The plant occupies a room in the basement and the drawing shows the method of heating, together with the plan of distribution. The air duct leading to the room above furnishes an example of the manner of admitting the warmed air to the rooms. The dampers _C₁_, _C₂_, etc., are controlled by separate motors. The motor _M_ is under the control of the thermostat _T_ in the room above. Any change of temperature in the room is corrected by the damper to admit cold or warm air as is desired. [Illustration: FIG. 173.--Plenum hot-blast heating system with temperature regulation.] The power-driven fan _F_ draws in outdoor air from an opening _A_, through a set of heater coils _H₁_, in which it is raised considerably in temperature. The heater in this case is a coil of steam pipes. The air--after being warmed--is taken into the fan and from it may be sent through a second set of coils _H₂_, to receive additional heat, or if already sufficiently warmed the air from the fan may pass under the second set of coils and receive no heat from them. Under the first heater coil is also a bypass which may be opened by the motor _N_ to admit cold air that is drawn directly into the fan. The movement of the air through these bypasses is under control of the thermostat, which causes the motor _N_ to open or close the bypass to suit the temperature of the room. When the bypass is opened the steam is shut off from the heater coils. Examination of the drawing will show that the air from the fan may pass through a second heater _H₂_, to the place marked _warm air_, or it may pass under the heater to the compartment marked _cold air_. The amount of warm and cold air which enters the duct is regulated by the position of the dampers _C_. The position of the dampers _C_, which is controlled by the motors _M_, is made to take amounts of cold or warm air to produce the desired temperature in the rooms. The motors _C₁_, etc., are under control of the thermostat in each room. Any change of temperature in the room will immediately affect the thermostat. The effect on the thermostat will so change the air pressure in the motor that the valve _C_ is moved to correct the difference in room temperature. If warm air is demanded, the motor changes the damper _C_ to close the cold-air supply and take air that must pass through the heater coils _H₂_. If only cold air is desired the damper will turn to shut off the course through the heaters and admit air directly from outdoors. =Humidifying Plants.=--Mechanical ventilation plants that are intended for washing the air may be made up of parts similar to that of Fig. 173, but in addition to the apparatus shown provision is made for the air to pass through a chamber filled with a spray of water. The air in passing through this spray is washed free of dust and at the same time absorbs water necessary for its desired humidity.