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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Ducting for extractor hood - urgent HELP please!! https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1240304313 Message started by Tika on Apr 21^st, 2009, 9:58am

Title: Ducting for extractor hood - urgent HELP please!!
Post by Tika on Apr 21^st, 2009, 9:58am

I am buying an extractor hood with max extraction rate of 550m3/hr which states it requires 5" ducting to outside (about 2m away). My builder has already fitted 4" ducting and insists that is standard and it will work perfectly okay. My local electrician says they only make 4" or 6 " ducting so it would have to be stepped down to 4" anyway - that it is safe and the only repercussion of using 4" is that it may not be as efficient. The extractor company say the warranty is invalid if you step down to 4" because the motor might blow up!! Who is correct? HELP please!!!

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Zambezi on Apr 21^st, 2009, 10:11am

Welcome to ATT Tika.
Ducting normally comes in 4 and 6 inch as you mentioned. Many extract hoods use 4" but some of the bigger ones use 6" (and you do get bigger ones that require even bigger but not normally found outside Chelsea or in resturants).
If they say 5" then I would go one size up not one down, so 6" would be better. If you restrict the flow (by putting a smaller duct on the extractor or bird nesting in the duct etc) then you may overload the motor, which could cause it to burn out.
PS make sure that the hood allows plastic or foil flexible ducting, some require solid/rigid non-combustible ducting.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Lectrician on Apr 21^st, 2009, 11:56am

Most of the models these days seem to have a 5" spigot but they also supply a 5" to 4" converter in the box that fits over the spigot.

If they do not supply one, I guess they intend you to use 5".

5" is available, my wholesaler stocks it.

How long is the duct run? If it is short with minimal joints or bends I dont see too much of an issue to be honest, although it is best to obviously go by the manual.

It is not easy to enlarge a 4" hole to 5", but can be done either manually with a bolster or using a core drill if you get the core started gently by using some kind of guide in the old hole. (assuming masonary wall).

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Tika on Apr 21^st, 2009, 2:25pm

Many thanks for your replies. My builder says he can't change the ducting because he's already made and plastered the false ceiling beneath it so I'm stuck with 4". If I only use the lower speeds would the extractor be okay or would I still risk burning out the motor?

The warranty is for 5 years - if anything went wrong, could they tell what size the ducting was if it's above the ceiling? I'm worried that I'll be left without any warranty at all.

Thanks for all your help. It's much appreciated.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Zambezi on Apr 21^st, 2009, 2:36pm

It is unlikely that the motor will burn out, it may have to work a bit harder than it normally would to push it through a slightly smaller duct. Most people don't leave the extract fan running for hours on end anyway, so I don't see too much of a problem using it in for short periods of time.
As for warranty, it depends on the manufacturer, some will come out to look at the appliance if it goes wrong, some will expect you to get a 3rd party to carry out the repairs and some will expect you to box it up and send it back to them. Unless they actually see the duct then there is no real way of telling.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Lectrician on Apr 21^st, 2009, 4:02pm

I wouldn't be worrying myself to be honest. How long is the duct out of curiousity? How many bends?

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by CWatters on Apr 21^st, 2009, 8:41pm

I'm fairly sure most are robust enough to survive being run with clogged filters anyway.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Twobarrows on Apr 21^st, 2009, 9:03pm

wrote on Apr 21^st, 2009, 9:58am:

I'd be more worried about the increase in noise rather than the fan blowing up; although the duct area is 36% less than recommended, it should accomodate this, after all it should allow for filters gradually restricting flow.
The biggest pain with extractor hoods is the bloody bulbs keep blowing. Is there an LED version of cooker hood bulbs available yet ?

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by LSpark on Apr 21^st, 2009, 9:33pm

Is there such thing as a cooker-hood bulb?.. most I've come across are standard lamps. And It's probably the combined high temperature and vibration together that shorterns their life.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Tika on Apr 22^nd, 2009, 12:49am

Thanks for all your replies. To answer Lectrician's question; the duct goes vertically up through the ceiling and then turns through 90 degrees to go about 2 metres across the top of a false ceiling to the outer wall.

I spoke to a lady at the extractor hood manufacturer's today and she was quite adamant that the warranty would be void if the ducting was stepped down from 5" to 4", which makes me wonder if they've had a lot of problems with their motors. They have their own service people to do repairs within the warranty period, but if the extractor is not fitted to their specification, they apparently charge quite a lot.

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by scotspark on Apr 23^rd, 2009, 4:29pm

the motor will be fine all you may lose is some air movement.

but 550m3/hr is a huge amount is this a commercial hood?

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Tika on Apr 23^rd, 2009, 5:29pm

No, it's not a commercial hood. Maybe I misunderstood. This is the technical information (the 90cm LA-GL):
http://www.luxairhoods.com/index.pl?s=techinfo
with the warning at the bottom!

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by scotspark on Apr 23^rd, 2009, 9:23pm

well they seem pretty adamant about it

but i wouldnt worry about it personally if its to late to change ro 5" solid ducting.

if it bothers you buy a different hood

Title: Re: Ducting for extractor hood - urgent HELP please!!
Post by Tika on Apr 25^th, 2009, 3:16pm

After much consideration, I thought you might be interested to know that I've decided to go ahead with this hood since I'm unlikely to want to use it on the higher speeds - to be honest it's more for show. I really appreciate all the advice and help given. Thanks (and fingers crossed!).