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?