A water heater expansion tank helps control the buildup of pressure in a hot-water system. Using an expansion tank can reduce damage to the water heater, pipes and fixtures.
When cold water enters a home water-heating system and is heated, it increases in volume due to thermal expansion. In some cases, this increase in water volume may exceed the capacity of the water tank. When this happens, the excess water volume flows out to the rest of the system. The increased water volume then causes an increase in pressure through the system.
In closed systems, which are set up to prevent water from flowing back into the city water supply, this increase in pressure can be particularly troublesome. Over time, the pressure fluctuations can cause serious damage. However, simply replacing the damaged components may not be enough.
To alleviate the problem, an expansion tank is used to provide a place for the extra water to go, reducing the pressure on the system. Many cities require homes with closed systems or areas where check valves are installed on water mains to have an expansion tank installed. Even if a system is not closed, it may benefit from an expansion tank. The tank provides extra storage space for heated water, thus effectively increasing the amount of hot water available to the home.