150mm cooker hood duct, is a reducer very noisy? + duct material

[TofuSpaceship]

Hi all,

we have a second hand Britannia cooker hood which comes with an outlet which is 150mm diameter.
The external diameter of the duct is 160/162mm and we are struggling to find someone to core it for us, as hire places usually go only up to 6" in size (150mm).
Industrial corers can do it but come out on a call-out charge of 250/300 quid.

I have heard from someone that also has a cooker hood with 150mm and used a reducer to 125mm that the noise is unbearable and the cooker hood is now not used most of the times!

Is that quite true?

Further, we bought a 150mm kit from Wickes which is flexi aluminium. I'm aware that flexi ducts are to be avoided, does this apply only to plastic flexi or alu flexi too? Can a rigid plastic duct can be used then, such as
https://www.screwfix.com/p/manrose-150mm-round-ducting-0-35m/686gy

 

[jj4091]

Never heard of using a reducer causing an unbearable noise. This is the company I always use for Flexi or rigid ducting depending on the applications. They sell all the different types & all the different accessories you could need https://www.tlc-direct.co.uk/Main_Index/Ventilation_Index/Ducting_2/index.html

 

[Lower]

If you reduce from 150 to 125mm you'll reduce the airflow that the extractor can achieve substantially.

Why not chain drill the hole to the size you need or buy a 170mm core drill from toolstation for £35?

 

[jj4091]

If you reduce from 150 to 125mm you'll reduce the airflow that the extractor can achieve substantially.

No it won't
https://www.energyvanguard.com/blog/what-happens-air-flow-ducts-when-size-changes

Why not chain drill the hole to the size you need or buy a 170mm core drill from toolstation for £35?

Good ideas

 

[Lower]

No it won't
https://www.energyvanguard.com/blog/what-happens-air-flow-ducts-when-size-changes

Yes it will. Whilst the airflow will speed up, the reduction in size creates a pressure increase that the fan has to overcome. The small fans used in cooker hoods don’t create much pressure which is why it’s important not to use flexi hose if possible and minimise the number of bends.

 

[jj4091]

I hardly think reducing the diameter by 17% will "substantially" have any effect at all.
Not getting into an argument that has been had on here many times before & will leave it up to the op. to decide.

 

[foxhole]

Flexi plastic Is not heat resistant taken a few down which were heavily perforated due to melting.

 

[TofuSpaceship]

If you reduce from 150 to 125mm you'll reduce the airflow that the extractor can achieve substantially.
Why not chain drill the hole to the size you need or buy a 170mm core drill from toolstation for £35?

Alas the 170mm core bit from TS that you are referring to is 170mm in length, the max diameter is 162mm which I fear it is going to be too tight.
When I speak to hire places, they quote core bits in inches, so they have either a 6 inch (152mm) or 7 inch (177mm).
The kit we bought from Wickes comes with its own wall grill which is 175mm square

If I look at the TLC shop, their wall grill is also 175mm square.
I will give them a call and check with them what's the outside diameter of their plastic rigid 6inch ducting.

I just cannot fathom that this matter is not simpler!!

 

[TofuSpaceship]

No it won't
https://www.energyvanguard.com/blog/what-happens-air-flow-ducts-when-size-changes

Insightful article, sometimes I wished I studied physics at uni! (probably I would have hated it).
I believe that explanation back the theory I heard about a reducer increasing noise, as the air needs to travel at higher speeds and it will create turbulence on the choke of the reducer

 

[jj4091]

Insightful article, sometimes I wished I studied physics at uni! (probably I would have hated it).
I believe that explanation back the theory I heard about a reducer increasing noise, as the air needs to travel at higher speeds and it will create turbulence on the choke of the reducer

The noise of the fan will drown out any noise that is created in the ducting, as I said I've never heard of it being a problem & nobody else has supported the theory that it is a problem.