This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
208. It frequently happens that radiators which are located close to the risers and have a free return circulation will take more than their proper share of hot water. This not only diminishes the supply for the more distant radiators, but the water thus passed through is discharged into the return main much hotter than it should be, and the motive force of the system is impaired thereby. To remedy this trouble, the system shown in Fig. 79 is sometimes employed. The distinguishing feature of this system is that the water is compelled to travel exactly the same distance in going to and from any radiator upon a given floor. Thus, a radiator situated close to the riser will have but little advantage over one situated a long distance away, on the same level.
The flow main is shown divided into two sections e and f which extend around the basement walls to the point b, where they unite. A return connection is made to the boiler by means of the pipe c, thus making a closed circuit. The return mains, however, run in the opposite direction to that usually employed. They begin at the radiators nearest to the boiler, instead of the most remote, as in the common way. Thus, they begin at the radiators g, and run along parallel with the flow mains, until they finally unite into a single pipe d.
It will be seen that the water passing to the radiator g runs only a short distance in the flow main; but, since it is obliged to pass through the whole length of the return circuit, the aggregate distance traveled by the water in going to and from the radiator h is precisely the same. Thus the frictional resistance to the flow of water to all of the various radiators on the same floor is practically equalized if the pipes are properly proportioned.