Bathroom extractor fan - ducting

8 Apr 2003
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United Kingdom
I will soon be installing an extractor fan in my bathroom.

I cannot get access to the outside wall so will install a ceiling mounted fan. However, to duct this to front or rear soffits through the attic will need over 4m of ducting so a pretty powerful fan would be needed.

There is a soil stack next to where the fan will be and this vents out through the roof. Is it possible to vent the extractor fan directly into this?

Would there any major problems with this? I suppose smells could be an issue but I will have a fan with electric shutters.

Hope someone can help.
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The shutters I have seen on ceiling fans are only plastic flaps that fall back shut when the fan is off and wont form an airtight seal - therefore I think you may get a back flow of stink! . . . so to speak. I would get a larger fan that will still extract well over longer pipe runs. B&Q have a 5" fan in some stores that list flow rates at different run lengths, which may be helpful. Look out for 2 identical fans labelled up as different manufacturers - only one has shutters.
There may also be some problem with building regs if you break into the soil pipe in the house without a proper one way valve.
I have got a 6 metre run and goes straight out to the exterior wall with a dome type vent as I find the shutter type vent can be heard in the bathroom by light wind outside and never has any problem.If you have no walls to go to you could vent it out the roof soffit vent or failing that straight up to the roof which involves tile cutting and flashing.

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