New cooker hood - "substandard" vent duct install

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Another one for a friend.

Just had a new kitchen fitted, but had "some problems" with the cooker hood - Siemens LC64GA520B. The manual isn't too descriptive about ducting, but does say :
"Round pipes
An inner diameter of 150 mm, but at least 120 mm, is recommended"
I read that as "at least 120mm, but 150mm is recommended.

First issue is that the fitter clearly decided that as the electrics are someone else's problem, he can ignore all electrical related things - so he boarded over the conduit put in specifically to get the supply cable to the hood and left the mains lead coiled up inside the duct cover (chimney ?) - he also boarded over the conduits put in for the under-cupboard lights :rolleyes: Anyway, I managed to get an extra cable up the conduit in the wall and through the ceiling into the cover.

But he left the flexi ducting just sticking into the attic (it's a bungalow) - probably because he was given a kit that wasn't sufficient for the job. The kit was a Hafele 7215 125mm 1M hose kit - the box was left in one of the cupboards. My mate "mentioned this" to the supplying company and they came back - so now it's ducted to outside, but neither of us think it looks "right".

Firstly, there was a "large amount of duct tape" on the top of the appliance. I know from experience that this duct tape usually dries out and peels off - leaving nothing but the dried up glue behind.

To see what was there, we pulled the tape off, and found this
There's a reducer (I assume it's the one shown in the appliance instructions), and the duct is a very loose fit on it. Neither were fixed with anything but the tape.

The duct kit has an adapter with it, but this doesn't fit either the reducer or the appliance spigot

In the attic, there is now an extra length taped on

which goes through a hole in the wall to a terminal

For some reason, they've fitted a plastic sleeve in the wall which only goes about 2/3 of the way through (the hole isn't large enough for the outer bit), and the duct is clearly fixed to the grill with more tape.


I've suggested my mate ask the supplier about this, but he's got to the stage of not wanting them back. Anyway, down to some questions ...

Given that the manufacturer recommends 150mm duct, is 125mm sufficient. The hood does certainly suck with the 125mm, but would it be significantly different with 150mm ?

Is joining the duct with this duct tape acceptable ? Or what should be used ?

Would it be better done with rigid apart from the last bit down to the appliance (to make lining stuff up easy) ?
I'm thinking it would be better if the rigid duct went right through the wall, and was in fact the duct itself, so self supporting and the grill is just to "finish the hole" on the outside ?

Should we be concerned about condensation in the duct given that it's in an unheated roof space ?


My thoughts are to remove the grill, remove the piece of rigid duct, open the hole out and fit a longer section of 150mm rigid, and refit the grill (squarely on the wall this time !). Inside, use a 90˚ bend, another length of rigid duct, and finally a short section of flexi duct to the appliance.


Still, it's better than the old hood that got pulled out. We think it was a "BIY" job by the previous owner, but that just vented into the attic :eek:
 
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NB - I do have a part roll of silver foil duct tape at work, that has what seems to be indestructible glue on it - at least it seems impossible to remove from anything.
 
IIRC, 15 Air Changes per Hour are recommended for kitchens but you'd need an airflow meter to check this.
 
I would not worry too much about the size of ducting but looking at the photo I wondered if rain could get into the ducting? Would it be better for it to initially slope upwards a bit before dropping down? Perhaps some string secured to a roof timber to hoist it up slightly. Duct tape comes in different qualities like most things but if you choose a known Brand such as 3M it should stay stuck and will seal despite the gaps. You could also run a bead of silicone round the joints before putting the tape round.
You could lag the ducting in the loft with fibreglass or something to avoid condensation problems.
 
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I realise that it might be unavoidable due to the location of your cooker, but having ducting running that length (and upwards ) before it exhausts externally is far from ideal. The longer the duct the more resistance to air flow (back pressure). You are also likely to have fumes condensing and running back down/congealing on the duct before being exhausted from your property.

Does the manufacturer's manual mention a recommended maximum run ? Normally it's 3 metres absolute max.
 
It's my mate's, not mine, but ...

Yes, it's more or less unavoidable. The alternatives are across and out through the soffit, or through the wall and up through the garage roof.
EDIT: Out through the soffit would be a PITA as there's really b***er all gap to get the duct through.

The manual doesn't really say anything about the ducting arrangements. The entire coverage of the subject is :
Pipe dimensions
Note: The device manufacturer does not assume any warranty
for complaints attributable to the pipe section.
■ The device achieves its optimum performance by means of a
short, straight exhaust air pipe and as large a pipe diameter
as possible.
■ As a result of long rough exhaust air pipes, many pipe bends
or diameters, the optimum extraction performance is not
achieved and fan noise is increased.
■ The pipes or hoses for laying the exhaust air line must
consist of non-combustible material.
Round pipes
An inner diameter of 150 mm, but at least 120 mm, is
recommended.
Flat ducts
The inner cross-section must correspond to the diameter of the
round pipes.
Ø 150 mm approx. 177 cm2
Ø 120 mm approx. 113 cm2
■ Flat ducts should have no sharp deflections.
■ Use sealing strip for deviating pipe diameters.
So as you see, apart from "shorter=better" and "smoother, straighter = better" it's not much help.
 
So as you see, apart from "shorter=better" and "smoother, straighter = better" it's not much help.
No, you need formulas so you can do your own install cost/install labor/end performance tradeoffs.

They may not be easy to get and everything is non-linear. It's worse than electrical engineering.

For ducting there is a Total Equivalent Length for pipes and fittings other than a short piece of round, straight pipe of diameter X but there may be another way to handle this.

Over here we have ASHRAE but they don't lend their books to libraries. If there is a tech library near you it might help.
 
Yes, but getting the books out and doing the calcs like that is perhaps, just a tad OTT for what is a simple application.

Boiler manufacturers usually state it in terms of "max total length = x m", and give the metre length equivalent of each fitting. Of course, with a gas boiler it's somewhat more important to get it right, and they tie you in to only using their own overpriced flue parts.
 
Dunno if you still need help with this, but I'm a bit of a pro when it comes to ducting :) The duct should be 125mm for that extractor. You need to use proper foil duct tape like you mentionned (Duck tape isn't duct tape, and neither is PVC electrical tape!) a pvc duct hose is no good (it'll melt) it needs to be foil, and through the unheated roof, the duct HAS to be insulated in some way (preferably insulated foil duct: http://www.screwfix.com/p/insulated-hose-127mm-x-10m/28054
or http://compare.ebay.co.uk/like/1702...&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=65

or rigid duct wrapped in reflective bubble insulation (which is used in lofts) because water WILL condense in it during the winter, causing mould and dripping

the grille outside has no damper or flaps either (drafty!)
 
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Well for now it's just been taped up with proper tape - so I don't think it's going to come unstuck ! Probably come back to it later - when the weather's cooled down and the attic isn't quite so warm.
 
Well for now it's just been taped up with proper tape - so I don't think it's going to come unstuck ! Probably come back to it later - when the weather's cooled down and the attic isn't quite so warm.

When I first installed a fan duct, I made the same mistake everyone else does, and used that white vinyl duct through the loft. In the winter, water started dripping through the ceiling (just the moisture in the warm air from the room condensing on the walls of the cold duct through the loft caused it to sag and fill with water. It also went all black and mouldy inside. I replaced it with insulated foil duct and all problems solved.
 

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