| First you have to understand a expansion tanks function. When new codes forced us to install check valves and backflow preventers on water service lines that made your water system a closed system. That meant that when the heated water expended it could no longer expand back along the water service line and would build up pressure in the syatem. this would kick off the T & P ,(Temperature and Pressure Relief) valve aqnd the valve would discharge hot water out the relief line. To prevent this we installed a Expansion,(Air) tank on the heaters cold water supply line for some place the water to expand to. you check a expansion tank exactly the same way you check a bladder tank on a pump. You tap the tank walls, if you hear a "clang" you have air inside and the T&P valve's protected. If you hear a "clunk" that indicates the tank's loaded with water and must be removed and drained to recharge it. If it can't be recharged it must be replaced. Good luck, Tom | 
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