Why does my furnace need an expansion tank? When water heats it expands and water can not be compressed, so without an expansion tank installed, pressure will build and cause the relief valve to blow off. The boiler expansion tank absorbs the expanding water by allowing the air in the expansion tank to compress, this prevents the pressure from building and allows the boiler to heat and cool without too mush pressure building up. Removing the furnace relief valve from the boiler or plugging it so that it does not leak is a very bad idea, this can allow the pressure on the boiler to reach dangerous levels causing an explosion that can destroy your whole block. So don't do that. Now on to the expansion tank.
For the most part Amtrol expansion tank is the most commonly used expansion tank. This is a # 30 it screws into the bottom of an air scoop This one will be easy to change, but if it is water logged it will come down hard once you unscrew it. It is very heavy when full of water.