On Tue, 18 Nov 2008 15:16:23 GMT someone who may be "BigWallop"
wrote this:-

You do have to allow a water trap at the
bottom of a vertical stack to catch any condensate that trickles down the
vent. Something that you can clean out and dry off when it needs it.

Manufacturers offer water traps which fit into a vertical duct just
above the fan. These have a water outlet which is intended to be
connected to a suitable drain.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54