This type solar water heater has pumps to circulate thermal media, (water, or glycol) to exchange heat from the solar collector to the water storage tank.

Thermal media circulate and collect heat from the collector and transfer to the drain back storage tank. The solar pump activates and heat transfer occurs only when it is required and as a result heating of the water happens in the controlled manner.

As the media drains to the drain back tank from the collector freely, the storage tank always needs installed below the collector and the interconnecting pipes must have slope.

  • Tanks can be installed separately from the collector
  • No overheating
  • Electric backup heater with auto temperature control
  • Remote monitoring & Control options

Drain back solar water heater

Drain back systems are closed-loop, indirect, active solar water heating systems. Glycol-water or mix of both will act as heat transfer media. The heat transfer media contained in an unpressurized, closed loop is pumped through the collectors. The media will transfer heat to the domestic water through a heat exchanger.

System components

  • Solar collectors, harp type solar collectors can only be used for Drain back applications
  • Storage tank, to hold the water being heated.
  • Solar Controller, to control the pumps by measuring the temperature deference of the water in the tank and the temperature of the solar collector.
  • Drainback reservoir—a tank in the solar thermal media looping and this will hold the thermal media.
  • Heat exchanger to transfer heat between the drainback loop and the domestic hot water tank.

When the pump is off, the thermal media drains out to the drain back tank as the collectors and pipes are installed at a proper slope, this will protect the collectors from both, overheating and freezing. Drain back type solar water heating can be used for residential water heating and swimming pool heating, space heating.

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Drainback systems have many advantages compared to other types of SHW systems. As it needs an air space in order to drain, the thermal media loop is not pressurized and therefore less stress is placed on pipe joints, threaded fittings, and gaskets.

If a break occurs in the thermal media looping, it will leak more slowly than if it was pressurized. Additionally, there are no motorized valves to fail, and the system does not rely on electricity to maintain freeze/over heat protection. If the power goes out, the pump shuts off and the thermal media drains from the collectors back into the reservoir and this will protect the solar collector from overheating or freezing.

Since drain back solar water heaters are of non-pressurized, less components when compared to pressurized close loop systems, for instance, expansion tank, check valve, pressure gauge, and an air vent are not required.

In comparison with pressurized systems which do not have any overheating protection by its own, and if the pump fails, fails or the system shuts off because it has reached its high limit, the system pressure will increase and can actuate the pressure-relief valve.

This can result in the system operating at reduced pressure, requiring additional glycol to be added. Additionally, glycol can become acidic when repeatedly overheated, risking corrosion to the piping and other components in the system.

Drainback systems simply drain the collector because the pumps shut off when the system’s high limit is reached and therefore it protects overheating and is free from all the above challenges of pressurized systems.

It is recommending to use distilled water as the thermal media as water has superior heat-transfer abilities compared to glycol, adding a slight boost to system performance. Using water as the thermal media also allows for the use of single-wall heat exchangers.

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