Direct Systems:

Direct systems: (A) Thermosiphon CHS system with tank above collector. (B) Pumped system with pump and controller driven by a photovoltaic panel

Direct or open loop systems circulate potable water through the collectors. They are cheaper than indirect systems and offer superior heat transfer from the collectors to the geyser, but have many drawbacks:

• They offer little or no overheat protection.
• They offer little or no freeze protection.
• The collectors will accumulate scale in hard water areas.

They are often not considered suitable for cold climates since, in the event of the collector being damaged by a freeze, pressurized water lines will force water to gush from the freeze-damaged collector until the problem is noticed and rectified.

Indirect Systems:

Indirect pumped systems: (C) Indirect system with heat exchanger in tank; (D) Drainback system with drainback reservoir. In these schematics the controller and pump are driven by mains electricity

Indirect or closed loop systems use a heat exchanger that separates the potable water from the fluid, known as the “heat-transfer fluid” (HTF), that circulates through the collector. The two most common HTFs are water and an antifreeze/water mix that typically uses non-toxic propylene glycol. After being heated in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable water. Though slightly more expensive, indirect systems offer freeze protection and typically offer overheat protection as well.