does not come on; also less total fuel is used to bring the smaller second tank up to temperature. Single-tank arrangements are not recommended because they frequently activate the heating element every time there is a draw of water, rather than wait for the solar collectors to provide additional heated water. The two-tank arrangement (fig. 15-9) avoids this control problem. Two-tank arrangements are suited to retrofits since the second tank (the water heater) is already there. A variation would be to use a heat exchanger (copper coil) (fig. 15-10) in the tempering tank collector loop for freeze protection. The tempering tank then becomes an inexpensive unpressurized tank.

Another method of heat storage in air systems is latent heat storage. Latent heat is stored in a material as it changes state from a solid to a liquid. Materials that have melting points near the temperatures supplied by solar collectors store heat as they melt and release it as they resolidify. The two materials that have received the most attention are salt hydrates and paraffins.

Storage Tanks

Water may be stored in a variety of containers usually made of steel, concrete, plastic, fiber glass, or other suitable materials.

Steel tanks are commercially available and have been used for water storage. They are available in many sizes and are easy to install. However, steel tanks are susceptible to corrosion and should be lined or galvanized. Dissimilar metal at pipe connections should be separated by high-temperature rubber connections or galvanic corrosion will occur. Steel tanks must be well insulated to reduce heat losses.

Fiber glass and plastic tanks are corrosion-resistant and installed easily. They are available in many shapes and sizes. Although many commonly fabricated tanks begin to soften at temperatures above 140F, there are more expensive, specially fabricated tanks available that can withstand temperatures up to 150F. The types of plastics needed to store large quantities of water at high temperatures can be more expensive than steel. Buried tanks must be protected from groundwater and resist buoyant forces. The tank must be reasonably accessible for repairs. In mild or warm climates, an outdoor location may be feasible.

Domestic Hot-Water Systems (DHW)

Domestic hot-water systems (without space heating) may use lined, insulated, or pressurized tanks similar to the conventional water heater. Appropriate temperature-and pressure-relief valves must be used. Since it is possible for solar collectors to reach hot temperatures, a tempering or mixing valve should be used. A typical two-tank installation with proper valves and connections is shown in figure 15-9.

To size the collectors and storage tank, you must estimate the hot-water consumption of the facility or building. The hot water consumption rate for a typical family home is 20 gal/day/person. When the hot-water consumption rate is more than average, use 30 gal/day/person. So, 80 to 120 gal/day should serve a typical four-person family.

Thermosiphon Systems

A variation of the DHW system is the thermosiphon system. It uses the principle of natural convection of fluid between a collector and an elevated storage tank. The advantage is no pump or controller is needed. The bottom of the tank should be mounted about 2 feet higher than the highest point of the collector. This is the main disadvantage because structural requirements often prohibit the weight of a water tank on a high point of the structure. Also, since the thermosiphon system is connected directly to the potable water supply, it cannot be protected from freezing. A