An expansion tank is added to a closed water heating system to prevent damage to the system caused by the expansion of heated water when the system is not in use.
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An expansion tank is attached to the cold water line near the water heater to allow heated water to expand and be stored safely. Once a heating cycle has ended or a fixture is used, the pressure is reduced and the water in the expansion tank flows back into the heating tank.
Choosing an expansion tank requires the installer to know water heater capacity in gallons, the water supply pressure in pounds per square inch (psi), starting and ending heated water temperature and the maximum pressure setting for the relief valve.
For residential use assume a maximum temperature of 160 degrees Fahrenheit and maximum psi of 150. A 30-gallon tank requires a two-gallon expansion tank. A tank between 66 and 82 gallons requires a five-gallon expansion tank. All other heating tank requirements vary between two-gallon and 10-gallon expansion tanks based on psi.
For commercial use, assume a maximum temperature of 180 degrees Fahrenheit and maximum psi of 150. A 52-gallon heating tank requires a five-gallon expansion tank and 100- and 120-gallon heating tanks require 10-gallon expansion tanks. All other sized heating tanks change requirements based on psi.