Solar water heater systems are designed to be active, with pumps or fans to promote the gathering of heat, or passive, with no mechanical parts added to the system. Solar water heaters have been around since just before the dawn of the twentieth century. Standard natural gas water heaters took their place in the early 1900's. Since the 1970's, new technologies have solved the many problems that have plagued the solar water heater industry. Difficulties such as oxidation, rusting, and inadequate amounts of hot water are not problems any more.
The difference between the different kinds of active solar water heaters is the way the heat collector protects itself against cold conditions. These systems are called: Recirculate, drain-down, drain-back, and heat exchange.
The recirculate system
This system uses an electrical pump to move water through the collector to prevent it from freezing. Moving water will not freeze, but some heat is sacrificed to do this.
The drain-down system
This system simply takes the water out of the collector by opening a valve when a thermostat indicates that the water is near freezing. If the electricity goes out for this system, then it will remove the water from the collector as well.
The drain-back system
This system uses a heat exchanger to store drained water from the collector when it nears freezing. Heat exchangers work with a storage tank and push water to your original water heater. The drain-back system has an electrical pump that works most of the time. If the pump is shut off for any reason, the heat exchanger stores the water from the collector and uses this water as the heating fluid. The heat exchanger can lose up to 10% of its heating efficiency.
The heat exchange system
This system is the only system which does not drain out any water from the collector when it nears freezing. The heat exchanger uses an oily fluid that prevents freezing to travel to the collector and keep the water warm. Some of these systems use compressed air to heat the water instead of an oily fluid.
All active solar water heater systems have pumps and fans working to keep them heating and moving water. These systems need maintenance on a regular basis. These systems should still be able to be bypassed by a standard water heater if necessary.
