Solar Water Heating

Solar water heating is arguably the most energy-efficient way of producing domestic hot water, as the primary energy source " sunlight " is free. Solar water heating has been used for many years in warm sunny climates, but it can work in locations as far north as Canada and Northern Europe. If you have a large family or use large quantities of hot water, a solar water heating system may be a cost-effective option. While the equipment has a higher initial cost than other types of water heaters, the energy savings can more than offset the cost over the life of the system.

Active vs. Passive Solar

There are two main types of solar water heating systems - active, which uses a pump to circulate the water between the tank and the collectors, and passive, which relies on natural convection to circulate the water.

Active systems can be either direct circulation or indirect circulation. Direct circulation systems circulate domestic water through the collectors and to the storage tank. These are best-suited for mild climates where temperatures seldom drop below freezing. Indirect circulation systems circulate a non-freezing heat transfer fluid through the collectors and then through a heat exchanger in the storage tank. These are preferred in cold climates where the pipes in a direct circulation system might freeze.

Passive systems are usually less expensive but less efficient. They can be either integral collector/storage systems or thermosyphon systems. The integral collector/storage type is typically used to preheat water for a conventional water heater, and is best-suited to climates where temperatures seldom fall below freezing. Thermosyphon systems rely on natural convection to circulate the water, so the tank must be located higher than the collector panels - the heated water from the panels flows upward to the tank and the cooler water returns to the collector for heating.


The primary components of any solar water heating system are one or more collectors to trap the sun's energy and a well-insulated storage tank.

There are three common types of collectors - flat-plate collector panels, integrated collector/storage systems, and evacuated tube collectors.

Flat-plate collector panels have a dark absorber plate behind a glass or polymer cover. Water circulates through dark-colored pipes running through the collector. As sunlight passes through the clear cover, its heat is absorbed by the absorber plate and piping and transferred to the water. The collector panels are usually mounted on the roof, facing south. They can also be mounted on a south-facing wall or on a stand on the ground (as when used to heat a swimming pool).

Integral collector/storage systems, also known as "batch" systems, have one or more black tanks or tubes inside an insulated box with a clear glass or plastic cover. They are often used to preheat water before it enters a conventional storage-type water heater. They can also be combined with a tankless or on-demand water heater.

Evacuated tube collectors consist of parallel rows of transparent tubes containing metal absorber tubes that absorb the sun's heat. This type is used mainly in commercial applications.

Storage tanks are typically large-capacity (80 gal. or greater) conventional water heaters (either electric or gas). The larger capacity allows for more "free" hot water during periods of overcast weather. When the solar collectors are unable to provide enough hot water, the back-up heating elements or burners make up the difference. A single-tank system uses the existing water heater for both storage and back-up, while a two-tank system pre-heats the water before it enters the main water heater tank.

Cost vs. Benefits

The cost-effectiveness of a solar water heating system depends on a number of factors, and should be evaluated by an experienced professional - these factors include:

  • Hot water use - the more hot water you use, the more likely a solar water heating system will pay for itself over time. They are usually most cost-effective for larger families or homes with a high demand for hot water.
  • Cost of the system - passive systems are typically less expensive, but may not be practical or appropriate in many cases.
  • Amount of available sunlight - solar applications obviously work best in locations with plenty of available sunlight. Ideally, collectors should be exposed to direct sunlight for the maximum possible number of hours each day, so proper location is critical to achieve optimum performance.

Payback periods will vary based on these and other factors, but tax credits and other incentives can significantly reduce the initial cost and shorten the payback time. Before purchasing a solar water heating system, you should investigate all potential incentives and factor them into your decision.

Installation and Maintenance

With any type of solar water heating system, proper installation is critical and should only be done by a qualified, experienced contractor. When selecting a contractor, look for one with a good deal of experience installing that specific type of system. Check with your state or county for licensing requirements, check references from past customers, and check sources like the Better Business Bureau for any complaints or problems.

As with any major system, proper maintenance is important. For active systems, be sure to discuss maintenance requirements with the installer and consult the owner's manual. Passive systems typically require little maintenance, as they are simpler, with fewer components.

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