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.
The circulation in a hot-water heating system is a movement of hot water from the boiler to the radiators, where it parts with some heat, and a consequent movement of colder water from the radiators to the boiler to become reheated. Without circulation heat cannot be conveyed from the boiler to the radiators. The velocity of circulation depends chiefly upon: (a) the difference between the mean density of the ascending current and that of the returning current; (b) the vertical height of circuit above the boiler; and (c) the resistance to flow due to friction, change in direction, etc. The theoretical velocity could easily be computed, but, as it is only imaginary, is of little value to the practical man; and, further, the actual velocity bears no definite ratio to the theoretical.