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.
207. This system is commonly called the one-pipe system, but the name is a misnomer. While it is practicable to operate a steam-heating system with a single main, and with single connections to the radiators, it is wholly impracticable to do so with hot water. The nearest approach that can be made to a one-pipe system of hot-water distribution is to connect both the flow and return branches of the radiators to the same main, substantially as shown in Fig. 78. The main is made of unusually large diameter, so that it acts as a reservoir, and the current through it is comparatively slow. The risers are tapped into the top of the main, and the returns are connected into the side or bottom, so that they deliver the cooled water into the lower part of the main.
It is necessary that the temperature of the water he maintained at a proper degree throughout the whole length of the main, so that the water supplied to the radiators farthest away from the boiler will be reasonably hot; otherwise, the radiators supplied with the cooler water must be made too large, in order to compensate for the low temperature of the supply.