Solar water heating systems almost always require some kind of thermal storage for solar heated water. Tanks used in solar water heating systems are available in two basic types: pressurized and atmospheric.

Pressurized tanks, constructed of stainless steel or welded steel with a baked-on glass liner, are made to withstand city water pressure without rupturing. Polyurethane foam insulation is used in between the metal skin and interior tank to minimize heat loss. Pressure relief valves are included in the design of pressurized tanks. If the pressure builds up too high inside the tank, the relief valve opens and water is released. Pressurized tanks range from about $500 for an 80 gallon capacity to about $17,000 for a 1,000 gallon capacity. The life span of a pressurized tank around 13 years but can vary depending on local water conditions and tank maintenance (changing the anode rods regularly).

Atmospheric tanks are basically just non-pressurized containers to hold solar heated water. Since they are not completely sealed from the surrounding atmosphere, they should be well insulated. An atmospheric tank that is filled beyond its capacity will overflow. In addition, atmospheric tanks will lose water through evaporation. A variety of materials are used in the construction of atmospheric and include stainless steel, EPDM lined steel, EPDM lined wood, polypropylene and fiberglass. Tank costs and longevity vary with material selection.

A two tank solar water heating system consists of a solar storage tank (it can be atmospheric or pressurized) and a pressurized backup tank. A one tank solar water heating system uses a pressurized tank which serves both as the solar storage tank and the backup water heater.

Simply stated, when using an atmospheric solar storage tank in your design, heat from the collectors is delivered first to the atmospheric tank, then on to the pressurized tank. When no atmospheric tank is used in the
design, with only a pressurized tank, the solar heat can be sent directly to the one pressurized tank.

Should you incorporate an atmospheric solar storage tank in your solar thermal design? Your solar equipment manufacturer should be able to help you decide what is best for your individual needs. However, if your tank capacity needs to be over 240 gallons, it may make sense to include an atmospheric tank for economic reasons.

See upcoming blog article for more information regarding atmospheric tanks.