Noisy Cooker Hood

[Deleted member 65149]

I have one of those very stylish angled cooker hoods (90cm Elica Black Shire) that came with our new kitchen installed a couple of years ago. I was warned that it was noisier than the standard horizontal hood because it needed to be higher powered, but I didn't realise how much noisier it would be. I thought we'd get used to it in time. After a few months I dampened some of the vibration generated noise by fitting some adhesive car sound deadening material to the inside of the motor/fan casing. It improved a bit, but not a great deal. Since then we've lived with the noise, only using the extractor when really necessary.

But instead of us getting used to it, it's got too much. So today I looked for possible ways to make it quieter, including possibly installing an in-line extractor in the ducting that goes through to the roof above the kitchen. But on a plumbing forum I discovered that the size of the ducting can make a difference to the noise levels. This made me read up the hood's installation instructions that within the smaller print said not to use ducting smaller than the 125mm outlet on top of the hood assembly. I gingerly clambered into the small loft space above the kitchen and saw that about 1.5m of flexible duct had been used between the hood outlet and the roof vent. It's 100mm diameter.

I thought that the difference between 100 and 125mm can't be enough to make a noticeable difference to the noise level, but then cast my mind back to the fluid dynamics I did at school about 60 years ago. What was the relationship between tube diameter and fluid flow? Mr Google came to my memory jogging aid with Poiseuille's equation. Plugging in the figures told me that dropping from a 125mm duct to a 100mm duct increases the flow resistance by nearly 2.5 times!!! That could easily make the motor and fans have to work a lot harder so noisier. The interference in a flexible duct can’t help either. When my currently aching back and Covid restrictions permit, I’ll be swapping the 100mm Flexi duct for 125mm smooth plastic pipe.

Hopefully that'll work. Any other realistic suggestions will be appreciated.

 

[W1ghty]

Does the noise increase as you up the fan speed setting ?
Is it possible just to disconnect the ducting for a few mins to see if the fan is quiet with no restriction ?

 

[190]

Mr Google came to my memory jogging aid with Poiseuille's equation. Plugging in the figures told me that dropping from a 125mm duct to a 100mm duct increases the flow resistance by nearly 2.5 times!!!

If the 125mm duct doesn't do the trick and you need to use an inline fan make sure to use a centrifugal fan or a so called mixed flow fan. Simple axial fans typically used in bathroom extracts are hopeless at pushing air through any length of ducting. A centrifugal fan which what should be in a cooker hood is far more efficient. Mixed flow fans are a cross between the two and work well with ducting.

 

[MrGreedy]

This is going to be interesting. I'm currently designing/planning a kitchen install, and a decent extractor 90cm is a must. I have been amazed at the different flow rates and noise ratings.

Is the predominant noise issue with fan blade, motor, or tunnel/ducting noise? I'm assuming fan blade.

Could you even go to 134mm or 150mm ducting?


This 90cm Matrix cooker hood had a max extraction of 350 cubic metres an hour and a max noise of 69dB

This 90cm CDA hood is 391 CM/Hr and 60dB on max

This Bosch 90cm cooker hood is over 500 metres an hour, but only 48dB!

There aren't any Neff units to compete with that Bosch on the noise front.

 

[Deleted member 65149]

Does the noise increase as you up the fan speed setting ?
Is it possible just to disconnect the ducting for a few mins to see if the fan is quiet with no restriction ?

There’s only a minor increase in noise at higher speed settings. But the hood’s spec does suggest that that’s the case. I’m going to try disconnecting the ducting but it will have to be up in the loft because disconnecting it in the kitchen will just add to the noise with air flow too close to the motor. I’m having back problems at the moment so don’t yet fancy crawling through the loft to do that.

 

[Deleted member 65149]

This is going to be interesting. I'm currently designing/planning a kitchen install, and a decent extractor 90cm is a must. I have been amazed at the different flow rates and noise ratings.

Is the predominant noise issue with fan blade, motor, or tunnel/ducting noise? I'm assuming fan blade.

Could you even go to 134mm or 150mm ducting?


This 90cm Matrix cooker hood had a max extraction of 350 cubic metres an hour and a max noise of 69dB

This 90cm CDA hood is 391 CM/Hr and 60dB on max

This Bosch 90cm cooker hood is over 500 metres an hour, but only 48dB!

There aren't any Neff units to compete with that Bosch on the noise front.

I don’t know where the predominant noise is from. It may be the fan or it may be the motor - but with the fan(s) actually mounted directly on the motor I would have to dismantle them to see. But even that wouldn’t be a fair assessment because I suspect the smaller ducting is causing increased resistance to the airflow and thus making the motor/fan assembly work harder.

I have wondered about going for even larger ducting, but that would mean adaptors at each end which in themselves would probably disrupt the airflow.

The hoods you’re looking at are the horizontal ones that don’t need such a powerful motor and fan assembly, so they’re usually quieter. But having said that, from my experience it’s well worth checking the specs for noise levels before you buy.

 

[Deleted member 65149]

If the 125mm duct doesn't do the trick and you need to use an inline fan make sure to use a centrifugal fan or a so called mixed flow fan. Simple axial fans typically used in bathroom extracts are hopeless at pushing air through any length of ducting. A centrifugal fan which what should be in a cooker hood is far more efficient. Mixed flow fans are a cross between the two and work well with ducting.

Excellent advice, thanks. I had wondered how to do it with inline fans because the only ones I saw from a quick look around were for bathrooms.

 

[MrGreedy]

Sorry, yes must have skimmed over that bit about being angled.

It can't help that the 'noise' from the fan or motor is more directly pointing towards the room.

 

[W1ghty]

I wouldn’t make any judgement until you disconnect the ducting in the roof . If it doesn’t go quiet then, you know the fault isn’t the ducting

 

[Deleted member 113659]

because the only ones I saw from a quick look around were for bathrooms.

That’s because they’re designed for bathrooms. Just moving/pulling clean moisture/air. Regardless of any filters in place they’ll be a certain degree of grease/grime from cooking. Not at all good for an in-line fan.

 

[Deleted member 65149]

That’s because they’re designed for bathrooms. Just moving/pulling clean moisture/air. Regardless of any filters in place they’ll be a certain degree of grease/grime from cooking. Not at all good for an in-line fan.

Thanks Ant. I did wonder about the grease issue too. Quite a bit of grease gathers in the motor/fan housing with the existing arrangement. All I need to do is lift the glass panel and remove the filter sheet. While that’s draining after washing it I can relatively easily wipe up the grease inside the unit with disposable kitchen towels. Getting up into the loft every month or two to do the job would be a thorough PITA.

 

[190]

I had wondered how to do it with inline fans because the only ones I saw from a quick look around were for bathrooms.

Mixed flow fans can be used for bathroom extract.

To clarify the difference, an in line axial fan will look like a simple tube with a propeller type fan blade.

A mixed flow fan will have a bulge in the middle and the fan blade will look like it can produce a centrifugal effect.

I have an inline axial fan in one bathroom and a mixed flow fan in another. The difference in performance is enormous. Put enough ducting on an axial fan and will will hardly flow any air at all. The mixed flow fan has a much flatter performance curve and will maintain a good flow rate through longer lengths of ducting.

 

[Deleted member 113659]

Is it possible to look around for a new extractor? There’s some awesome ones available and all are advertised with m3 extraction and Db’s.
Ours does 750m3 ph and has a 6” outlet and is not too bad noise wise.

Silly question but guessing cooker is not on an outside wall?

 

[Deleted member 65149]

Is it possible to look around for a new extractor? There’s some awesome ones available and all are advertised with m3 extraction and Db’s.
Ours does 750m3 ph and has a 6” outlet and is not too bad noise wise.

Silly question but guessing cooker is not on an outside wall?

A new extractor was one of my thoughts.

The cooker is on an outside wall so extracting that route is possible. I‘d just prefer not to be knocking holes through a first floor wall when a reasonably easy route is ducting to a roof vent. I’m getting too old to be climbing ladders!

 

[Wagon]

We have an angled Neff one through ducting , its not noisy
Neff

 

[Deleted member 113659]

A new extractor was one of my thoughts.

The cooker is on an outside wall so extracting that route is possible. I‘d just prefer not to be knocking holes through a first floor wall when a reasonably easy route is ducting to a roof vent. I’m getting too old to be climbing ladders!

Tim. Please consider core drilling wall. The extractor will be much much more efficient the shorter the route. So through the wall is 300-450mm. The motor will be under less strain. Best looking for an extractor with 150 outlet.

if you haven’t got core drill and diamond cores then ask someone who has. 10 minute at most for a lovely neat hole. I promise you’ll notice the difference immediately.