HOT-WATER HEATING There are to-day in vogue several systems of heating by hot water, and yet only two gen eral fundamental principles may be said to be involved in all the various systems—namely, gravity and forced circulation. In the gravity systems, circulation is caused by the natural effect of gravity operating through the differ ence in weight of the water in the flow or ascend ing column and that in the return or descending column—that is, the difference between the spe cific gravities of the two columns, without which the water would not circulate. In the systems employing forced circulation, the water is driven or pumped through the system, the necessary power for this purpose being supplied by a cen trifugal or rotary pump.
Hot-water heating is most frequently used in this country for the warming of residences, although it is often adopted also in buildings of larger size. The gravity principle is the one commonly used in residence heating, by reason of the simplicity of its system and the ease of management it affords. It is in the larger class of buildings that the principle of forced circu lation is generally practiced, owing to the size of the apparatus and the consequent extreme length of the mains or lateral supply pipes, and also by reason of the further fact that with this style of installation it is usual to supply a large number of radiators from a single riser.
We shall designate the two methods as low pressure and With the low pressure—the method commonly adopted—it is usual to carry the water in the system, in mild weather, at from 100° to 125° F., increas ing the temperature as the weather gets colder to possibly 180°, which is the maximum degree allowed under the ordinary steam-fitter's test; in other words, the fitter is invariably asked to supply a heater and a sufficient amount of radi ation to warm the building to 70° in the coldest weather, with the water at a temperature of not more than 180° at the heater. With the later type of accelerated or assisted systems of gravity hot-water work, the temperature may be carried to 240°, this equaling a pressure of 10 pounds on the system.
The heater used for low-pressure hot-water work is quite similar to the type of steam boiler already described, excepting that the steam space in the dome of the boiler is omitted in water constructions, and the internal waterways are sometimes (as they always should be) made smaller. The same features of construction essential to the production of a good, efficient steam boiler must necessarily be included to se cure a good hot-water heater. The circulation of the water within the heater itself should be positive throughout, and as nearly vertical as possible.
There are three general methods of piping employed in the installation of low-pressure hot-water work—namely, the ordinary two-pipe system, the overhead system, and the circuit sys tem, which are described in detail in succeeding pages.
To the steam-fitter or mechanic about to in stall a low-pressure system, a few words of cau tion regarding some features surrounding the work are advisable: First, the greatest care should be exercised in properly grading the piping as to size and pitch.
Second, it is of the utmost importance that the proper type of pipe connection be used at each point on the pip ing, for it is an old axiom among heating men that "as a circulation is once established, so will it continue." For example, if either one of two radiators supplied from the same pipe connection should receive the full circula tion, the other would be "robbed" of its supply and thereby prevented from doing its share of the work; and this unsatisfactory condition will continue until the form of connection is changed.
Third, provision must be made for extracting any ac cumulation of air from the system, and for keeping the system always as free as possible from air, which is regarded as the arch enemy of the heating man. A pipe or radiator air-bound is out of active service, as water will not circulate through any portions of the system in this condition.