Extractor fan backdraft shutters fouling on flexible ducting

27 Oct 2020
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United Kingdom
Hi, I have had an extractor fan installed in the ceiling of my bathroom, with flexible insulated ducting going out through the soffit outside the bathroom. I thought I could feel a draft coming down from the fan when it was running, and then eventually realised that the backdraft shutters aren't opening because the ducting (which was an extremely tight fit over the cylinder of the fan in the loft, I'm not sure what you call it) is touching them, because as the ducting concertinas the ducting material is folding inwards (as it normally does) but this is touching the backdraft shutters. My fan is an Airvent 100mm Quiet Extractor Fan Humidistat, which I have since found out is allegedly for wall installation only - but I looked at other fans, such as the 'Xpelair Simply Silent 100mm Contour Extractor Fan Humidistat', which says it's fine for ceiling mounting, but its backdraft shutters are right at the end of the fan 'cylinder' (or whatever it's called) as well, so I think they would also foul on the ducting I am using. Has anybody else had this problem? The ducting I am using is from Toolstation, 'PVC Flexible Insulated Ducting Hose 100mm x 10m'. It was extremely tight when fitting it over the fan cylinder, but I presume it must be designed for all 100mm fans? The ducting itself only has to go about 1.3m from the bathroom ceiling to the soffit, so I could try normal flexible ducting, but I thought that insulated ducting would be better (as the house is going to be let out, and I want a long lasting solution). I have drawn a simple diagram of the problem, but I couldn't draw the backdraft shutters in Microsoft Publisher, so you'll have to imagine them!
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Could you swap it for a rigid pipe or does it need to flex?
Thank you both for your replies, that's exactly what I did! I bought sone 100mm i.d. plastic pipe (Toolstation item '100 Round Pipe 100mm x 350mm') (i.d. means "inside diameter", I hadn't realised this when looking up extractor fans) and cut it so that it was about two inches longer than the fan cylinder, slid it over it, then fitted a plastic 100mm pipe connector to the top (Toolstation item 'Straight Round Pipe Connector 100mm'). This meant that I then had an inch or so of 100mm outside diameter connector on which I could fit the very tight inner ducting (of the insulated ducting.) Then I put a cable tie around the inner ducting, and pulled the outer ducting and insulation down to meet the loft insulation, which was all around the pipe cylinder.
To finally check everything was working correctly, I turned the fan on and held up an orange smoke pellet, and saw all the smoke get sucked up into the fan, and come out of the vent in the soffit. I would like to try this again with a helper holding the smoke pellet under the fan while I am in the loft, checking for any leaks up there.
Thanks for your help!
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Here are some photos of the two parts I bought - the 100mm by 300mm pipe, and the 100mm pipe connector. Hopefully this will help other people who had the same problem I did.

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