PROBLEM: Expansion Tank Has Excess Water and Inadequate Air
There are two basic types of hot water boiler expansion tanks. There is the older style large horizontal steel tank and the newer style smaller diaphragm tank (shown in photo above).
In older boiler systems the steel expansion tank may be found in the attic or suspended between joists in the basement. In newer systems the diaphragm expansion tank may be attached to the boiler piping near the boiler. The expansion tank must be properly charged with air to prevent water in the system from boiling and exceeding the desired 12 psi of pressure.
For tips on how to drain these two tank types, please see the tutorial How to Re-charge a Hot Water Boiler Expansion Tank.