Extractor Fan duct size

22 Feb 2011
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United Kingdom
Hi all. I am currently fitting an extractor fan and have bought some ducting from Wickes to do the job. The ducting will run about 3m from the shower to the outside wall above the ceiling. When I bought the ducting I assumed that the extractor fan was 100mm. Unfortunately I was wrong! All of the fittings for the fan are about 115mm. My question is this. If I use a reducer to reduce the duct size to 100mm, will I reduce the efficiency of the fan. I have found a reducer but cannot find a ducting supplier that supplies all of the 115mm diameter parts I need. Any advice would be most appreciated. Mark.
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Thanks, I was unsure whether it would put more stress on the fan - driving air down a smaller hole. I'll crack on then.....

PS. why does the fan state 125mm when it only measures 115mm? - just wondering.....
Reducing the duct size will make the fan less efficient, but probably more importantly, significantly noisier.
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Dear Mountainwalker, and anyone else who cares to reply,

Now I'm confused. How much noisier will the fan be above ceiling. I was about to fit; now I have 2nd thoughts. On inspection the fan inside the housing is about 100mm with a little clearance each side so I guess the air that it is pushing is about 100mm - so its efficiency should not be compromised, but I am worried about noise.

A simple job to fit this extractor fan is spiraling into a more complex venture. To miss lead flashing on the outside wall I have had to drill through and then strengthen a floor joist. I wanted to reduce the amount of wood cut from the joist, hence the reduction to 100mm from 115mm (the fan says 125mm but it measures 115mm - why?).

Also the only ducting I can find is 125mm. I hope this too will measure 115mm......

Comments? Any help would be most appreciated.
You're worrying about nothing. if it were a commercial size extractor then worry about reducing the duct size a bit. For a domestic fan it will make no noticeable difference to lifespan, noise or extraction.
Whatever size you finally decide on, make sure you use rigid PVC piping and not that concertina crap which since you are (sometimes ) talking about 115 mm, is what I suspect you have at the moment.

(the fan says 125mm but it measures 115mm - why?).

Lots of cheapskate manufacturers/importers use the same instructions for all models and leave you to sort it out since its a long way to Bang-Shi City to complain.

Others are simply so poor in things like service and QC that they forget/ don't care about updating the leaflets when they modify the product. This is why you are sometimes puzzled trying to find the fourth wire which should be yellow and your product only has three black wires.
Thanks for your info. I'll crack on anyway. Regarding the sizing. The fan has 125mm written into the plastic but only measures 115mm. The ducting that fits is the concertina (crap) type but I can't find rigid ducting that fits. I plan to use short length of the concertina then reduce to 100mm then go rigid to the outside wall. That should hopefully do the job. :D

110 mm rigid pipe is freely available from builders merchants.


10% reduction in area will definitely make it noisier because you will speed up the flow and maybe cause turbulence. It's just the same as changing to a faster fan speed : that creates a lot more noise.
OK I get the hint.... No more groundhog day. Just had to be sure - the manufacturer of the fan is telling me that I should not reduce the size of the duct because they cannot guarantee the performance because they haven't tested the fan in anything but a straight through configuration using the concertina ducting as supplied. Wanted to get your expert opinions before I press ahead - but if its too much trouble to reply or help then don't bother or go elsewhere..... :LOL:
the manufacturer of the fan is telling me that I should not reduce the size of the duct because they cannot guarantee the performance because they haven't tested the fan in anything but a straight through configuration using the concertina ducting as supplied

Ha ! What a joke that is. Just what performance are they guaranteeing exactly ? That it shifts "x" m3/hr - well just trying measuring that . That it extracts smells and odours ? Just try and say you are unsatisfied and see how forthcoming they are.

Also, looking at the above comment, it seems they have based adequate performance on a straight pipe-run with no deviations/elbows which would kick your situation ( and most other peoples') into touch as far as a complaint would go.

Let us know what you finally do.
this exactly what the manufacturers said.....

"I am sorry to have to inform you, that the HC115 shower kit cannot be used with any other components. It has been made to be used with the 3m of flexi duct pulled taught and with no additional bends or alternative components. If you add bends to this product it will no longer meet the 15l/s air flow rate as required by The Building Regulations: Part F.

I suggest you purchase a different product to meet your installation requirements. If you purchase a centrifugal fan, such as a SF90 or RF90, these product have better pressure handling and can be ducted using flat ducting."

They go on to say......

"Both the fans I specified for you are suitable for wall and ceiling installations.
The HC115 has not been tested with other components, and so we cannot guarantee the performance.
The Ducting supplied with the HC115 is 125mm the fan motor and 90degree bend is 115mm. I do not know of any 115mm ducting suppliers, and again suggest you change to a centrifugal fan which will ensure you reach the performance you need.

I have tried to find pipe fittings to fit the 115mm but no luck so I have decided to give up and send the HC115 back and purchase another, but no idea what yet. I'll let you know..... Any ideas? What a bloody saga. Simple job?.....
:evil: :cry:

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