Cooker Hood - Maximum Duct length?

I would like to change a cooker hood from recirculation to extraction to outside. The hood is mounted on an internal wall, however as the property is a bungalow it may be possible to run a rectangular duct in the attic and exit via a soffit.
The duct length would be about 3 metres. Is this practical?
I have looked at a Neff installation manual but it does not specify a maximum length. Just states the obvious that shorter, straighter and larger is better.
--
Michael Chare
duct in the
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On 03/12/2015 21:09, Michael Chare wrote:

Would it be possible to run a 150mm circular duct? If not the whole way, just the majority of it until you get to the soffit?
--
Dazza

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On 03/12/2015 21:31, gremlin_95 wrote:

Quite possibly. I just thought that a rectangular duct would lie on the rafters more easily. I need to make a proper inspection as the roof is at a shallow angle and getting to the edge is difficult.
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Michael Chare

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On 03/12/2015 22:06, Michael Chare wrote:

Yep, shouldn't be a problem. As said, solid ducting is much preferred to flexible stuff.
--
Dazza

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The duct needs to slope slightly so that any condensation in the duct falls away from the house.
You may also need to consider insulating the ducting to prevent condensation forming on the outside of the duct.
But 3m is fine for most rigid ducts.
--
Adam


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On Sunday, December 6, 2015 at 3:07:59 PM UTC, ARW wrote:

Ah yes, the thing that is annoying me is did I slope the duct - I just cannot remember. Surely I must have done. Anyway, no Legionnaires disease yet ! Simon.
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It's not Legionnaires disease that is the problem but condensate dripping back into the cooker hood. Or you can blame the slopes:-)
--
Adam


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On 03/12/2015 21:09, Michael Chare wrote:

I'd have said that 3 metres would be fine. Mine is about 2, with no obvious problem. Probably less "resistance" if you use the rectangular PVC duct rather than the flexible corrugated aluminium duct throughout.
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On 03/12/15 21:33, newshound wrote:

+1 - use round or rectangular solid duct as far as possible. May need some 100x50 flexi to go over the wall plate under the eaves - but then it is at least a short bit.
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On 03/12/15 21:09, Michael Chare wrote:

3m is unlikely to be a problem.
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On Thursday, December 3, 2015 at 9:09:49 PM UTC, Michael Chare wrote:

We have about 3m of 125mm duct, 2 gravity flaps in the 1m vertical bit, changing to rectangular to output through the soffit, etc. No problems. Just avoid the flexible stuff like the plague ! Also, of course it totally depends on the power and pressure producing abilities (centrifugal / axial etc.) of the fan ... Simon.
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On Thursday, December 3, 2015 at 10:33:42 PM UTC, sm_jamieson wrote:

Use the recommended diameter ducting and as said before avoid the flexible stuff as it plays merry hell with the performance, as will an excess of bends.
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On Thursday, 3 December 2015 21:09:49 UTC, Michael Chare wrote:

You will also need to let air into the kitchen to replace that extracted.
Be specially careful if you have any heating appliances with flues, you could suck poison gases into the house when the kitchen extractor is running
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Actually you could not be more incorrect.
--
Adam


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On 08/12/2015 19:26, ARW wrote:

I've become more familiar that I'd like with the HETAS regs recently - and this is mentioned as a potential problem.
(In practice as we have so many draughts, and the flue draws really well so it's not an issue.)
Andy
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On Tuesday, December 8, 2015 at 9:40:32 PM UTC, Vir Campestris wrote:

We smelt an ashy or coal-like smell with the cooker hood running the other day. I shut the underfloor vent to the Baxi Burnall grate, and the smell went away. The hood must have been drawing air through the vent and the ashes and into the room ... Simon.
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On Tuesday, 8 December 2015 20:27:04 UTC+1, ARW wrote:

That is clearly true.

I hate to say this, but I think harry may be half-right this time.
Clearly something like a boiler with a balanced flue will be fine - but anything connected to an *un*-balanced flue (in other words, an ordinary chimney) will need adequate ventilation to ensure that the extractor doesn't suck the flue gases into the house.
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