The following freeze protection information is provided by Febco for the backflow preventers they manufacture for irrigation systems. The guidelines are not specific to the Febco models and can therefore be applied to most any outdoor-mounted backflow preventer, regardless of manufacturer.
These are great devices for spring and fall seasons - when you want your irrigation system active, but the temperatures may dip below freezing. They are NOT an alternative to winterization. Similar devices are available from several manufacturers.