Water expands about 4% when heated from room temperature to 140°F, and that volume has nowhere to go in a closed system. An expansion tank absorbs it into an air cushion behind a rubber diaphragm, which is what keeps the relief valve from weeping every time the burner fires.
Heating tanks sit on the boiler loop and are sized to the system water volume and the temperature swing. Potable tanks go on the cold inlet of a water heater and must be rated for drinking water, with a stainless or polymer-lined connection instead of the plain steel used on hydronic models. Well tanks are a different device again — they store a usable volume of water so the pump does not start on every tap.
The tank has to be pre-charged to match the system fill pressure before it is installed, with no water in it. Charging it after installation reads the water pressure, not the air, and gives a tank that is effectively full and useless.
