Drainback Solar Systems

This system design has its origins in the Dutch Water regulations, which originally did not permit the only the single skin of the solar heat exchanger to separate the glycol-water mix of the solar loop from the domestic hot water (considered as drinking water). The heat transfer medium adopted in the solar loop was plain water and this necessitated changes to be made to the system design to prevent freezing or boiling. The solution adopted was to allow the water to drain back to a holding reservoir when heat was not required from the collector. Once the demand for heat returned, the controller would operate the pump to fill the loop and collect heat as a normal system would. Once the pump switches off the water drains back under gravity (all pipes must have an adequate fall). There is therefore no water to freeze in the collectors in cold weather, nor to boil in hot weather when the full contents of the cylinder have been brought up to temperature. The drainback volume may either be an external container as in the Imagination system below or an expansion volume in the top of the domestic hot water cylinder as in the Solar Pure system below.

Precautions must be taken with the design of these systems to overcome the following issues.

• Prevent cold water from being pumped to an empty panel in stagnation conditions (this would result in severe water hammer)

Examples of Drainback Solar Systems