2 gal. Thermal Expansion TankBuilt with heavy duty butyl diaphragm system to absorb the expansion of water volume and help prevent dangerous pressure build up. This Everbilt Expansion Tank will absorb pressure increases when properly installed on your closed water system. Most local codes now require an Expansion Tank on residential closed water systems. Water pressure over 80 psi can shorten the life of your water system and appliances. California residents see Prop 65 WARNINGS