In Fig. 174 is shown a heating and ventilating system in which the air conditioning is accomplished by automatic regulators for both temperature and humidity. The plant occupies a room in the basement, and a room directly above illustrates the conditions that prevail in all of the other rooms of the building. The principal features of the plant are the fan _G_, which supplies the air; the hot-air furnace _H_, which furnishes the heat; and the water spray _S_, which provides the moisture with which the air is humidified. [Illustration: FIG. 174.--Furnace blast system of heating, with temperature regulation and humidity control.] The air is drawn in at _A_ to a room in which a motor-driven fan _G_ forces the supply through the heating apparatus into the building. The air after leaving the fan passes through a cold-air duct _C_ to the heating surfaces _H_ to be warmed. The air in passing over the heating surfaces is raised to a degree considerably above the temperature of the rooms. The hot air leaving the heater _H_ enters the tempered air chamber _T_ through the passage _K_. A damper _M_ provides means for also admitting cold air to the chamber _T_ directly from the fan. The thermostat, located at _O_, is connected with a pneumatic motor _V_ (similar to Fig. 170) which regulates the supply of cold and hot air from _K_ and _M_ to suit the desired temperature of the air supply for the rooms above. The arm of the motor _V_ is so arranged that an upward movement opens the cold-air and closes the hot-air passages; the downward movement produces the opposite effect. The motor _V_ thus controls the temperature of the air. In this system the air is humidified by a direct water spray marked _S_ in the drawing. A part of the hot air from the heater _H_ may escape through the damper _W_ and absorb water on its way to the duct _D_, which takes the air to the room above, where it enters through the register _E_. This air as it comes from the heater, being hot, will absorb a larger amount of water than the air could hold when cooled to room temperature; for this reason only a part of the air supply is humidified. The supply of the hot humid air is admitted to the duct _D_ in such quantity as will produce the desired degree of humidity in the rooms. The degree of room temperature is governed by the thermostat, in the room, which, by means of the motor _N_, controls the damper _F_. This damper admits hot humid air and the tempered air from the chamber _T_ in proper proportion. At any time the humidity of the air in the room reaches the maximum amount for which it is set, the humidostat, through its motor, closes the valve _R_, which controls the water supply to the spray nozzle, and the moisture in the air is reduced until a further amount is demanded. With apparatus of this kind the temperature and humidity may be kept practically constant.