frank li wrote:Freezing is usually addressed by draining fluid back inside the house or with the use of antifreeze. In batch heaters, you can shutter them with an insulated cover at night or on very cold and over cast days, they can be freeze tolerant (ambient temp) without the insulating cover.
For hydronic solar collectors, its easy to let the water back in by gravity. If your pump is pv driven, it cannot not fail in pump mode and run all night in freezing weather and destroy stuff. Antifreeze is most common in the north.