Height limitation for natural circulation thermosiphon

[rollingstein]

Suppose I have an overhead tank that heats hot water by natural circulation via a thermosiphon (& a Heat Exchanger with some source of waste heat), is there a limitation on how tall this loop can be?

e.g. Can there be an elevated tank at 30 feet with a heat exchanger at ground level that heats the water via a thermosiphon? What's the right way to analyse this?

Even more fundamentally, does a more elevated tank make a thermosiphon harder or easier to get going?

 

[CWatters]

Not an expert but I can't think of a reason why there would be a limit on height (unless perhaps you want to heat a space station or something). As i recall the flow rate increases until the pressure difference (due to the different densities in each leg) can no longer overcome the pressure drops in the pipework. I think starting will be harder with height because the percentage of water heated is smaller yet pipe losses are higher.