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Solar Water Heaters

Solar water heaters—also called solar domestic hot water systems—can be a cost-effective way to generate hot water for your home. They can be used in any climate, and the fuel they use—sunshine—is free.

How They Work

Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems, Active and Passive.  Active systems have a circulating pump and controls.  Passive Systems just warm up the water you normally heat so you don't have to heat it as much.  Passive systems have no moving parts.

Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.

Three types of solar collectors are used for residential applications:

  • collector-storage systems

    This type of  water heating system has many black tubes or tanks or pipes under one or more insulated glass or plastic covers.  Cool water starts at one end of the collector and is heating as it passes through the collector.  The warmed water then goes on to the conventional backup water heater where it is stored before use.  Some collector-storage systems have a dark plate that absorbs the heat of the sun and no cover and are used in heating pools.

  • Flat-plate collector

    Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic covers. Unglazed flat-plate collectors—typically used for solar pool heating—have a dark absorber plate, made of metal or polymer, without a cover or enclosure.

  • Evacuated-tube solar collectors

    This style of collector features parallel rows of glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin has a special coating that absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for commercial applications.

There are two types of active solar water heating systems:

  • Direct circulation systems

    Pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes.

  • Indirect circulation systems

    Pumps circulate a non-freezing special fluid through collectors and a heat exchanges.  This heated fluid then heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.

Illustration of an active, closed loop solar water heater. A large, flat panel called a flat plate collector is connected to a tank called a solar storage/backup water heater by two pipes. One of these pipes is runs through a cylindrical pump into the bottom of the tank, where it becomes a coil called a double-wall heat exchanger. This coil runs up through the tank and out again to the flat plate collector. Antifreeze fluid runs only through this collector loop. Two pipes run out the top of the water heater tank; one is a cold water supply into the tank, and the other sends hot water to the house.

Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems:

  • Integral collector-storage passive systems

    These work best in areas where winter temperatures are not as cold and don't often get below freezing for a long period of time.  They work better for installations that need a lot of hot water throughout the day.  

  • Thermosyphon systems

    Water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems do not require a pump, but are quite reliable.  They are heavier systems and require special attention by the installer to make sure the roof support is adequate for the load.  They are usually more expensive than integral collector-storage passive systems.

Illustration of a passive, batch solar water heater. Cold water enters a pipe and can either enter a solar storage/backup water heater tank or the batch collector, depending on which bypass valve is opened. If the valve to the batch collector is open, a vertical pipe (which also has a spigot drain valve for cold climates) carries the water up into the batch collector. The batch collector is a large box holding a tank and covered with a glaze that faces the sun. Water is heated in this tank, and another pipe takes the heated water from the batch collector into the solar storage/backup water heater, where it is then carried to the house.


For more information about solar water heating systems,  see the following information:

Selecting a Solar Water Heater

Before you purchase and install a solar water heating system, you want to do the following:

For information about specific solar water heater models and systems, see the Product Information resources listed on the right side of this page (or below if you've printed the page).

Installing and Maintaining the System

The proper installation of solar water heaters should be done by a professional contractor.  There are many factors to consider including the amount of solar heat available in your area, and local building codes.  Working with a licensed contractor will save you time and money.  

Proper maintenance of the water heating system will keep it running for years.  While passive systems don't require much maintenance, active systems should be looked at on a regular basis for leaks and problems.  Most systems come with an owners manual that will serve as a guide for the recommended maintenance.  Plumbing and other conventional water heating components require the same maintenance as conventional systems. Glazing may need to be cleaned in dry climates where rainwater doesn't provide a natural rinse.

Regular maintenance on simple systems should be done every 3-4 years by a solar contractor. Systems with electrical components are very reliable and rarely need parts replaced.  They should still be checked on a regular basis.  For additional information about maintenance, see the following:

When screening potential contractors for installation and/or maintenance, ask the following questions:


  • Is your company licensed or certified?
    Make sure that the professional installer you hire is licensed to install systems in your state.   Your local city or county building department will tell you if a license is required in your city.   You should confirm licensing with the California State Contractors Licensing Board.   The state website also has information about any complaints that may have been issued against licensed contractors.

  • Does your company have experience installing and maintaining solar water heating systems?
    Choose a company that has record of installing the type of system you are looking at and make sure they will also service what they sell.

  • How many years of experience does your company have with solar heating installation and maintenance?
    Water Heating Systems have been around for years.  Get a list of past customers who can provide references for the contractor you choose..
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Redding, CA 96002

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