Sounds a stupid question but I seem to be getting conflicting answers as to whether I can reduce the extractor hoods we're looking at down to 4" or not. One hood manufacturer (LUXAIR) said reducing from 5" to 4" (100mm) would invalidate warranty. NEFF have said SOME of their hoods can be reduced down but only to a minimum of 120mm. I want to continue using the existing 100mm outlet pipe that runs through the ceiling void if at all possible, but don't want to screw up the extractor hood motor. Q. I know efficiency will go down / noise will go up but will either be really noticeable? Q. Are the manufacturers just playing on the safe side when stating a minimum outlet size of 120mm or should I really not entertain connecting up to a 100mm pipe? There's a screwfix product - Manrose Round Central Reducer White 125-100mm (10684) which would seem to be perfect for the job. Thanks in advance for any advice you can offer. Daz.
Putting a 5 inch outlet from a fan through a 4 inch hole can make the fan vary noisy. There is also to much back pressure which will put strain on the fan and cause it to fail. This don't stop folks though as some don't own a 5 inch hole cutter so just fit and hope for the best.
100mm & 125mm round ducting has had it's day,I would only fit 150mm pipe for duct or the larger rectangular ducting with a round adaptor, by using larger size ducting it future proofs you, & allows you fit a wider range of more powerful hoods. and put a slight fall on the ducting to the outside, will allow any moisture to run out, rather than lay in the duct & go mouldy.
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