How Does It Work?
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. They do this in a way that is over three times as efficient as PV modules and doesn’t take up a large amount of space on the roof.
There are several types and designs of solar water heaters, but in the end, they 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
And unlike a traditional "off-the-shelf" water heater, they have a few more parts and pieces that come together when installed. The diagram shows one type of solar water heater common in California, and it works as follows:
- 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, which is then pumped to the solar tank where it heats the water. 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 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 hot water is used.
- The water in the solar tank heated by the sun flows into the inlet of the backup water heater as hot water is used. If needed, the backup water heater boosts the water temperature up to the setpoint. However, if the water from the solar tank is already warm enough, the backup does not turn on at all. Backup water heaters ensure there is always enough hot water, regardless of 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 – natural gas or electric, tank-type, or tankless.
- The whole system is linked by a controller that monitors the temperature of the water in the solar tank and the rate of hot water usage. Most solar water heaters use smart controllers that allow the water in the solar tank to heat well above the setpoint of the 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 the 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 the plumbing system to avoid scalding risks.
Depending on the geographical location, local weather, time of year, and system type, a solar water heating system can provide all the needed hot water in a household. But if needed, the backup water heater will turn on from time to time, automatically.
Operation and Settings
Although solar water heaters may seem more complex than traditional water heaters, they can be easy and reliable to operate. A few simple guidelines can help get the most out of the system:
- All solar water heating systems should be installed by authorized, licensed personnel with the right qualifications and tools.
- Authorized contractor/personnel will setup, test, and commission the new system according to the manufacturer’s installation manual. They should demonstrate how to operate the specific system and what issues or alerts to watch for.
- The water temperature should be checked and adjusted as necessary. Most manufacturers recommend a set point of 110-125°F. Consult the manual to see how to adjust the setpoint.
- If there is a problem with the solar water heater, it will be installed with valves to allow bypassing it and using the backup water heater alone.
- Refer to the manufacturer's user manual for a more detailed operation guide specific to the solar water heating system.
How Do I Know It Is Working?
Because solar water heaters work quietly and automatically, owners may wonder if their system is working correctly and providing all possible savings. There are several ways to determine if a solar water heating system of the type shown above is functioning properly, depending on the type. One or more of the following can indicate that it is working:
- The pump/pump station is running.
- The flowmeter in the solar loop shows a reading.
- The temperature gage on the solar tank shows an elevated temperature
- Temperature gages on the solar loop show that the fluid coming from the collector is warmer than the fluid entering the collector.
- The controller or connected mobile app shows the current operation and energy savings.
- The utility bill drops!
Bear in mind that the pump will not operate at all times. Overnight, when there is no solar energy, the pump turns off and will not restart until the collector heats up again in the morning. Later in the day, the pump may also turn off if the solar tank reaches its maximum temperature. If you check these items and are concerned that the solar water heater is not operating, contact your installer or service provider. Rest assured that even if the solar water heater is not operating as it should, your backup water heater will automatically provide you with the hot water you need.
Maintenance
Like any other water heater, preventive maintenance is necessary for retaining system efficiency and prolonging operating life. Regular inspections are advised as described in the manual for the solar water heater. Typical maintenance may include:
- Clean the collectors as recommended in the user manual. Regular rains should keep the solar collectors clean from most dust and debris. In most cases, it is not necessary to clear snow from collectors. Most collectors are designed to melt away any snow/ice atop the collectors. DO NOT pour water on the collectors. DO NOT try to remove snow and ice with sharp objects to avoid damage to the collectors.
- After extreme weather events, check the collectors and piping for any damage and tighten mounting bolts if necessary. If any damage is found, contact the installer or service provider.
- At least once a year, or in the case of malfunction, the following visual checks should be carried out:
- Pump and overall system operation
- Condition of outdoor piping and insulation
- Solar collectors
- Solar tanks and backup heaters
- If the solar tank or backup heater includes an anode rod, check and replace it as recommended by the manufacturer.
- Periodically check the floor around the solar tank and backup heater for leaks. If a leak or abnormal noise is detected, contact the installer and consider bypassing the solar system.
- Flush the solar tank and backup water heater as recommended in the manufacturer’s operation manual. In coastal areas or areas prone to limescale formation, it may be necessary to flush more often.
- Heat-transfer fluid must be checked at least every three years and changed if necessary (or sooner if required by the manufacturer of the system). Contact the installer for this important maintenance procedure.
Follow the maintenance procedures for specific solar water heaters. Consider setting up a maintenance agreement with a qualified service provider to ensure regular maintenance appropriate to the system. With proper maintenance, most solar water heaters can last 20 years or even more. The solar system may even outlast the backup water heater.
Warranties
Solar water heating systems and backup water heaters are usually provided with warranties covering major components and installation. ENERGY STAR® certified solar water heaters also have warranties. Refer to the manual/warranty guide for more information on the warranty for the system and directions on how to file a warranty claim if needed.