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.
Much has been said and written as to the respective advantages of upward and downward ventilation, and I am inclined to think that the more general condemnation of downward ventilation has been arrived at without full con-sideration of the facts. A statement previously made - that the flue from a fireplace is almost the only (if not the only) air-outlet from most rooms - can easily be verified by any one, particularly when 1 fire is burning in the grate. Open a window, or other "ventilator", wherever it may be about the room, and it will be found that air will as a rule enter In feet, only whan the suctional power of the wind outside can overcome the draught up the Hue, will air leave the apartment in any quantity except up the Hue. provided of course that the door is shut, so that other influences within the house may be excludeth Inasmuch as fireplaces are invariably placed near the floor, and in all modern fireplaces the lowest point of the flue is generally less than three feet above the floor, extraction of air in such cases must be in a downward direction from the ceiling towards the fireplace, unless inlets occur in improper positions, - e.g. about the lower portion of the room, - when quick horizontal movements of cool air direct from the inlet to the fire (in other words, draughts) may be set up, to the discomfort of the occupants.
There is nothing more important in the attainment of comfortable ventilation than to estimate rightly the functions of the fireplace flue. Most smoke-flues are built about 14 inches by 9 inches, and. when pargetted, will measure 120 square inches; but often at the throat, and more frequently by the chimney-pot, this area is reduced to between 60 and 80 square inches, or (say) half a square foot. As the lengths of flues vary considerably, and the question of no fire or more or less fire in the grate exerts a varying influence upon the amount of air which in a given time will pass up the flues, it will be seen how greatly must differ, even under similar conditions of the external atmosphere, the rate at which the air of various apartments will change, and, as the conditions vary outside, greater differences still will be found in the amount of change of air that will take place within the various rooms. Nevertheless, in any apartment provided with an open fireplace, the smoke-flue is the dominant means for securing an outflow of air.
It is possible to vary the size of the flue opening by means of a register, an appliance which fortunately is now less employed than formerly, because it is, as previously mentioned, too often wrongly used to entirely close the flue, and it is only in the smallest rooms and in cold weather that the ordinary smoke-flue can be considered too large for the purpose of ventilation.
Regulation of the amount of the change of air in an apartment is better effected at the inlet than the outlet; consequently all inlets should be capable of easy regulation. They should also be of larger area than the outlet, viz. the fireplace flue.
Windows and doors should in every dwelling be regarded, in addition to their other functions, as definite appliances for ventilation. Generally they serve as inlets for air. but at times as outlets. When an open fire is burning in a room, they are almost invariably inlets only, and in the majority of rooms in which there is a fireplace, windows and doors are by far the most effective appliances whieh can be employed for ventilation, provided they are suitably placed, and can be easily opened, more or less, at the discretion of the occupants. Although it may not be comfortable to have windows and doors constantly open when the rooms are occupied, there are many times when they may be partly opened, and on other occasions, when the rooms are unoccupied, they may all be thrown open, so that a thorough change of air may take place, regardless of draughts.
In stating this, there is no intention of condemning as useless other inlet-appliances when intelligently fixed and regulated, but there is a danger, which many people fall into when such appliances are provided, of trusting to these alone, and making them the excuse for not freely opening windows and doors; whereas all such appliances, which necessarily cannot be of very large dimensions, ought only to be regarded as supplementary to the open windows.
Doors have been mentioned above as appliances for securing change of air to an apartment; as such they are frequently and usefully employed, but a distinction must always be made between outer doors and those to the separate rooms, and it must be ascertained whether at the time they serve as outlets or inlets. If change of air takes place by way of an inner door, it must be noted that, should air pass out of the room thereby, it will probably proceed thence through the hall and by the stairway into some other rooms, the doors of which may be open, so that, if the air has become fouled in the first room, its foulness will become in part communicated to the hall, staircase, and other rooms; and when air enters a room by the door, it comes from other parts of the house, and the question will then arise whether it has in any way become contaminated in its passage.
The position of the door of a room, and the side upon which it is hung, will have a considerable effect upon the comfort of the occupants of the room, bacause, even if each compartment of a house be separately supplied with a suitable air inlet and outlet, the balance of power is so sensitive that, as the door is one of the largest openings round a room and the one most frequently opened and closed, it must exert considerable influence upon the air movements within the room: it must therefore be regarded as a ventilator. Fig. 603 indicates various positionsw and directions in which doors may be hung, the best being those in which the incoming air has the greatest distance to travel from door to fired
The advantage of securing change of air throug, the doorways is that, in cold weather, the air is more likely to become tempered by passing through portions of the building, and the change is consequently less unpleasant to the occupants than when air is allowed to enter direct from the exterior. The air can also be easily regulated in quantity by opening the door more or less, but as the privacy of the room is thereby lessened, such an expedient is not at all times convenient; the difficulty can, however, be overcome by the use of fan-lights over the doors, made to open, as shown in Plate II.
Fig. 608. - Four Arrangements of Dorrs and Fireplaces, showing current of Air when the doorse are open.
The ventilation of all rooms in a house through the hall and stairway has been advocated, and openings have in some instances been provided above the doorwavs. fitted with hinged casements or with flaps or louvres, and if a good supply of fresh air be admitted to the hall and stairway, warmed in cold weather, and all chance of contamination within the house be avoided, such a system may be carried out with good effect. But when open fires are employed in some rooms only of the house, and the air-supply to the hall and stairway is inadequate, the stronger suctional power induced up the flues in such rooms will have a tendency to draw air from other rooms where there are no fires, perhaps down the smoke-flues, and may thus cause a sooty smell throughout the house; or where the staircase is of considerable height, the tendency is for the wanned air to rise to the top, and, if it can find exit there, it will cause a pull upon the air of the apartments instead of supplying air thereto. In fact, there are many houses not specially arranged to be ventilated as above described, in which this tendency of a lofty stairway to act as an upcast shaft and draw air from the rooms, becomes most troublesome, and is a frequent cause of smoky chimneys, or at least of down-draughts in flues where there are no fires alight. Con-Bequently it is. as a rule, better to provide separately for the ventilation of each room, as well as of the halls and stairways.
Suggestions have been given how this ventilation may best be accomplished, but it is impossible to lay down absolute rules which will in every case be effective. Conditions are so widely different that, to obtain success, each case must be treated on its merits, and little more can be done than explain the salient points which should demand attention, and ever to insist upon the necessity for intelligent regulation if the comfort of occupants is to be secured.