This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
It is often desirable to produce a continuous circulation in the distributing pipes so that hot water may be drawn from the faucets at once, without waiting for the cooler water in the pipe between the boiler and the faucet to run out
This is accomplished by connecting a small pipe with the hot-water pipe near the faucet, and connecting it with the bottom of the boiler as shown in Fig. 17. This makes a circuit, and a constant circulation is produced by the difference in temperature of the water in the supply and circulation pipes.