A schematic diagram of indirect water heating systems is shown in Figure 5.11.
In this system, a heat transfer fluid is circulated through the closed collector loop
to a heat exchanger, where its heat is transferred to the potable water. The most
commonly used heat transfer fluids are water–ethylene glycol solutions, although
other heat transfer fluids such as silicone oils and refrigerants can be used. When
fluids that are non-potable or toxic are used, double-wall heat exchangers should
be employed; this can be two heat exchangers in series. The heat exchanger
can be located inside the storage tank, around the storage tank (tank mantle), or
external to the storage tank (see Section 5.3). It should be noted that the collector loop is closed; therefore, an expansion tank and a pressure relief valve are
required. Additional over–temperature protection may be needed to prevent the
collector heat–transfer fluid from decomposing or becoming corrosive.