Hood extractor ducting - large to small to large

[kawrou]

We are installing a fan which has a 150mm ducting outlet. Unfortunately the path will only allow a 100mm hole to be drilled. After the hole there will be a 90 degree bend followed about a 1.7 meter straight path out.

I was wondering if it would be better for that 1.7 meter length to stay as 100mm ducting or to turn it into a larger rectangular ducting?
Something like 150mm reduced to 100mm and then increase to maybe 220 x 90mm rectangular duct.
I’m not sure if increasing after being reduced would provide any benefit or not.
Any advice appreciated.

Thank you.

 

[Murdochcat]

Reduce to 100 mm and don't use flexible vent pipe

 

[foxhole]

We are installing a fan which has a 150mm ducting outlet. Unfortunately the path will only allow a 100mm hole to be drilled. After the hole there will be a 90 degree bend followed about a 1.7 meter straight path out.

I was wondering if it would be better for that 1.7 meter length to stay as 100mm ducting or to turn it into a larger rectangular ducting?
Something like 150mm reduced to 100mm and then increase to maybe 220 x 90mm rectangular duct.
I’m not sure if increasing after being reduced would provide any benefit or not.
Any advice appreciated.

Thank you.

Kitchen or bathroom fan?

 

[kawrou]

For kitchen hood. Thanks

 

[jj4091]

As Murdochat says reduce to round 100mm ducting (non flex ) & use it all the way. You will also have to spend less on adaptors & tube. You will no doubt get responses telling you not to reduce as it will affect fan performance & burn your motor out (been loads of threads on this topic over the years ) but I have yet to see any negative affect doing it that way.

 

[Mike13]

keep a slight slope down from inside to outside to allow steam/grease to make its way slowly outside.

 

[kawrou]

As Murdochat says reduce to round 100mm ducting (non flex ) & use it all the way. You will also have to spend less on adaptors & tube. You will no doubt get responses telling you not to reduce as it will affect fan performance & burn your motor out (been loads of threads on this topic over the years ) but I have yet to see any negative affect doing it that way.

Thank you for the reply.

That’s what I was worried about. After finally getting a fan that could be ducted outside only to then somehow affect the performance somehow.

 

[kawrou]

keep a slight slope down from inside to outside to allow steam/grease to make its way slowly outside.

Thank you. Will keep that in mind.

 

[Wayners]

Reducing to 100mm will cause back pressure. Fan will get noisy on high setting and the strain will cause it to get hot or even break. Probably run fine on lower speeds

 

[kawrou]

Reducing to 100mm will cause back pressure. Fan will get noisy on high setting and the strain will cause it to get hot or even break. Probably run fine on lower speeds

Yes, I understand that. I think on the highest setting the 100mm pipe will affect it. That’s why I wonder what happens if you increase the pipe diameter after reducing it and whether it makes it better or not.

 

[foxhole]

Yes, I understand that. I think on the highest setting the 100mm pipe will affect it. That’s why I wonder what happens if you increase the pipe diameter after reducing it and whether it makes it better or not.

Reduced mine to 100mm. Not too noisy and still working fine 10 years later .