How to replace cooker hood ducting?

[9namjam9]

My 350m3/h cooker hood is 4.5m from the exterior wall, and is currently connected to an exterior vent with 100mm aluminium ducting inside a bulkhead. I'd like to have better extraction, 500m3/h or more, and it seems like I need 150mm duct for higher powered hoods. Unfortuately the floor has been glued and screwed down over the bulkhead now, so I was thinking maybe I could connect the 150mm duct to the old 100mm duct and drag it through the bulkhead? It might be tricky because there is quite a bit of insulation in the bulkhead now, and additional timbers to level the floor mean the duct has to bend around a timber as it exits. Then I would need to core a bigger hole in the exterior wall - is it possible to cement up the old hole in the cavity wall and core a hole about 10cm to the right, so the duct doesn't have a kink at the end, or would that weaken the wall? Any other ideas?

I would go straight out behind the hood, but there are timbers, pipes and a waterproof membrane that make it complicated...

Edited December 20, 2021 by 9namjam9

[ProDave]

Picture of the bulkhead in the kitchen?

 

That floppy duct that is there is about the worst sort you can have.  I would replace it with 110mm rigid (drain pipe) but you will really need access into the bulkhead.

 

That is going to be one hell of a noisy cooker hood, just what are you cooking?

[9namjam9]

10 minutes ago, ProDave said:

Picture of the bulkhead in the kitchen?

 

That floppy duct that is there is about the worst sort you can have.  I would replace it with 110mm rigid (drain pipe) but you will really need access into the bulkhead.

 

That is going to be one hell of a noisy cooker hood, just what are you cooking?

Yeah I ditched the crushed plastic duct and replaced it with new aluminium 100m duct , before I realised it wasn't wide enough for most modern extractors. By then the floor had been put down. I was thinking maybe I could push/pull rigid ducting through a new hole in the wall, though as I said, there's a lot of RW3 in there now that might make things tricky..

Why would it be noisy? 

[saveasteading]

Anything other than replacing that ducting would be a bodge, and defeat the object of the better fan.

Changing the corrugated to a solid plastic pipe would make more difference than increasing the corrugated size, or any mix-up of old and new.

The new plastic pipe would go in the same hole.

Maybe just maybe you could push a new 100mm pipe in from the outside, if there was a gap in the insulation.

OR Use a rectangular duct in the kitchen as it looks a bit tidier than round.

 

BUT for a proper job you have to lift the floor, then use a 100 or 150 solid pipe.

 

The hole in the wall is the easy bit once you decide what to do.

 

[9namjam9]

8 minutes ago, saveasteading said:

Anything other than replacing that ducting would be a bodge, and defeat the object of the better fan.

Changing the corrugated to a solid plastic pipe would make more difference than increasing the corrugated size, or any mix-up of old and new.

The new plastic pipe would go in the same hole.

Maybe just maybe you could push a new 100mm pipe in from the outside, if there was a gap in the insulation.

OR Use a rectangular duct in the kitchen as it looks a bit tidier than round.

 

BUT for a proper job you have to lift the floor, then use a 100 or 150 solid pipe.

 

The hole in the wall is the easy bit once you decide what to do.

 

Is rigid duct that much better than corrugated? also, do you know where I can get 5m of rigid plastic 150mm/125mm/100mm ducting? I can't seem to find any more than 2m long

Edited December 20, 2021 by 9namjam9

[Roys]

Standard 110mm drainage pipe comes in 3 m lengths about £20 a length from memory, that is what is usually used forgoing through the wall for extractors. Easy to join together.

Loads of places sell it.

[saveasteading]

28 minutes ago, 9namjam9 said:

Is rigid duct that much better than corrugated?

Yes, in the same way that drains use smooth pipes, and corrugated are only for sokaways.

The roughness causes turbulence all the way.

 

28 minutes ago, 9namjam9 said:

where I can get 5m of rigid plastic 150mm/125mm/100mm ducting?

 

At a proper builders' or drainage merchants.

Or buy in 2m / 3m lengths with socket ends, or plain pipes with connectors.

Also, the junctions at the hood and the outside louvre will be better joints.

 

Edited December 20, 2021 by saveasteading

[9namjam9]

1 hour ago, saveasteading said:

Yes, in the same way that drains use smooth pipes, and corrugated are only for sokaways.

The roughness causes turbulence all the way.

 

 

At a proper builders' or drainage merchants.

Or buy in 2m / 3m lengths with socket ends, or plain pipes with connectors.

Also, the junctions at the hood and the outside louvre will be better joints.

 

I found this study that shows compressing flex duct even by about 10% halves its cfm compared to the same diameter rigid (or flex at full extension). I think I’ll try shoving some 125mm rigid pipe in through a new hole.

https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/
 

 

Edited December 20, 2021 by 9namjam9

[saveasteading]

I wonder if the existing fan will turn out to be strong enough after all, with an improved duct.

110mm drainage duct is 100 internally. You should also try to make the bend from the fan to the duct as smooth as possible.

 

There might be another plus in that these paper/ plastic wrapped spiral ducts get punctured easily.

[wozza]

Using 110mm soil pipe with that extractor will be a bodge, using flexible duct of any size is also a bad idea (noisy due to turbulence from the irregular shape)

 

IIRC my last extractor stated that the duct should be 150mm, but would allow 125mm. 

 

Have you considered using oval / rectangular duct - the high end stuff has vanes in the bends to reduce turbulence (turbulence reduces airflow and creates noise)

Edited December 21, 2021 by wozza

[9namjam9]

I just realised that noggin in the bulkhead could obstruct a rigid pipe... I'm wondering if I can remove part of the bulkhead joist that's against the wall and core thru directly behind the extractor. Not sure how structural it is..