A thermal expansion tank is a small tank used in conjunction with a closed water heating system. This tanks are typically used in conjunction with traditional water heater, tankless water heater or water heating system to protect closed water heating systems and domestic hot water systems from excessive pressure. The tank absorbs excessive water pressure caused by thermal expansion which can damage a water heater and portions of the home’s plumbing system. It serves as a overflow receptacle, absorbing excess water volume created by expansion that occurs when heating water. They are typically partially filled with compressed air to act as a cushion.
When water is heated it expands. For example, water heated from 90°F to a thermostat setting of 140°F in a 40 gallon hot water heater will expand by almost one-half gallon. This is because when water is heated, its density decreases and its volume expands (see below). Since water is not compressible, the extra volume created by expansion must go someplace. During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. Hence, system pressure increases.
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