What is a Thermal Expansion Tank? Do I Need One?
What is thermal expansion?
When cold water in a water heater is heated, it expands and the pressure can rise from a normal 60psi up to as much as 150psi, which is excessive. The relief valve on your water heater may go off and cause a minor flood. A thermal expansion tank has air inside which is compressible. The heated water can expand and the pressure stays steady.
Thermal expansion tanks are required on water heaters if the home has city water with a pressure regulating valve.