Expansion Tank

An expansion tank is a small pressurized tank, usually 2–20 gallons, installed on the cold-water supply line upstream of a water heater or on the outlet line in closed-loop heating systems. When water is heated, it expands but cannot escape back into the street main because check valves and pressure reducers lock it in place. Without somewhere for that expanded water to go, pressure spikes dangerously—the tank gives it space to move, absorbing the extra volume and preventing pressure relief valve leakage or tank rupture.

These tanks use an internal diaphragm or bladder to separate water from a precharged air chamber. The air side is pre-pressurized to match the static pressure in the system, usually 75–80% of the cold-water supply pressure. As hot water expands, it compresses the air, then the air pushes back during cooling cycles, pushing water back toward the heater. If the air charge leaks out or the diaphragm ruptures, the tank fills solid with water and stops working—you'll hear banging or see a slow pressure creep.

Code requires expansion tanks on any closed-system water heater in most jurisdictions. Without one, you're burning out relief valves, getting thermal shock, and shortening heater life. Check the pre-charge pressure with an air gauge when you install; it should match the cold-water system pressure at the lowest point in the line.