The folks at Occidental Arts and Ecology Center and the Water Institute have published a useful booklet, Heating Water With the Sun: A Thermal Drainback Solar Hot Water Heater Demonstration.
"This solar hot water heating system is designed to heat water with the sun using a non-pressurized open loop of plumbing that is not tied into the plumbing of the bath house. In this indirect system, roughly 200 gallons of water gets pumped from a special solar thermal storage tank (see next photo) to the panels and back to the tank several times a day getting hotter with each cycle. These panels do not store water to be used directly by users in the bath house as they do in a passive batch system. Instead, they are designed to heat the finite amount of water that gets repeatedly pumped through a storage tank and uses a heat exchanger to heat the actual water to be used in the building. These panels are designed to run the water through as much piping as possible to maximize the amount of time the water is exposed to the heat of the sun. They are placed on a south-facing roof at an angle that ensures the fullest exposure to the sun."
The booklet includes an extensive list of resources and can be downloaded as a free PDF. Great stuff!
Link: Heating Water With the Sun: A Thermal Drainback Solar Hot Water Heater Demonstration