The next time you are faced with replacing an old steam boiler that was attached to a two-pipe air vent system, try the old-timer's trick. Remember that the replacement boiler is smaller in size and holds less water, thus requiring a boiler feed tank that acts as a reservoir for the new boiler. The feed tank helps prevent the boiler from shutting down due to a low water condition, or flooding the boiler if a feeder is present. With the boiler feed tank, all the system surges take place in the receiver, allowing the boiler to maintain a steady water line.
Now the steam can reach down into those former wet returns and shove all that water back and forth in the piping, eventually showing up at the vent pipe, filling the boiler room with steam. Of course, the water hammer is incredible.