I have a hot water heating system connected to several radiators throughout the house. While all diagrams I have seen of radiator plumbing have a separate flow and return pipe, this one has a single loop of pipe from the furnace outlet to its inlet. Each radiator branches off this main loop and returns to it a little further along the loop.
My first instinct was "this doesn't make any sense", that there should be very inefficient, if any, circulation through the radiators (though they do clearly heat up).
But I'm noticing that each radiator has one plain 3-way split from the main loop, and one joint that has something else going on: (Flow direction is left to right; this picture is the only example where I can fit both pipe connections in the same picture)
I tried some googling to find what this device might be called, but no luck. My best guess is it's a valve that prefers flow from the return pipe but will open to through flow if the return pipe does not have sufficient pressure (i.e. radiator is shut off). Can anyone identify this device? Is this a typical water radiator setup?
I'm hoping to learn a little more about how these systems are connected and if there's anything I should seek to change if I end up redoing any of this plumbing (for ex finishing the basement)!