ENERGY STAR® Solar Thermal Water Heating System

Receive a $2,500 to $5,000 rebate on ENERGY STAR certified solar thermal systems with a natural gas water heater backup.

Visit socalgas.com/Rebates for details and to apply.

 

Getting To Know Your Solar Water Heater

 

Solar water heaters are often confused with solar electric energy generators (such as solar PV panels). Instead of generating electricity, they collect thermal energy from the sun to produce the hot water needed in homes, offices, and even industries. And they do this in a way that is over three times as efficient as PV modules and won’t take up a large amount of space on your roof.

Now that you have a solar water heating system in your home, it is helpful to understand the basic way they work. There are several different types and designs of solar water heaters. But in the end, they all do the following things:

  • Collect thermal energy from the sun
  • Use the collected solar energy to heat your water
  • Store any extra energy for use later when you need it. 
     
solar water heating loop diagram


And unlike a traditional “off the shelf” water heater, they have a few more parts and pieces that come together when installed in your home. The diagram shows one type of solar water heater common to California, and it works as follows:

 

solar water heating house diagram

 

  • Solar collectors are mounted to the roof, usually oriented toward the south, and tilted to catch the maximum amount of solar energy during the day.
  • Pipes making up the solar loop connect the collector to the solar tank, and a pump moves the fluid in a loop. The collector warms the fluid, it is pumped to the solar tank where it heats the water, and then the cooler fluid flows back up to the collector to be reheated.
  • The solar storage tank contains the heating heat exchanger and the tap water. For an indirect system like this one, a heat transfer fluid is used that does not mix with your tap water. The heat from the solar loop is transferred to the water through the heat exchanger. A cold water line connects to the solar tank to refill it as you use hot water.
  • The water in the solar tank heated by the sun flows into the inlet of the backup water heater as you use hot water. If needed, the backup water heater boosts the water temperature up to the setpoint. But if the water from the solar tank is already warm enough, the backup does not turn on at all. Backup water heaters are there to make sure you always have enough hot water, no matter the weather. (Note: Some solar water heaters may not have separate solar tanks and backup water heaters. These combine the two into a single tank to save space.) Any kind of backup water heater can be used – gas or electric, tank-type, or tankless.
  • The whole system is linked by a controller that monitors the temperature of the water in your solar tank and the rate you are using hot water. Most solar water heaters use smart controllers that allow the water in the solar tank to heat well above the setpoint of your water heater. This allows solar water heaters to store as much solar energy as possible during the day to reduce or eliminate the need for the backup heater to run. For example, if your hot water setpoint is 120°F, a solar tank controller may allow the water in the solar tank to heat up to as much as 160°F (depending on the tank) during the day. Once it hits the maximum temperature, the pump turns off. A mixing valve then adjusts the temperature to a safe setpoint as it moves into your plumbing system to avoid scalding risks.

Depending on the geographical location, local weather, time of year, and system type, your solar water heating system can provide all the needed hot water in a household. But when you need it, the backup water heater will turn on from time to time, automatically.