Two Pipe STEAMsystems are often found in "older" homes with big solid systems as well as institutional buildings like schools and commercial units of the big citiessometimes have a vent on the radiator, and sometimes they don't. If you fall into the category of Two Pipe STEAM, you are the caretaker of a quixotic but simple confluence of pipes running under your feet. They are a marvel of old school engineering.
Like Hot water heating systems, there's one pipe in and one pipe out. Steam comes in on one side and once it cools and condenses, it trickles back out the other side of the radiator, all the way to the boiler, to repeat the process. | 
