Water to Water heat exchangers for use of domestic hot water heating

– Water-to-water heat exchangers use a heat transfer fluid (often propylene glycol and water) that circulates through the solar collector, absorbs heat, then passes through a heat exchanger to transfer its heat to potable water in a storage tank. In cold weather, heat-transfer fluids, such as propylene glycol antifreeze, prevent the solar collector from freezing. Water-to-water heat exchangers have either one or two barriers (single or double walls) between the heat-transfer fluid and the domestic water supply. 

A single-wall heat exchanger is a tube or pipe surrounded by fluid. Heat-transfer fluid can either flow through or surround tubing, while potable water can be the other fluid. A simple way to build a heat exchanger is to put a small pipe inside a larger pipe and circulate the fluid inside the outer pipe. Heat exchangers with double walls have two walls between the two fluids. If the heat-transfer fluid is toxic, then two walls with drainage between them and leak detection are required. These are often used even for non-toxic heat transfer fluids, such as propylene glycol (antifreeze). In case of leaks, the double walls protect the potable water supply. Double-wall heat exchangers increase safety.  An example of a double-wall, liquid-to-liquid heat exchanger is the "wrap-around heat exchanger," in which a tube is wrapped around and bonded to the outside of a hot water tank

– Solar heating systems with air-heating solar collectors do not usually require a heat exchanger between the collector and the air distribution system. The systems that use air heater collectors to heat water use air-to-liquid heat exchangers, which are similar to liquid-to-air heat exchangers. The radiator on the front of a large truck engine looks similar to these. In the secondary circuit, hot tap water can be produced in one of two ways: • Instantaneous heating - The water is heated as soon as it is requested. Hot water can be produced instantly using either a secondary heat exchanger, such as a brazed heat exchanger (BHE), or a bi-thermic heat exchanger. Heating that is semi-instantaneous - Hot water is not always produced on demand. Rather, hot water from the primary circuit is stored in a high-temperature tank.