Effects of Thermal Expansion within "Closed" Water Systems

The Cross-Connection Program encourages the use of backflow prevention assemblies to protect the public’s water distribution system. A backflow prevention assembly is installed on a building’s water service connection to create a "closed" water system. A close system is created because the assembly prevents the reverse flow of water from a building to the water main. This effectively prevents the flow of pollutants or contaminates into the public water system.

When cold water within a "closed" system expands due to heating, it increases pressure. This is known as thermal expansion. This increased pressure may cause leaks in pipes or faucets, "popping" of the water heater pressure relief valve, collapse of the water heater’s center flue, or other plumbing system problems.

To prevent pressure increase from thermal expansion, the District of Columbia’s plumbing code requires that a thermal expansion device be placed on the potable water system when a backflow prevention assembly is installed. To determine what type and size of device needed, please consult a licensed plumber.

For questions related to thermal expansion, backflow prevention or cross connections, please contact WASA’s Cross Connection Program at 202-612-3440.

Below is a diagram of a water heater without a thermal expansion device and a water heater with a thermal expansion tank, a common thermal expansion device.

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