The closed steel expansion tank has no moving parts (see Figure 10-43). It is normally two-thirds filled with water and one-third with air. As heated water expands and its excess volume enters the tank, it compresses the air at the top of the tank. The compression of the air in the tank results in an increase of system pressure, which is indicated on the boiler pressure gauge.
When the system water cools down, its volume contracts, and the air in the tank expands back to its original volume, causing system pressure to fall. To sum it all up, the rise and fall of system pressure is created by the expansion and contraction of the air in the expansion tank.
One problem encountered with a closed steel expansion tank directly connected into the system is that the system water can absorb the air and send it to the radiators and convectors by gravity circulation. Installing a gravity-flow check valve on the expansion tank will prevent gravity circulation.