This historic book may have numerous typos, missing text, images, or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1906. Not illustrated. Excerpt: ... short ducts and low velocities. The latter are used chiefly for exhausting air, although they may be employed to advantage for supplying air to buildings of relatively small size. 3. Plenum And Exhaust Or Vacuum Systems Of Ventilation Compared. The plenum system is a pressure system designed to force air into a building, creating a slight pressure therein, and overcoming to a certain extent the in-leakage of cold air. The air-supply may be taken from a source the surroundings and condition of which are known, thus insuring its purity, and when necessary this air may be filtered by passing it through cheesecloth screens. The exhaust or vacuum system is a system of ventilation designed to exhaust the air from a building, creating a tendency toward a slight vacuum therein. With the exhaust method alone, the air which takes the place of that drawn from the rooms, comes from uncertain sources which may not be favorable to a pure supply. It is a common mistake to suppose that with the exhaust system all the air required to take the place of that removed will flow in or be drawn in through the flues leading from furnaces or indirect radiation when these systems of heating are used. As a matter of fact, the air which is to take the place of that removed, tends to follow the lines of least resistance, and much of it is drawn in around doors and windows, causing uncomfortable draughts. On the other hand, all the air supplied with a plenum system will not pass out of the room through the vent-flues provided for the purpose, but instead, much of it will pass out around windows and doors, especially if theso happen to be on the lee side of the building. The above considerations lead naturally to the conclusion that the best arrangement is to combine the plenum and e...