Tom:

Here are some comparison numbers from spec sheets, etc.
http://jones.colorado.edu/jones/PropaneFacts.html

I have a Carver P4.  I would rate it "marginal" at cold
temps.  But remember these British furnaces come set up
for higher propane pressure, so work less well at our
stock 11" of water pressure.  I have a Propex 2800 which
I have not got around to installing.  Also, I think Karl
has kits with slightly larger orifices to make them work
right with our propane pressure, so the numbers in my
table might be accurate then, but less accurate with 11"
of pressure.

Richard
Boulder