YouTube - ‪Solar Water Heating - Part 3: Heat Pumps‬‏
The water is pumped from outdoors. So, if you were to use Propane,
and had the unit located so any leak would roll downhill away from your basement..?.
That could be pretty safe..
I like the idea of recirculator loop (heat booster) they added on..
I think it's an add-on, otherwise they would have put it inside.
I'm just guessing, but it seems like they might have a few gallons
in the exchanger+ a small tank.
Found some links:
http://www.itssolar.co.za/domesticheatpumps.php
Some docs are there in pdfs.