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.
Too much stress cannot be laid upon the fact that in most British homes, when- open fires are provided in almost every room, the smoke-flues from the fireplaces must be regarded as the exits for air. Under ordinary circumstance-they are sufficient for the purpose; when a tire is burning in the grate, the smoke-flue is practically the only way whereby the air of the apartment finds its way to the open. It is therefore useless to provide other openings as exits. without the employment of heat or mechanical power, because such will almost invariably be found to act as inlets and not as outlets, and will be liable to cause discomfort by admitting too directly large volumes of cold air in improper directions.
To place a mica-flap ventilator (tigs. 582 and 583), as an outlet, in the external wall of a room with an open fire, is a very common mistake; when the windows and doors are closed, the suction up the flue will almost invariably close the flaps, so that air cannot possibly eacape in that direction, and air will rarely force its way out past such flaps, even when doors and windows are freely Opened The only useful position in which a flapped ventilator can be fixed is in direct connection with the flue from the fireplace, towards the upper part of the room; then, provided the throat of the flue is throttled and there is a good draught upwards, the heated air from the upper portion of the room will be curried off. This, however is only an advan tage when a room becomes overheated, and an objection to ventilatora so placed is the almost certain disfigurement of the walls and ceiling by smoke being at times drawn into the room. Better ventilation of the apartment could be secured by the employment of a suitable and well-placed air-inlet, with the fireplace flue acting as the sole outlet.
Fig. 583 -Section of Mica-flap Air outlet with Double Flaps
If it is really considered desirable to employ a flapped ventilator, one has been devised which works within a segment of a circle, so as to avoid noise from the movement of the flaps, whenever the forces within and without vary. This form (Fig. 584) is, however, unsuited for connecting with smoke-flues, because the flaps can never be made to fit so tightly that smoke will not pass around the edges; consequently its use is very limited.
Fig 584 - Improved Mica-flap Air-outlet.
Fig. 586.-Projecting Air outlet lor Fixing in Walla
For rooms without fireplaces and ventilation-flues, outlet ventilators in the external wall may be of service. A ventilator of this kind is shown in Fig. 585; it can be supplied with a valve shown in the plan and section, to prevent any inrush of air. Another form is shown in Fig. 586, which has not the disadvantage of projecting beyond the face of the external wall, but which may not be absolutely free from back-draught.
Extract-cowls on outlets, such as are shown in figs. 587 to 597, are designed (a) to act as wind-bafflers so as to obviate a direct blow down the outlet-shaft. (6) to prevent the inlet of wet, and (c) to accelerate the outflow of air by the suctional force of wind acting upon the cowl.
Fig 586 -Flush Air-outlet for Fixing in Walla.
Rival claims by numerous manufacturers have caused experiments to be made with a view to demonstrate the relative value of the various forms employed, and the conclusion arrived at, - after many trials carefully made by Commissioners appointed both in England and in America, - is that an open tube, with a protecting cap to prevent the entrance of wet, gives as good a result as any of them, if varying states of the weather are taken into account. When a strong wind is blowing, change of air can, as a rule, be secured within a building without special means, and to add to the facility of obtaining change of air at such times may simply mean greater discomfort to the occupants. In a still atmosphere, when change of air would be of advantage, the cowl exercises no power whatever, and unless provided with a valve, as in Fig. 597, there will certainly be a down-draught instead of an up-draught, if a suctional influence, such as an open fire, be employed within.
Nervertheless, outlet-cowls may be usefully employed in connection with buildings in which there are no open fires or other upcast flues, provided there are also suitable inlets. In many buildings - such as schools, churches, chapels, public halls, Ac. - which are only occasionally occupied, changes of air can thereby be effected at times when no one is present. Such changes of air will greatly assist in keeping the buildings in a pure and healthy state for occupation when required. Means, however, should always be conveniently placed for regulation, or even closing both the inlets and outlets; otherwise, in cold weather, discomfort from draughts will be experienced. There should, as a rule, be only one outlet to an apartment, suited to the size thereof, for if the outlets be multiplied, particularly when fitted with cowls, one may draw upon the other at times when the action of the wind is irregularly exercised upon them Several openings from the apartment connected to one cowled outlet, as in Fig. 587, may be advantageously used for withdrawing air from different parts of the same room.
Fig 587 - Walker's Extract - cowl with Two Shafts.
Fig. 588 - View of Lobster -back Rotary Cowl.
Fig . 589 -Rotary Ventilator fixed in Window pane.