As of 2012, any home built after 2012 is required to have an expansion tank installed at the top of the water heater. Thermal expansion tanks allow extra space for the excessive pressure to travel when the water heater is in use. They are built to expand and contract when the water heater releases the excess pressure. If the water pressure gets too high it can damage valves in plumbing fixtures, joints in supply pipes and the water heater itself. Expansion tanks work by equalizing the pressure throughout the system, which creates more space for excess water to enter. Some manufactures may even void you water heater warranty if you don’t have an expansion tank for your water heater.