DIY Doctor

routeing for extractor fan ducting

Postby chris_on_tour2002 » Wed Aug 13, 2008 4:07 pm

posted this on electrics and plumbing as not sure where best it suits...

apologies, its a bit long-winded...

i removed an extractor fan from a bathroom ceiling before plastering. there is no ducting - the extracted air just goes up into the space between joists for the ceiling and the floor above (bathroom is on the ground floor). clearly this is not good especially as there are electrics running close by.

to run ducting to the outside is going to be a pain (i can see why they left it, though of course that does not excuse it) i would need to run a duct of around 4 metres or slightly more and go through a double skinned red brick wall. the vent will be in an unsightly place right next to a leaded window on the first floor (pretty chalet bungalow, keen not to spoil the look).

dont really fancy boring a 100m hole if not necessary. there is another route i could take into a roof space for the ground floor extension. the roof space is appx L3m x D2m x H1.5m and is a pitched lean-to roof. so the overall area is around 4.5m³ - the roof space is vented so has a cross flow of air. halogen lights are present in the ceiling below. the roof space is insulated with kingspan. so my question is, can i put the ducting in this space?

is it legal? i suspect i know the answer, and i am sure that there must be a reg stating what can and can't be done. if anybody knows the ruling i'd be grateful to be pointed in the right direction!

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Simply Build It

Postby sparx » Wed Aug 13, 2008 6:39 pm

Hi Chris,
I think I follow the layout you have....first option to cut out single brick at end of joist run and install rectangular air duct with brick looking outlet, at bathroom end fit rect/round adaptor, this would be best but whichever way you go a standard bladed fan will not give any flow over 4+ metres of duct it will need a centrifugal 'drum' type impeller fan.
To go up to roof may be easier but it must be led out under soffit or facia if just vented into loft even with lights etc giving warmth there will be a serious danger of condensation in winter, never mind bldg regs!!
one of the reasons I hate retro fitting fans never as easy as bldr. tells me (& customer) it will be!
good luck SPARX
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Postby chris_on_tour2002 » Wed Aug 13, 2008 9:52 pm

thanks for that sparx, i didn't think it was a particularly good idea to have it vent into the roof space.

brick-shaped outlet would be a good idea, unfortunately the exterior is rendered and painted white.

i looked at going through the soffit and there isn't the room in the eaves.

also i was aware of the issue with length of ducting and the power output of the fan. a larger more powerful fan will be required, but on an earlier post i enquired about the quietest extractor out there. power and silence, don't think the two will go together! and as i am doing this for the mother-in-law, i need to get it right!

i'm thinking i'll take the extractor out, plaster over the hole and tell her to open the window!

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