Drainback and Glycol solar hot water systems have been battling for supremacy for many years. These systems have their own special application for certain climates and both have the pros and cons.
The Drainback solar hot water is a standard system in solar industry. The Drainback system takes advantage of using water for its solar fluid. They also have an external heat exchanger that is separate from the water heater. The Drainback system is completely freeze proof when installed correctly. Arizona has many times during the winter that can freeze solar panels. When the system turns on to heat the water in the water heater, it pumps the water in the external heat exchanger up to the solar collector and back down from the collector to the heat exchanger. While this is going on another pump takes take the cold water from the water heater and pumps in through a copper coil that sits inside the heat exchanger. The water from your water heater never comes in contact with the solar fluid (water). When the system is down heating the water in your storage tank (water heater) it shuts both pumps down. All the water being ran through the solar collector now drains back into the external heat exchanger, preventing any freezing from happening in the collector. The Drainback system is a very effective product and Arizona is one of the most installed systems.