Ask The Trades - Ducting for extractor hood

Tika

GDPR opt-out

Ducting for extractor hood - urgent HELP please!!
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!!!

Back to top

« Last Edit: Apr 21^st, 2009, 10:01am by Tika »

IP Logged

Zambezi

GDPR opt-out

Re: Ducting for extractor hood - urgent HELP please!!
Reply #1 - 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.

Back to top

IP Logged

Lectrician

Administrator
Trade Member
Author

Offline

Ask The Trades
Posts: 8815

Total Thanks: 109
For This Post: 0

Braunton, North Devon
United Kingdom
Gender: male

Trade: Electrician

Re: Ducting for extractor hood - urgent HELP please!!
Reply #2 - 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).

Back to top

Need to post a picture? Click HERE for info!

IP Logged

Tika

GDPR opt-out

Total Thanks: 109
For This Post: 0

Re: Ducting for extractor hood - urgent HELP please!!
Reply #3 - 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.

Back to top

IP Logged

Zambezi

GDPR opt-out

Total Thanks: 109
For This Post: 0

Re: Ducting for extractor hood - urgent HELP please!!
Reply #4 - 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.

Back to top

IP Logged

Lectrician Administrator Trade Member Author Offline Ask The Trades Posts: 8815 Total Thanks: 109 For This Post: 0 Braunton, North Devon United Kingdom Gender: Trade: Electrician	Re: Ducting for extractor hood - urgent HELP please!! Reply #5 - 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?
Back to top	Need to post a picture? Click HERE for info! IP Logged

CWatters Super Member Offline "Daddy fick it" says James Posts: 5150 Total Thanks: 58 For This Post: 0 Gender:	Re: Ducting for extractor hood - urgent HELP please!! Reply #6 - Apr 21^st, 2009, 8:41pm I'm fairly sure most are robust enough to survive being run with clogged filters anyway.
Back to top	IP Logged

Twobarrows

GDPR opt-out

Total Thanks: 58
For This Post: 0

Re: Ducting for extractor hood - urgent HELP please!!
Reply #7 - Apr 21^st, 2009, 9:03pm

Quote:

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!!!

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 ?

Back to top

IP Logged

LSpark Global Moderator Trade Member Author Offline Posts: 8069 Total Thanks: 3 For This Post: 0 London, UK United Kingdom Gender: Trade: Electrician	Re: Ducting for extractor hood - urgent HELP please!! Reply #8 - 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.
Back to top	IP Logged

Tika

GDPR opt-out

Total Thanks: 3
For This Post: 0

Re: Ducting for extractor hood - urgent HELP please!!
Reply #9 - 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.

Back to top

IP Logged

scotspark Trade Member Offline mmmmm it's gonna cost you!! Posts: 3074 Total Thanks: 10 For This Post: 0 scotland Scotland Gender: Trade: Electrician	Re: Ducting for extractor hood - urgent HELP please!! Reply #10 - 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?
Back to top	brown to red and blue to f****&&&&ommited!!!!!!!!!!!!!!!!!!!!!!!!!!! IP Logged

Tika GDPR opt-out Total Thanks: 10 For This Post: 0	Re: Ducting for extractor hood - urgent HELP please!! Reply #11 - 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!
Back to top	IP Logged

scotspark Trade Member Offline mmmmm it's gonna cost you!! Posts: 3074 Total Thanks: 10 For This Post: 0 scotland Scotland Gender: Trade: Electrician	Re: Ducting for extractor hood - urgent HELP please!! Reply #12 - 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
Back to top	brown to red and blue to f****&&&&ommited!!!!!!!!!!!!!!!!!!!!!!!!!!! IP Logged

Tika GDPR opt-out Total Thanks: 10 For This Post: 0	Re: Ducting for extractor hood - urgent HELP please!! Reply #13 - 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!).
Back to top	IP Logged