Mechanical ventilation may be accomplished by power-driven fans, either by exhausting the air from the building or by forcing air into it, and under some conditions a combination of the two methods is used. [Illustration: FIG. 171.--Exhaust fan for induced ventilation.] [Illustration: FIG. 172.--Ventilation apparatus in which is included the heater coils, the fan and the motor.] The exhaust method of ventilation is that in which air is blown out of the building by a fan; and the supply, to replenish that taken away, is conducted into the building through ducts prepared for the purpose. In some cases the induced air supply leaks into the rooms through the joints in the doors and windows, and through the accidental crevices. In Fig. 171 is shown a simple exhaust fan installed to produce such a change of air. It is suitable for kitchens and other places where it is desired to eliminate smoke or gases rather than to produce a supply of air. With this apparatus the air of the room is blown out by the rotating fan and new air to take the place of that exhausted is drawn in at any convenient opening. =The Plenum Method.=--That form of mechanical ventilation by means of which air is forced into the rooms is known as the plenum method. It is the most positive means of air supply because its action is attended by a slight pressure above the outside air; it is continuous in action and the amount of entering air is under control. The escape of the expelled air is made through vent flues especially constructed for the purpose. =Ventilation Apparatus.=--Fig. 172 illustrates the form of apparatus used for ventilating buildings where no attempt is made at washing or humidifying the air. Enclosed in a sheet-iron case _C_ is a fan which is driven by the electric motor _M_. The capacity of the fan, for the delivery of air, is made to suit the requirements of the building. In this case the fan is secured to an extension of the armature shaft of the motor. Connecting with the case which encloses the fan is another sheet-iron box _H_, containg coils of heating pipe. The heating apparatus is designed to change the temperature of the entering air to suit the requirements of the building. This represents the draw-through or induced-draft type of ventilation apparatus. The air delivered by the fan induces a flow of outside air which is drawn through the heating coils and discharged through the opening _E_. At this point it enters the main ventilation duct from which it is distributed by branch conduits throughout the building. The temperature of the air sent out from the fan is regulated by the steam valves of the heater coils to suit the prevailing conditions. Under some installations of this character the ventilating air is made to furnish the heat necessary to warm the building as well as to provide its air supply. As ordinarily used, however, the temperature of the ventilating air is the same as that of the room.