I have an older system in my house. I use hot water heat with the old cast iron radiators and the expansion tank hanging between the joist in the basement ceiling. Last winter my system pressure got to high and the relief valve opened. I called around to different plumbers and long story short, they told me the expansion tank was bad and had to be replaced. I had a plumber come over, disconnected the old tank and replaced it with a new expansion tank about a 1/4 size smaller than the old one. The new tank is like the one you are describing, the diaphram kind. It is the only one on the system and works perfect. When the system is cold the psi is about 12 and hot it runs roughly 20-21psi. I do know that the size of your boiler determines the size of expansion tank required. If the new tank is the proper size for your system, that is all you should need. Perhpaps your old tank went bad and that is why a new tank was added. My old tank had a hole in it and kept filling up with water after I drained it. Those tanks that hand from the ceiling should remain empty and when the water in your system heats up, it will expand into the tank to keep system pressure down and once it cools and pressure drops, the tank should empty itself. But if the tank is already waterlogged, the water has nowhere to expand causing a system overpressure. This is what happened to me and when the new diaphram tank was added. The problem was corrected and the old tank removed. Just check with a plumber and find out if the new tank is the correct size. If it is, I would think that is all you would need. I am not a plumber or an expert in this field, I am just sharing some information from my own expereince with this closley related issue. I hope I was of some help. Good luck.