The ProgressivTube is constructed to withstand periodic freezing conditions.  Due to the large mass of water volume, it takes extended periods of below freezing temperatures before damage would occur.  See http://solardirect.com/swh/pt/pt1/why_progressivTube/why_progressivtube.htm for map of regions where freezing is not a typically an issue.  In climates that have occasional days or nights of freezing temperatures, the unit can be outfitted with a freeze valves for protection during unpredicted freeze conditions (see http://shop.solardirect.com/product_info.php?cPath=69_71_84_72_86&products_id=157 for information on freeze valves).  In climates where there are extended days and nights of freezing temperatures, the unit would be drained; typically for the entire period as shown in the map.  The effectiveness of the ProgressivTube in these colder climates compared to Closed Loop systems that can operate in freezing temperatures is compared at http://solardirect.com/swh/pt/pt1/why_progressivTube/why_progressivtube.htm.  The ProgressivTube Passive Solar Water Heater typically outperforms other systems, and the economics are very favorable.