The Thermal Expansion Tank 2.1 Gal is utilized on potable water lines between the backflow preventer or pressure reducing valve and the water heater for protection against water thermal expansion in residential applications. As the system pressure increases, water enters the polypropylene-lined water chamber of the tank, expanding a diaphragm into a pre-charged air chamber to keep the system pressure below the relief valve setting. Materials include a carbon steel shell with an epoxy finished coating, and corrosion-resistant stainless steel connection with a polypropylene liner and Butyl rubber diaphragm.