52b8100c_na___Solar_Hot_Water__A_Primer__3
[0] Draindown [1] Open loop; Designed to drain water when near freezing [2] Can be powered by PV [3] 
Freeze protection is vulnerable to numerous problems; Collectors & piping must have adequate slope to drain; Inappropriate for use with hard water
Arizona Solar Center - Your Guide to Solar and Other Renewable Energy Sources in Arizona - Solar Hot Water: A Primer
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