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The system in your house is probably equipped with the type of expansion tank that was used before captive air tanks were invented. There's nothing wrong with continuing to use it. Over time the air in the tank is gradually absorbed by the water. You should drain the tank at least once a year, usually at the start of the heating season. There should be a valve on the pipe to the tank. Close that valve. Then connect a hose to the drain spigot and open it. Most installations have an air bleed on the shutoff valve to the tank, if there is one open it as well. When the tank stops draining close the drain and the air bleed, open the valve to the tank. That's it!