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