52b8100c_na___Solar_Hot_Water__A_Primer__3
[0] Thermosiphon [1] Typically open loop; May be closed loop with heat exchanger & antifreeze [2] Simple; Requires no electricity for operation [3] 
Collector must be located below tank; Inappropriate for use with hard water (open loop system)
Arizona Solar Center - Your Guide to Solar and Other Renewable Energy Sources in Arizona - Solar Hot Water: A Primer
3