With modern plumbing codes mandating backflow prevention, thermal expansion can cause pressure buildup in domestic water systems. When demand is put upon a potable water system, hot water is drawn from the water heater. Cold water from the supply line enters the water heater to replenish it. The colder water is heated to replace the hot water used. With the installation of a backflow preventer, check valve or pressure reducing valve on the supply line, the water heater and the system piping form a closed plumbing system under pressure. As the water is heated, thermal expansion occurs. Pressure increases until the relief valve opens and the expanded water "spills" from the water heater. This "spillage" results in wasted BTU"s and a potential safety hazard for the homeowner. Closed Potable Hot Water system without THERM-X-TROL Backflow preventer, pressure reducing valve or meter causes expanded (heated) water to build pressure causing the relief valve to open resulting in: Wasted BTU"s Shortened water heater life. The Amtrol THERM-X-TROL ST-5 Expansion Tank is designed to eliminate this problem by providing control of maximum pressures at a level below the relief valve setting. It also provides an additional space in the system to accommodate the increased volume of water created by thermal expansion, returning it to the system when hot water delivery is demanded. Closed Potable Hot Water System with THERM-X-TROL Expanded (heated) water is absorbed by THERM-X-TROL which means: Water heater and fixtures are protected Eliminates BTU and water waste, saving money and energy.