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.