> How low can the outside temperature go (outside westy propane tank) and
> a propex furnace, at or near sea level, still operate?

Propane freezes at -304 degrees F, so don't worry.  ;-)

Actually, what matters is its boiling point, since you need it
to go from liquid to gas to run your furnace.  That is -43 F.

The Vanagon propane tanks have a two stage regulator to better
control pressure in cold temps.  If there is moisture in the
propane, it can freeze when the pressure is reduced.  A two
stage regulator is better at producing an even, desired pressure
in cold weather than a single stage.  BBQs don't care....

I have a Carver P4.  The lowest temp that I have started it
at is 23 degrees F.  Because it expects higher pressure than
we use here in the US, it takes a few tries to start at cold
temps.  Maybe 4, a few times 6.  The Propex is probably
similar unless it has an orifice sized for 11" of water,
NA propane pressure standard.  (British standard pressure
equals about 15" of water.  See http://jones.colorado.edu/jones/PropaneFacts

Richard