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