On Fri, 24 Aug 2007 10:12:58 -0700, "
mused:

OK, thanks for all that...

But...woah!!! - "back pressure calculations" - OK, can anyone help me
by explaining this? (sorry for being ignorant)

You know, pressure, at the back.

Also, "Bear in mind when you are pumping air out of the house, fresh
air has to come in somewhere. "

If the fan is running (eg 20 mins after leave room) and all windows
and doors shut..can this damage the fan - ie does it have to work
harder??

Technically yes, but the main thing if the doors and windows are all
closed is that it isn't actrually drawing any clean air in so you may
as well just turn it off as it's just spinning and not achieveing
much.
--
Regards,
Stuart.