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Active Solar Hot Water


Select the type of solar hot-water heater according to climate, cost, and operations and maintenance preferences.

There are five types of solar water-heating systems; these are described in the Solar Hot Water Heating section of Choose Building Components.

  • Thermosyphon Systems
  • Direct-Circulation Systems
  • Drain-Down Systems
  • Indirect Water-Heating Systems
  • Air Systems

Consider a pre-heat or full-temperature system.

  • A low-temperature solar water-heating system can be sized to provide only hot-water preheating. When hot water is needed, the warm water from storage is boosted to full temperature with a conventional gas or electric-based hot-water system. These systems can be relatively simple, with reduced collector size, lower insulation levels, and small boosting system, making them attractive options.


  • Higher-temperature solar hot-water systems can be designed to provide full- temperature hot water. A conventional gas or electric backup system is used only when there is no sun for extended periods. A high-temperature system can save more in fuel costs, but with the tradeoff of more expensive equipment.

For systems using water as a collection medium, consider the following issues:

  • Prevent stagnation. If a system is allowed to stagnate in direct sun, very high temperatures can quickly result, causing collector materials to deteriorate rapidly and causing closed piping or storage tanks to burst from excessive pressure. Stagnation can be avoided by venting or slow circulation of some water to keep the collectors cool; a drainback system can also be used.


  • Provide freeze protection. Freeze protection is important, even in nonfreezing climates, because an extreme weather event can cause substantial damage to a system. In desert climates, systems can freeze even on relatively warm nights because their heat radiates outward to the cool night sky, dropping the system temperature to freezing. The strategies used to protect a water system from freezing are determined by the main system type (e.g., direct-circulation, draindown, or closed-loop).


  • Avoid calcification and corrosion. Calcification is the buildup of minerals inside a collector and its pipes caused by "hard" water circulating through the system. Open systems that circulate city water are especially vulnerable. Mineral scales eventually clog the system, reducing flows and pump efficiency. Water can also be slowly corrosive of both metals and organic compounds. Gaskets and sealants can be quite vulnerable. Closed-loop systems can compensate with buffering chemicals to maintain a neutral pH.


  • Plan for leaks. Any failure of a water-based system is likely to result in a leak. Provisions should be made to contain all possible leaks and prevent water from harming other building components. Electrical equipment, and any personnel working on the electrical equipment, should be protected from exposure to leaking water.


  • Select a heat-storage strategy. Almost all water systems involve thermal storage. This is typically done by collecting the heated water in storage tanks for use as needed. The simplest systems circulate the heated water directly. More complex systems use one or more heat exchangers to isolate system components, adding the potential for more sophisticated levels of control.


  • Minimize pumps and pump energy. Systems using pumps can require significant energy usage. Each pump also requires control logic that raises the complexity and cost of the system. Failure of a pump by stagnation or freezing can result in significant damage.


U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585
www.eere.energy.gov www.energy.gov