This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
These are made in different ways, but the most common and reliable method is shown at (a) in Fig. 15. The cold supply pipe e has a branch f taken off, to supply the boiler. The return pipe g furnishes a supply to the waterback a, and the sediment cock It is used to empty the boiler when necessary. The hot water enters the boiler through the flow pipe c, and rising to the to] conveyed to the fixtures through d. The only objection to this arrangement is that it requires a long time for the water in the boiler to become hot, because the hot water continually mixes with the cold. To overcome this trouble, c often connected to d over the boiler, as shown in (b). But the latter method is also objectionable, in that circulation between the range and the boiler will cease when the service pipe is shut off, because the hot water will siphon down to the vent hole in the top of the inner tube. The waterback will then blow steam through c and d.