On Fri, 24 Aug 2007 20:01:13 +0100, Andy Hall
mused:

On 2007-08-24 18:12:58 +0100, " said:

OK, thanks for all that...

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

In effect this is what is presented when a fan is connected to a duct -
a resistance to flow.

A centrifugal fan will do a better job than an axial one with this
environment but is noisier. If there is direct outside access an
axial one is a better choice for this reason.



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??

Kind of the same thing. You won't burn it out but it won't have been
able to move as much air. You can hear a fan getting slightly noisier
whne closing a well sealed door. THe point is not to try to
hermetically seal the house. You can go for heat eschange type systems
where the heat from outgoing air os partly transferred to incoming at a
heat exchanger but these are quite expensive and challenging to install
neatly after the house is built

The ones I fit, and I haven;t fitted one for a while but I assume they
are still the same, just sit in a 6" hole in the wall in exactly the
same way a standard 6" fan would.
--
Regards,
Stuart.