-> TD> of a LPG system?? Yes, as long as you get one set up with orfices
-> for LPG.
Which you can do easily enough.  BTW, I just stumbled over some
interesting data about that.....
Natural gas has a specific gravity of around .8  (Remember air is 1)
Natural gas produces approx 950 t0 1150 BTU's per cubic foot.  It
requires about 10 to 1 air to fuel mixture for complete combustion.
Propane has a specific gravity of 1.5.  PRopane produces approx 2500
BTUs per cu ft. It requires well in excess of 20 to 1 air to fuel
mixture.
Butane has a specific gravity of 2.5 to 1.  It produces in excess of
3400 BTUs per cu. ft. and requires at least 24 to 1 air to fuel mix.
There's also some important considerations concerning the pressures
required to shove the different gasses through pipe.  I don't have the
accurate numbers at hand, but suffice it to say, I recently boughta
multifuel cook-top unit to replace the "Dead Short" surface unit we used
in the kitchen for 25 years, and the pressures required for the new unit
to work effectively on Propane were MUCH higher than for natural gas.
(Part of this was of course the size of the orfice needed for LPG, but
flow through the pipe was stated to be a major contributor.)
Additionally, Butane producing 3400+ BTU's per cubic foot is almost neck
and neck with electricity which produces 3500 BTU's per KW.  Also since
the heat is DIRECTLY in contact with the pan, it takes much less TIME
for the Gas unit to boil water, so much fewer Kilowatt HOURS are
dissipated, thus saving buck$ that way too.  (As a matter of fact my
wife is still cooking on ELECTRIC time so I'm getting a lot of BURNT
offerings out of the kitchen these days.)
-> require *no* electrical power.
No EXTERNAL elect power.  The juice produced by the thermocouple keeps
the solenoid valve energized as long as the pilot is burning.
(Gotcha Russ!)  
-> Couldn't hurt to have it inline during the summer while being able to
-> bypass it during the winter.
I'm toying with that subject at the moment.  Something has to be done
about the radiant leakage of collected heat!  I'm losing a LOT of stored
heat when the sun goes down.  Water that will fry your butt at 4 PM is
too cool to bathe in by 8 PM, (Local sundown)  That's rediculous!
Besides which, getting the HOT water to come DOWN off the roof to the
storage tank is a major pain.  All my really HOT water goes back UP
there.
-> TD> Is this sort of thing practical on a small scale, let's say
-> 250-400 TD> Gallons?
OK, where ya gonna KEEP this water?  Very pertenent question!
 ^..^
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