A thermal expansion tank is a small, pressurized tank with a compressible air cushion (diaphragm) that is installed on the supply side (cold water inlet) of a water heater.
An expansion tank is always highly recommended if you have a ‘closed-loop system’ caused by any kind of check valve or pressure regulating valve installed on your house’s water supply line. A common analogy compares having high water pressure in your home to having high blood pressure.
Expansion tanks work by equalizing pressure throughout the system. … As hot water enters the heating system, the pressure in the system increases. As pressure increases, the diaphragm in the expansion tank is pushed down. This compresses the air in the tank, creating more space for excess water to enter.
The cost to install a thermal expansion tank can range from $300 to $400.