If metal the foam might help quiet the noise.

Also you could mount the mvhr on rubber washers.

[JohnMo]

46 minutes ago, MPH243 said:

Do you have solid metal ducting between mvhr and silencer or foam like this 

https://www.epicair.co.uk/products/Ubbink-aerfoam-150mm-2m-pipe-insulated-duct?utm_source=google&utm_medium=cpc&utm_campaign=20379623923&gad_source=1&gad_campaignid=20375261885&gbraid=0AAAAAD8qV0kJRL43zBaZRBZ16PVsqJvqB&gclid=Cj0KCQjwrojHBhDdARIsAJdEJ_cB4QtbWmQTL0iscIrgurD10fWtrsWbSvCQmMconP2pdEfZATfGfoAaAn-MEALw_wcB

 

If metal the foam might help quiet the noise.

Also you could mount the mvhr on rubber washers.

I have Ubbink between unit and outside. And solid plastic stub and a couple 45 Deg at MVHR and manifold ends. Silencer is nearer 1m long, now I look at a photo.

 

 

 

[Mike]

6 hours ago, Mattg4321 said:

3. Do something with the manifold box, which seems like it could be part of the reason for the noise, or at least an option for a solution.

One option might be to replace it with the Zehnder Comfowell CW-S 520, which has inbuilt acoustic attenuators (and can also take 10 x 75mm ducts).

[Nick Laslett]

@Mattg4321, you can buy these semi-flexible attenuators direct from Lindab. 
 

https://www.lindab.co.uk/Products/ventilation/Sound-attenuation/circular-attenuators/slfa/?sort=popularity&display=16&page=1

 

Unlike the BPC product, they actually have a spec sheet with the attenuators reduction for different frequencies. The BPC product might be great, but need more details.  

You can get an audio tools app for your iPhone/iPad, which will tell you the frequency you are trying to reduce. There maybe some free ones, but this is the one I recommend. 
 

https://apps.apple.com/gb/app/audiotools-db-sound-audio/id325307477

 

They have a video on their site demonstrating the FFT function. 
 

https://studiosixdigital.com/audiotools-modules-2/acoustic-analysis-modules/fft/

There are many MVHR ducting attenuators out there, but the good ones will provide the frequency attenuation charts. Good site here with multiple examples of different types. 

 

https://en.ventilatory.net/duct-elements/silencers

 

 

 

Good BuildHub thread here about MVHR attenuators.

 

 

Edited October 5 by Nick Laslett

[Nickfromwales]

I've always stayed as far away from rigid ducting as possible, and ALWAYS used the box attenuators.

 

Never a single issue, so I'll carry on with these and semi-rigid radial ducting.

[Nick Laslett]

BPC used to do foam dampers at the plenum. They have something similar here. 
 

https://bpcventilation.com/collections/acoustic-solutions/products/quiet-vent-adjustable-flow-control-damper

 

The previous version was talked about on BuildHub, years ago. 
 

 

 

 

I did find these products, which I liked the look of:

 

https://ventmann.co.uk/products/inline-foam-damper?srsltid=AfmBOoq-8qHhLQ_amR3s3V39sjjcbggu86fNVxxn-ttVyy9-eZ3zq-lI

 

 

Edited October 5 by Nick Laslett

[Nickfromwales]

8 minutes ago, Nick Laslett said:

BPC used to do foam dampers at the plenum. They have something similar here. 
 

https://bpcventilation.com/collections/acoustic-solutions/products/quiet-vent-adjustable-flow-control-damper

 

The previous version was talked about on BuildHub, years ago. 
 

 

 

I did find these products, which I liked the look of:

 

https://ventmann.co.uk/products/inline-foam-damper?srsltid=AfmBOoq-8qHhLQ_amR3s3V39sjjcbggu86fNVxxn-ttVyy9-eZ3zq-lI

 

 

But these are for controlling flow rates, not for acoustic attenuation?

 

The issue seems to be at source, so needs resolving before getting into the distribution ductwork?

[Nick Laslett]

9 minutes ago, Nickfromwales said:

But these are for controlling flow rates, not for acoustic attenuation?

 

The issue seems to be at source, so needs resolving before getting into the distribution ductwork?

I agree. It is quite confusing. @Bitpipe implies in this thread, that he got the foam inserts from BPC and used them at the plenum to reduce sound. But he might have had the old fashioned Vents with no attenuation properties. 
 

Quote

Anyway, I pinged BPC and they sent me a foam insert with removable sections for the plenum and it does work well - not completely eliminated but enough not to be a nuisance anymore.

 

 

[Nickfromwales]

3 minutes ago, Nick Laslett said:

I agree. It is quite confusing

Reducing airflow reduces (attenuates) sound.

 

A bit is lost in translation there imo, and attenuation (true) should begin at the point it's (the offending noise) created, eg at source.

[Mattg4321]

Thanks all. It seems I’m not the first to come to the conclusion the BPC flexi silencers are not good, so it’s either replace that - probably with something homemade due to the awkward position of it. 
 

Or, line the manifold with acoustic foam to see if that can work as a second silencer. Which is probably the easier option, so the one I’ll try first. 

[Nickfromwales]

1 hour ago, Mattg4321 said:

Thanks all. It seems I’m not the first to come to the conclusion the BPC flexi silencers are not good, so it’s either replace that - probably with something homemade due to the awkward position of it. 
 

Or, line the manifold with acoustic foam to see if that can work as a second silencer. Which is probably the easier option, so the one I’ll try first. 

I think it would be fairer to say these aren’t good in general; BPC sell these but so do a lot of other companies / suppliers. 

[dpmiller]

they have their uses. Do you need to silence the actual airflow, or isolate pipe lengths from each other to reduce propogation? Quite different requirements...

[Nickfromwales]

3 hours ago, dpmiller said:

they have their uses. Do you need to silence the actual airflow, or isolate pipe lengths from each other to reduce propogation? Quite different requirements...

If the sound is from the unit, you attenuate at source, but if its transmission of vibration which then manifests itself downstream then a totally different issue to investigate and resolve.

 

@Mattg4321 if the unit is top hung on a wall bracket, pull the bottom of the unit forward and slide a towel up behind it, and see if that makes any difference. 

[Mattg4321]

1 hour ago, Nickfromwales said:

If the sound is from the unit, you attenuate at source, but if its transmission of vibration which then manifests itself downstream then a totally different issue to investigate and resolve.

 

@Mattg4321 if the unit is top hung on a wall bracket, pull the bottom of the unit forward and slide a towel up behind it, and see if that makes any difference. 

Tried that and it made no difference. I think vibration would be dealt with by the flexi silencer anyway. 
 

I’m pretty sure the noise is from the unit and the silencer isn’t dealing with it effectively. The fact I have spiral ducting and a metal box as a manifold aren’t helping either, when some others have silencing material in their manifolds/foam ducting. 
 

I just need to find time to take it apart now

[Nickfromwales]

Can you post some pics so I can make some suggestions? Unit > manifold etc plz. 

[S2D2]

Any recommendations for where to buy a good rigid silencer that could mitigate OPs issue?

 

I thought these did the vibration decoupling between mvhr unit and ducting? https://bpcventilation.com/products/quiet-vent-125mm-150mm-rubber-flexible-circular-duct-connector-2-x-clips