Steve,

The water does move, albeit slowly. I had a problem with long delays in
getting hot water to my fixtures. My supply line (because I have a 40
to 60 psi well supplied system) is a 1" dia copper pipe so I needed to
dump about 6 gallons to move hot water from the water heater to the most
distant bathroom. A few years ago I ran a 1/2" return line at basement
ceiling height the length of my basement from that most distant bathroom
back to the water heater, a distance of about 150 ft. I teed the return
into the water heater behind the drain valve giving me an elevation
difference of 9 to 10 ft, insulated the supply and return lines and hot
water is now quickly available.

Recall that Henry Ford was able to cool his Model T quite well with a
thermosyphon and the height difference there is only about a foot.

RB

Steve wrote:
For a thermosyphon to work the water heater needs to be lower than the
fixtures and that usually requires the water heater to be in the basement.
Even then the syphon won't work if there is any horizontal runs or dips..

I have worked on hot water heating systems, as a steamfitter, that were
actually single pipe systems. The hot water rises up the pipe and the cold
water goes down as. Actually the water never really travels or circulates.
The BTUs of heat migrate up through the column of water.. These primative
system are generally very simple with the hot water boiler at the bottom of
a column (pipe) in the basement and each hot water radiator is tee'ed off at
each floor level of the house.. I haven't seen one of these sytems for 50
years. Sorry for the 'flash-back'.

Steve