Is Gravity Drainage Sufficient for All Drainback Systems?

System Requirements for Effective Drainback

In a drainback solar hot water system, the fluid in the collectors is allowed to drain back into the storage tank at night, when the system is not in operation. For effective drainback, the system requires a certain level of head pressure to ensure that the fluid drains back into the tank. Typically, a drainback system requires a minimum of 6-12 inches (15-30 cm) of head pressure to ensure efficient drainage.

Gravity Drainage Limitations

Gravity drainage can be sufficient for small solar collector arrays or systems with low elevation changes. However, for larger systems or those involving elevated tanks, gravity drainage alone may not be enough to ensure efficient drainage. In such cases, a drainback pump or other auxiliary pump may be required to provide the necessary head pressure to facilitate drainage. For example, a system with a 1,000-gallon (3,785-liter) storage tank elevated 10 feet (3 meters) above the collector array may require a pump to provide the necessary head pressure for effective drainback.

System Design Considerations

When designing a drainback solar hot water system, it’s essential to consider the elevation change between the collector array and the storage tank, as well as the size and number of collectors. By taking into account these factors, system designers can determine the necessary head pressure required for efficient drainage and select the appropriate components, such as pumps or valves, to ensure reliable operation. For example, a system with a 2,000-gallon (7,571-liter) storage tank and 10 solar collectors elevated 5 feet (1.5 meters) above the tank may require a pump with a flow rate of 10 GPM (38 LPM) to provide the necessary head pressure for effective drainback.