Here's a great recent Samba thread on the underfloor install.
http://www.thesamba.com/vw/forum/viewtopic.php?t=601703&highlight=propex
Stephen 

     On Monday, September 29, 2014 9:06 AM, David Bjorkman <ddbjorkman@VERIZON.NET> wrote:
   

 Hi All;

I am thinking of going the propex route.  I have 2 questions for those more propex knowledgeable than I.
1)  I have heard that the propex heaters have some problems with altitude.  Their performance, and mayhaps ability to operate, diminish with altitude.  Anyone up in the Rockies or another high point able to confirm or deny what I have heard?
2)  If I go with the installation, I am wondering about placement.  I already am unhappy with the loss of my overhead storage space to the A/C (which I enjoy, I'm so conflicted) and so do not want to lose anymore interior space to the propex.  I do not want to remove the rear hot water heat unit so am thinking about the under the floor installation.  Anyone have experience with this kind of placement and the rigors of snow, road salt, and other hazards that the heater will run into if placed under the vehicle and into the elements?

Thanks;
Dave B.
 '87 Westy with FAS inline 4