In Oregon, we must protect solar water heating systems against freezing.  There are three options:

• Use a drain-back system, which automatically empties the water out of the collectors back into an indoor tank when the collector temperature drops too low.
• Use an antifreeze solution (non-toxic propylene glycol) in the collector.  A heat exchanger is used to transfer the sun's heat from the antifreeze solution into house's potable water system. In the heat exchanger, the solution never comes into contact with the water you use.
• Manually drain the entire system when cold weather begins.  This method is not advised for solar domestic water heaters.  It carries the highest risk since it’s not automatic, and allows your solar water heater to be used only during summer and early fall.  Solar pool heaters, which operate only in the summer months, must be manually drained each fall.