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.
The system of hot water piping consists of a free circulation of water from the heater to the radiators and back to the heater again. The supply pipe is taken off at the top of the heater and main and branches are run with an incline upward, the return pipes being parallel to these and connected with the bottom of the heater. Risers are taken off the tops of the mains and return pipes connected to the return mains in a like manner. (Fig. 65.) As this system depends upon the force of gravity to keep up its circulation an expansion tank (Fig. 66) must be provided, high enough above the highest radiator to insure free circulation, and the pipes should be run so that any radiator may be shut off without interfering with the general circulation. This tank should be supplied with a glass gauge and an overflow to some convenient place, and an open vent pipe should be taken from the top. The connection from the heating system enters at the bottom and an automatic connection should be made with the main water supply. A water connection should also be made at the heater to be used when the system is first filled, as by this means the air is driven upward and discharged through the vent on the tank.
The expansion tank should be large enough to contain a gallon of water for every forty square feet of radiation.