The expansion tank does the job of maintaining even pressure inside the boiler by giving its water a place to expand and a cushion to absorb the pressure and then regulate it. Inside the expansion tank, a diaphragm divides the chamber into two sections. This system prevents air from entering the water.Click to see full answer. Also question is, how do I know if my expansion tank is working? How to Tell If an Expansion Tank is Working Tap on the tank. It should be hollow about half way up. Check the temperature by feeling the top and bottom of the tank. The tank should be warm on the bottom and cool on the top if it is working properly. Use an air gauge to check the pressure inside the tank. Check to see if your release valve is spurting water. Additionally, what happens when an expansion tank fails? If the expansion tank is missing or not operating properly, pressure in the system may cause the boiler’s pressure relief valve to vent water. Or, air bubbles being released by the heating water may gather somewhere else in the system, causing a blockage that stops the flow of hot water. Also Know, where does the expansion tank go on a boiler? Usually the expansion tank is mounted on the hot or supply side of the boiler and usually it’s located before any circulator pumps. If you see a tank that looks like the unit below, your boiler has an internal-bladder type hydronic heating boiler expansion tank.How much water should be in a boiler expansion tank?The expansion tank is usually about half full of water and half full of air–by volume. One tank full of air at atmospheric pressure will be about half its volume when pressurized. When in doubt, drain it. First, shut the valve between the tank and the boiler, or the rest of the system.