[Illustration: FIG. 175.--Direct steam heating system with mechanical fan-blast ventilation, temperature regulation and humidity control.] Fig. 175 shows another arrangement of a similarly controlled plant in which steam is used for humidifying the air. The air is admitted at _A_, from whence it passes through a steam-heating coil _S_, which raises it to a predetermined temperature. The steam jets are arranged at _H_, for providing the necessary moisture. The humidostat through a motor valve _V_ governs the amount of steam that is permitted to enter the humidifying chamber. A thermostat located in the air duct at _B_ controls the temperature of the air sent to the rooms by regulating the amount of heat given out by the steam coils _S_. This control is made still more sensitive by use of a cold-air bypass. The damper _D_ is opened by a motor valve to admit cold air at the same time the steam is shut off from the heater coils. In this plant the ventilating air is not intended to supply all of the heat to the rooms. A thermostat on the wall controls the room temperature by regulating the amount of steam admitted to the radiators. In the ventilating plant previously described, all of the heat for the building is supplied through the ventilating system; in the plant shown in Fig. 175, the heating apparatus which warms the building is entirely separate and may be used when the ventilating system is inoperative. The humidity is controlled by admitting saturated air to the warmer air of the rooms in such quantity as will produce the desired mixture. The humidostat, on the left-hand wall, regulates the quantity of moisture by opening or closing the steam valve _V_ as occasion requires. Another example of air-conditioning plant similar in principle to that just described is often called the dew-point system. It depends for its action on a definite dew-point temperature at which the air is saturated with moisture, before being heated to room temperature. The air to be conditioned is first warmed, by passing through a set of tempering coils, to a degree at which it will contain the necessary moisture when saturated. After saturation the temperature is raised by a second set of heating coils to the room temperature, the moisture contained being right to give the desired humidity. To illustrate, suppose that it is desired to maintain a constant humidity of 50 per cent. saturation at 70°F. in the building. The temperature at which the air must be saturated, to contain 4 grains of moisture per cubic foot, is found in the table on page 199 to be 48°F. The entering air must first be raised to that temperature by the tempering coils. The air then enters the spray chamber where it absorbs moisture to saturation, by contact with a multitude of water particles. This saturated air now passes through a second set of heated coils and takes up heat sufficient to raise it to the finished temperature.