In areas where freezing is very rare, batch systems (aka integrated collector storage; ICS) are suitable. With ICS, a large volume of water (30+ gallons) is stored in one or more collectors, heated, and drawn out as needed.
Because the end-use water is heated directly without the need for pumps, these systems can be very efficient. They require fewer components, and are common in parts of the Sun Belt states and in tropical climates. The thermal mass of the water can prevent freezing from occurring. However, in marginal areas, or locations with a greater chance of freezing, this may not be sufficient. The piping to and from the collector, which has much less mass than the collector itself, is the weak point and most likely to burst in a freeze.
Drainback systems are commonly used in freezing climates. When this type of system is not operating—i.e., not actively pumping heat transfer fluid through the collectors—the HTF drains from the collectors into a storage tank located in a conditioned space, eliminating the potential for freezing. For more information on drainback SHW, see HP97 and HP138.