52b8100c_na___Solar_Hot_Water__A_Primer__3
[0] Drainback [1] Closed loop; Cold climates [2] Very good freeze protection if used with antifreeze; No problems with hard water; Simplest of reliable freeze protection systems; Fluid not subject to stagnation temperatures; Simple to homebrew; can be powered by PV [3] 
Heat exchanger & antifreeze reduce efficiency; Collectors & piping must have adequate slope to drain; Requires larger pump to lift
Arizona Solar Center - Your Guide to Solar and Other Renewable Energy Sources in Arizona - Solar Hot Water: A Primer
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