There are two type of tanks. The bladder type which hangs on the heating pipes and the older steel tank which hangs between the floor joists. They work completely different. One is called an air control system and the other is an air elimination or air removal system.
The purpose of the expansion tank or compression tank as it may also be called, is to absorb the expansion of the water as it is heated up. When you raise water temperature you raise water pressure. Since water is a non-compressible the air charge in the expansion tank acts like a pillow and compresses as the water is pushed into the tank. This keeps the pressure in the boiler from rising to high and blowing off the pressure relief valve. As you may have figured out the amount of air in the tank is important in controlling water pressure.
There are time when the air charge gets too low and the pressure rises dramatically when the boiler runs. Too much air is not much of a problem. The old ceiling mounted tanks when drained properly by allowing air to enter while draining, will allow the tank to take on air at atmospheric pressure. The drain valve is closed and the water valve is reopened the tank fills to the appropriate pressure, controlled by the pressure reducing valve.
The bladder type tank has to have the air charge checked and adjusted periodically. The air charge must be equal to or a couple of pounds above the system fill pressure.
Let's discuss each type of expansion tank starting off with the bladder type tank as this is most commonly replaced with the new boiler.
The bladder type tanks today have a diaphragm between the water and the air charge. They are usually mounted on the air separator with an automatic air vent on the air separator to eliminate the air thus air elimination system. They may also be installed with a micro-bubbler type air eliminator to get rid of the air in the hydronic system. The water side is open to the heating system. There is an air schrader valve on the bottom of the tank to check the air charge. When installing this tank the air charge must be checked before water is added to the heating system or at least before the tank is charged with water. The air charge must match the system fill pressure and up to 2 lbs over is OK. If you need to operate above 12 psi as this is the standard air charge raise the air charge to the higher water pressure. For example. If you are heating a three story building you will normally need 18 psi to get the water to the third floor. That means before you put water in the system raise the air charge in the tank to 18 - 20 psig.
Once the tank is installed and the system is filled the tank should have semiannual maintenance to verify the air charge is at the proper level. The tank air charge cannot be checked with water pressure on the heating system. The water pressure must be at "0" psig. The system radiation type and water volume dictate the tank size. Manufacturers will have charts to size the bladder type tanks. When the tank is properly sized and the air charge is correct the pressure should not change but a few pounds as the system heats up from a cold start to 180f.