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Simple physics tells us that when a fluid heats, it expands in volume. For a home water heater, this means that when a full tank of water is heated up, it may require more space than is availvable in the tank.
What is a Thermal Expansion Tank?
The thermal expansion tank serves as an overflow receptacle by absorbing excess water volume created by the tank water heater when heating water, as well as absorbing fluctuations in the incoming water supply pressure. Since water expands when it is heated due to thermal expansion, the water heater creates extra water volume every time it heats water. It's estimated, for example, that the cold water in a standard 50 gallon water heater expands to 52 gallons when heated to 120° F. This extra water volume can create excess pressure in the plumbing system, and if the increased pressure is enough it may, over time, cause damage to the water heater, plumbing fixtures and the water pipes themselves. (This is normally not a problem on modern on-demand, tankless water heaters. Only tank style water heaters are subject to this problem.)
Closed Private Water Supply Systems (household plumbing)
In a closed system, which is a private water system equipped with a one-way valve such as a backflow valve, check valve, or pressure reducing valve that doesn't allow expanding water to push back into the District's water supply. The extra water volume that results from heating puts stress on other parts of the plumbing system and can result in broken pipes, leaky taps, relief line leaks, or a damaged or leaking water heater. If the water pressure is serve, there can be even more serious damage to the water heater, possibly posing a safety hazard. Therefore in this kind of situation, it isn't enough to replace the water heater, because the pressure fluctuation will continue to stress the plumbing system. Nor does the standard temperature and pressure-relief valve adequately correct the water heater thermal expansion problem.
Open Private Water Supply System (household plumbing)
In an open system where expanding water can push back into the District's water supply, there is rarely any problem with thermal expansion.