Ducting Options / Plan For Island Extractor Hood

[Cej238]

Hi,

I have a hob on my kitchen island with an extractor hood. The performance of the hood is really awful - noisy, sounds turbulent and is moving very little air. The issue is the convoluted path the current soft foil ducting takes - illustrated by the red line in the attached picture.

Current: Soft 150mm foil ducting attaches to the rear of the extractor fan. It turns and passes through a gap in the extractor bracket (which is about 120mm), then turns upwards into the roof cavity, then turns to attach to another piece of soft ducting that runs approx 3m horizontally through the roof cavity to the side of the house.

I've thought about alternate layouts, but I think the current 'side vent' in the house is always going to be too long and involve too many twists and turns to be viable. I'm thinking that I should vent the extractor vertically through our flat roof (yellow line on the diagram).

The challenge is that the extractor is fitted exactly underneath a steel. So the ducting will have to route around it. This means passing through the extractor bracket which as it currently stands is less than 150mm. I might be able to remove a piece of the bracket to allow 150mm clearance, but am also interested either in squashable ducting or low profile ducting.

• Can anyone advise on my options here? The soft ducting seems awful and I realise I will want to minimise any 'squashing' - but how compressable is "semi rigid" ducting? Can I easily squash solid ducting? Are there any oval or square ducting that I can use that will interface with a 150mm aperture?

Aside from squeezing through the approx 120mm gap, what are the merits of semi-rigid vs rigid ducting? On the face of it, I need two straight pieces and two 90 degree turns. I might even be able to get away with two 45 degree turns. I really want to avoid completely dismantling my roof and dropped ceiling box. On that basis, semi rigid seems like it will be much more forgiving... I can imagine cutting the hole in the roof in slightly the wrong place and a run of solid ducting not connecting up.

• What is my best bet? Perhaps I should use semi-rigid, attached to some solid 150mm to run vertically out through the roof?

I would also be grateful for advice about how to break through the roof? (Currently 3 layer felt) and how to patch it up?

And finally, I've googled various flat roof vents - there seem to be a few different designs. Is there a particular type that is recommended?

I'll be working with a builder/roofer for at least some of this, but they are a bit reluctant to propose a plan guaranteed to work - so need to take charge of the plan myself and buy the parts.

Very grateful for any help.

Thanks!

 

[foxhole]

Anything would be better that what you have , max length for max performance is often around 5m which is reduced by 1m for each 90degree angle .

 

[Cej238]

Agreed. My real issue is what to buy noting that for my skills at least, making changes to the roof/drop down box is a bit of a challenge. Have you used the semi-rigid ducting before? Is it rigid enough for to be a massive performance hit? It would certainly seem easier to fit than fully rigid smooth ducting.

Thanks!

 

[foxhole]

Agreed. My real issue is what to buy noting that for my skills at least, making changes to the roof/drop down box is a bit of a challenge. Have you used the semi-rigid ducting before? Is it rigid enough for to be a massive performance hit? It would certainly seem easier to fit than fully rigid smooth ducting.

Thanks!

The flexi is easy to use, all bends can be as long radius as you have room for minimising loss of performance.A bit noisier than rigid.The more places you can secure the less it flaps about [which can cause tears, in both meanings of the word].

 

[Cej238]

Thanks. I think I'll order a length of semi-rigid and see how it goes.

Does anyone have any thoughts as to what what I need to get in vented through the roof?

- Is there a recommended approach to cutting the hole? I'll get the roofer we used previously to repair the hole/felt, but naturally want to minimise damage or ensure it is 'fixable'.
- What hardware would be a good bet to connect the duct to on the roof?

Many thanks!

 

[foxhole]

If you are using a roofer get him to the cut and install into the roof.He will be able to source suitable outlet for your roof type.