Water radiator system has single loop instead of flow & return pipes -- does this make sense?

Question

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)!

Answer 1

The heating system that you have is called A "mono-flo" hot water system. The tees in your system can either be standard or restricted tees. The usual installation practice is to have one restrictor tee on either the supply or return and a normal tee on the other supply or return pipe. When I was still installing these systems I would usually install 1 tee if the radiator was above the supply pipe and 2 tees if the radiator was below the supply piping. However the use of 1 or 2 tees is usually dependent upon the size of the radiator. A very large radiator would require 2 tees to provide adequate flow. These systems can be zoned at each radiator or room if desired. If you desire to learn more about these systems, just type in " a mono-flo heating system" using any search engine. (I use google)