Ducting for Kitchen Extractor Fan

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We have an Icon 60 150mm extractor fan in the kitchen which uses 6 inch flexible ducting. The ducting needs replaced as it has a couple of holes which I just discovered (the builders had covered them in tape 5 yrs ago which is now failing). The ducting run is roughly 4 m long and vents through the soffit. The outside grille happens to be an addvent 4 inch grille & the ducting looks like it has just been placed over the top without being secured on - I'm guessing a 6 inch grille should have been used but the soffit isn't wide enough.
Just wondering what is the best thing to do here?
My 1st thoughts were to replace with again 6 inch flexible ducting & use a reducer to reduce the ducting at the end to 100mm to fit the external grille but then i thought it would be difficult/impossible to do as I'm guessing the hole in the soffit would be too small to pull the 6 inch ducting through from the outside. I think it would be too tricky to fit it from the inside due to how narrow the roofspace gets.
Could we simply use 4 inch/100 mm flexible ducting with a reducer at the fan connection or would the airflow be too poor? This certainly seems the easier option but I'm no expert and I'm guessing it would reduce the efficiency of the fan
Any other suggestions? My husband's friend is an ex bathroom fitter and we were going to ask him to do the job.
Thanks.
 
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You are right in general, but if the reduction is within a few cm of the outlet end of the duct, for this very short section the air speed will increase over the reduced diameter section so shouldn't add to the duct air resistance too much. Although this may increase the noise as it passes through the vent. However, it appears that this would be difficult for you to install logistically.

If it were me, I would fit two 4" vents side by side and 'Tee' the 150mm down to two 100mm branches, one to each vent.

It's good that you are rectifying the situation if some of the warm damp air kitchen air is escaping into the roof space because the existing duct is not secured properly then it can cause condensation and rot of the roof timbers.
 
Thanks. That sounds like a good solution. Would you mind putting a link in for the relevant parts involved as I wouldn't know where to start? Thanks so much.
 
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That's brilliant - thanks for your help. Could we just attach the 100mm ducting to the reducer with tape & cable ties? (The link mentions using a threaded hose connector).
 
Sometimes the duct goes inside the fitting so you can't use a cable tie, but duct tape will be fine. Where the duct goes over the fitting you could use a cable tie if you wanted, but generally duct tape is what's used
 
Just as an afterthought how feasible would it be to just connect the reducer to the 150mm ducting (near the end of the run) and then run the 100mm ducting from it to the grille?
 
Hi Emma
I think that is what I'd do - at least give it a try! It could save quite a lot of faffing about in inaccessible places and the smaller outlet has been used before - presumably without any issues.
I leave just enough ducting to fall through the soffit to allow me to connect the grille with cable ties, duct tape or whatever before bunging the grille back.......best to get that as good a seal as you can.
John :)
 
Thanks John, hope you're well. I'll suggest it to the fitter when we can pin him down. Best wishes Emma
 
I thought you had ruled doing that out? Have I missed something?

My 1st thoughts were to replace with again 6 inch flexible ducting & use a reducer to reduce the ducting at the end to 100mm to fit the external grille but then i thought it would be difficult/impossible to do as I'm guessing the hole in the soffit would be too small to pull the 6 inch ducting through from the outside. I think it would be too tricky to fit it from the inside due to how narrow the roofspace gets.
 
I thought initially the reducer would need to fit onto the external grille but now I'm wondering about it being attached to both ends of the ducting in the loft space where it would be easier to access (150mm one end & 100mm other end). Is that doable?
 
I don't know what the maximum length of duct that can be connected to the Icon 60 is, but the Icon 60 has a maximum airflow of 260m3/hour. At this flowrate, the resistance of 100mm duct is approximately 7 times higher than 150mm duct. So what that means in practice is that adding 1 metre of 100mm duct would create the same air resistance as adding 7 metres of 150mm duct.
 
Thanks, I wasn't aware of that. I take it that the method you mentioned wouldn't affect the air resistance as you still have 2 100mm ducts taking the steam etc out?
 
That's why there are two of them. 2 x 100mm ducts can handle twice as much air as 1 x 100mm duct

The outlet area of 150mm duct is approx. 175 cm2

The outlet area of 100mm duct is approx. 80 cm2, but because there are two of them 80 x 2 = 160 cm2

So using 2 x 100mm ducts provides just slightly under the same rate 1 x 150mm duct which is why I suggested it.

Icon seem keen that the diameter is maintained.

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