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.
Ordinary dwellings in this country are more frequently warmer than the osier atmosphere, and open fires are commonly employed during the colder months of the year; consequently, unless each room receives a separate and adequate supply of air direct from the exterior, there is risk of down-draught in some flues, particularly if others are more lofty. It is therefore advisable to make as little variation as practicable in the height of the flues in a single dwelling, although, even if all the flues are carried to the same height, they will draw on one another if there is variation of temperature in the several flues and air-communication at the base, without separate air-supply to each apartment Frequently this is the cause of annoyance resulting from "smoky chimneys", of flues with a down-draught tendency, which probably could be cured by admitting a supply of air from the exterior to each room separately.
The section of a house, shown in Fig. 561, will illustrate this. A fire lighted in either of the rooms would have a tendency to draw air by way of the staircase down one of the other flues, provided that the windows, doors, and other openings to the exterior are closed. This other flue may even suck down smoke emitted from the flue at the base of which there is a fire. On attempting to light a fire in the other room, it will be found that smoke will not freely ascend the flue. Close the doors of both rooms and partly open the windows, and if smokiness within the house resulted simply from the suction of one flue upon the other, the fires will both burn briskly and without smokiness.
A central hall provided with an extract-ventilator at the top, as in Fig. 562, will, either from the air thereof becoming warmer than in some of the rooms around, or by wind acting upon the outlet, have a tendency to draw air from the rooms, so that, unless there are other inlets, air will be sucked down the flues and result in a sooty smell about the premises.
These two examples alike indicate the necessity for air-inlets, relatively proportioned to the outlets, to each separate compartment throughout the building.
As change of air is essential to ventilation, there must be movement of air.
Fig 561- Section of House, showing Danger of Down-draughts in Flues when Air-Inlets are not provtded.
When this movement is too rapid, or is ill-directed, uncomfortable draughts are the result. Even warm air moving quickly will cause discomfort, and may be more pernicious than colder currents, because, unless such warm air is in a proper hygrometric condition , it will cause excessive evaporation from the body, and by evaporation a lowering of temperature takes place. To obviate draughts and yet give the necessary change of air, substituting that which is ventilator, showing Danger of Down-draughts in Flues when Air inlets are not provided fresh and wholesome for that which has become vitiated, is essentially good ventilation. The sizes, nature, and positions of inlets and outlets must be carefully con-aidered in all schemes for ventilation. Supposing that a force be employed for propelling air into an apartment which measures 20 feet by 15 feet by 10 feet high (equal to a cubic capacity of 3000 feet), so as to change the air six times in an hour, a volume of air equal to 18,000 cubic feet per hour would be required; this might be introduced through an aperture one foot square, if travelling at the rate of 300 feet per minute = 5 feet per second. The principal outlet is supposed to the fireplace flue. The question will arise, where should the inlet be situated ? If placed on the opposite side of the room, as in Fig. 563, the incoming air will take the most direct course to the outlet, and between the two there will the considerable draught, and far more rapid change than in other parts of the room If. however, the opening be trumpet-mouthed inwards, there will be better diffusion; but this will be still more improved if such an inlet is placed on the same side of the room as the outlet, as in Fig. 564, and the risk of setting up unpleasant draughts will be reduced to a minimum, unless the incoming air a very low temperature, Special air-inlets, other than windows and doors, should be situate at about two-thirds the height of the room, and shaped so as to give the incoming air a to be, and, as regards its temperature, in a condition which will tend to their comfort. As it approaches the fire, the suctional influence exerted by the rarefaction of the air passing up the flue will gradually increase its velocity until it reaches the mouth of the flue, up which it will rush to the open.
Fig 542 - Section of House with Ertract.
, with square Inlet opposite the Outlet slight upward tendency; then, if* they are placed on the same Bide of the room as the outlet - viz., the fireplace, which is near to the floor-level - the incoming air will distribute itself about the upper portion of the room. With a tire in the grate the surfaces of walls, ceiling, furniture etc, will be warmer than the incoming air, and this, by contact with such surfaces, and by mixing with some of the warmer air of the apartment, will acquire a higher temperature than when it entered; it will have to travel a considerable distance within the room, whereby its velocity will be reduced, and if the inflow is continuous, and there are no other disturbing influences, it will advance at a low rate of speed towards the outlet, through the lower portion of the room where occupants are most likely
Fig. 564 -Plan and Section showing Air current in Room, with Trumpet mouthed Inlet on the same said as the Outlet.