This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Before deciding upon the position of a special air-inlet, particularly to an existing room, it is important to ascertain the direction of the air-currents in the room with a new to ascertaining that position from which the incoming air will be most equally diffused without creating draughts. This is not always easy to ascertain, because of the invisible nature of the atmosphere, and the very slight influence it exerts when its movement is slow.
One method adopted to assist in the detection of air-currents is the employment of volatile essences, the odour of which may be easily recognized, such as oil of pepperment Their power of diffusion, however, is so rapid, and the course of air-conveyance at times so obscure, that it is difficult to ascertain with certainty, by their use, the course of air-currents. Smoke is also employed for the purpose, but the heat evolved in the production of the smoke often de-terminea its direction upwards when otherwise the movements of the air have a horizontal tendency. If air-currents are moderately strong, a lighted taper is useful as a test by observing the deflection of the flame.
In some houses it will be observed how much more quickly and completely kitchen-smells are conveyed throughout the building than in others; this generally results from the want of independent ventilation to the several apartments, although in some instances the evil arises from the genera] arrangement of the plan and the relative positions of fireplaces and doors.
The provision of separate air-inlets for every room is undoubtedly a great step towards preventing the air from one room (the kitchen, for example) being drawn into any of the other rooms. The principal varieties of such air-inlets will now be described and illustrated.
Tobin tubes (Fig. 565) have been largely used for air-inlets, but as usually supplied and fixed they cause so much discomfort, that frequently they are kept closed or quickly removed. One disadvantage they have which has not received sufficient attention: viz. the external gratings are, for ground-floor rooms, often placed too near to the ground-level, and at times I have found them situated close to sources of impurity, such as gully-gratings and accumulations of refuse. There is not the slightest necessity for a long vertical tube; the direct upward tendency given to the air passing through such a tube at a high velocity causes it to rebound from the ceiling and come down like a cold shower upon the occupants of the room. Wall-papers and ceilings alxo become disfigured by dirt brought in with the air. The length of tube is also liable to become fouled.
Fig, 563 - View of Tobin Air-inlet Tube.
Fig. 566 shows a short form of Tobin tube, with a valve a, perforated baffle-plate B, and a projecting piece 0 to prevent discoloration of the wall above. Canvas bags are also provided for inserting in these tubes, but unless they are regularly cleaned, they become very dirty, and prevent the entrance of air.
Simple openings through the outer walls, expanded inwards and provided with a flap or louvre-regulation, suitably placed on the same side of the room as the, fireplace, are by far the best air-inlets which can be supplied to supplement window-openings, because they are simple and easily kept clean. So long as it is compatible with comfort to admit external air direct to the room, without first raising its temperature, the air may be admitted by such openings, and be equally distributed throughout the apartment in sufficient quantity.
As a rule inlet-openings are far too small, and consequently when the extract power in the outlet-flue is considerable, air enters with too great velocity and causes draughts, or air is drawn from other source The sectional area of an ordinary chimney-pot is about half a superficial foot, and the velocity therein will, with a fire burning, often be at the rate of 600 feet per minute, which is equal to a withdrawal of 300 cubic feet of air from the room in that space of time. In order that all the air required should be admitted by the specially-provided air-inlets, and that on entering the room the velocity may not exceed 5 feet per second, an absolutely clear opening of 144 square inches should be provided for a hole straight through the wall, but with a grated hole increasing in size from the outer to the inner face, the outer opening might be 15 inches by 12 inches, filled with an iron or terra-cotta grating, with perforated zinc of 1/8-inch mesh on the inside, together with a regulating flap. The opening on the inner face of the wall might be 24 inches by 18 inches, and would be best kept clear, but might be inclosed by a wire-guard fixed with buttons, so as to be easily removed. Such openings would be little noticed if coloured to the same tone as the walls of the apartment, and they can easily be kept clean. If placed just below a picture-rail, or provided with a slight ledge above, as shown in Fig. 567, marking of the walls by the incoming air will be practically avoided. For ordinarily calm weather, the inner Hap should be entirely open, but when Strong winds are blowing against the outer face of the opening, it may be necessary to partially or even wholly close it, for it is difficult to devise a covering for the exterior which would baffle every wind that blows.
Fig. 566 - Short Tobin Tube.
A, valve; B baffle plate; c, lip to prevent discoloration of wall above.
Fig. 567 - Trumpet-mouthed Air-Inlet