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1209.3 Systems With Open Expansion Tanks
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An open expansion tank shall be located not less than 36 inches (914 mm) above the highest point in the system and shall be sized based on the capacity of the system. An overflow with a diameter of not less than one-half the size of the water supply or not less than 1 inch (25 mm) in diameter shall be installed at the top of the tank. The overflow shall discharge through an air gap into the drainage system. Isolation valves shall not be installed in the piping between the heat-distribution system and the expansion tank. Tanks shall be located in accordance with the manufacturer's instructions unless otherwise specified by the system design. Each tank shall be equipped with a shutoff device that will remain open during operation of the heating system. Valve handles shall be locked open or removed to prevent from being inadvertently shut off.