What are the advantages of the gravity drain system?
Gravity systems deliver hot water at less than half the cost per liter compared to pressure systems. All glass heat pipes never corrode even with mineral rich water, and survive shipping better, probably because they are closed on the end like an egg rather than open like a thermos-see the installation manual, all glass heat pipe end photo: http://www.freefuelforever.com/Gravity%20System%20Installation%20Manual.pdf High pressure is possible out of a gravity system-just put it up higher from the point of use. There are other tricks as well, such as a pump downstream of the collector, or a standpipe on the air vent to increase pressure.
