Any change of temperature of air produces a change equal to 1/491 part of its volume, for each degree variation. If a cubic foot of air is raised in temperature 1°F., its volume is 1/491 part larger than the original volume, and its buoyancy in the surrounding air is increased correspondingly. Air that has a temperature higher than that surrounding it will tend to rise because it is lighter. The air rising from a hot-air register or from a heated surface are illustrations of this condition.

Since the change of volume - or what is the same thing, its tendency to rise-increases 1/491 for each degree difference in temperature, the upward velocity of highly heated air will be very great. In warm air that fills a chimney flue or a room, the same tendency exists, the warmest air rises to the highest point and if the air can escape, as in the case of a chimney, a draft will result.

The draft of a chimney, in quiet air, is due to the difference in temperature between the air inside the flue above that outside the house. A chimney that does not "draw" and causes a stove to "smoke," will often produce sufficient draft after the flue has been warmed. The upward movement of the warmer air in the flue produces a constantly increasing velocity, until it reaches the top of the chimney. This is an accelerated velocity that may be calculated by use of the formula given in physics, to express the velocity of accelerated motion. The well-known formula V= √2gh may be modified to express the conditions existing in a flue and permit of the calculation of the quantity of air discharged.

The upward flow of air in a chimney flue being due to the difference in temperature of the air in the flue over the outside air, the flow of air from the rooms will continue as long as the difference in temperature exists. Moreover, the air that is discharged from the rooms will be replenished from the outside, and for the air sent out of the flue a corresponding amount will be brought into the rooms through any openings that exist - door or windows or through cracks or crevices, depending on the completeness with which the house is closed. In no case is a house air-tight. The air pressure registered by the barometer is always the same inside as that outside the building. During cold weather, when the windows and doors are closed, the air is escaping through the chimney and also through every little crack and chink in the top of the rooms where the air is warmest. The colder air is entering at the same time through the joints about windows, door casings, through the crevices in the walls and particularly through the open joints made by the baseboards and the floor. This latter entrance of cold air is one of the commonest causes of cold floors. The shrinkage of the baseboards and floors from the quarter-round moulding which forms the joint leaves openings through which cold air is freely admitted from partitions and outside walls. The cold, heavier air remains near the floor because it can rise only when heated or forced upward by a draft. If the same air were permitted to enter at points near the ceiling and mingle with the warmest air in the room, a more uniform temperature would result, as well as better ventilation. The entering cold air, mixing with the warm air at the top of the room, would be reduced in its temperature and weight. The heavier air in falling would diffuse with the air beneath it and thus improve the general quality of the atmosphere.

It is important to remember that the discharge of air through a chimney flue will depend, in considerable amount, on the rate the new air is able to enter the house. In a new, tightly constructed house, the flue is often capable of discharging air much faster than it can enter, when it must find its way in through accidental openings. In such cases an open door or window immediately improves the draft of the stove.

The ventilation in the average dwelling is and must be accomplished by natural draft that is generated through difference in temperature of the air. The possibility of providing an acceptable system of continuous ventilation is confined to the draft of the chimney or to a flue provided especially for that purpose. Such being the case, the dimensions of flues constructed for ventilation should be the subject of investigation. The work that a chimney or ventilating flue has to do is continuous and will last throughout its lifetime; its proportions should therefore be considered with more than passing care.

It has been stated that the method of calculating volumes of air that will pass through a flue is based on the formula used to express the velocity of accelerated motion. The fundamental formula must be changed to suit the conditions produced when air is heated and made buoyant by expansion.

As has been stated, the change in temperature of air 1°F. causes an increase or decrease 1/491 Part of its volume for each degree change. Any portion of air, warmer than that which surrounds it, tends to rise because of its lighter weight; the tendency to rise increases with the difference in temperature. The draft of a flue is caused by this condition of difference in temperature between the air inside the flue and the outside atmosphere.

In order that this general condition may be expressed in the simplest form let: T = the temperature inside the flue in degrees F.

t = the temperature outside the flue in degrees F. H = the height of the flue in feet.

The quantity T - t /491 expresses the difference in temperature in degrees, divided by the change of volume for each degree. This gives the constant upward tendency of the air in passing through the flue. If this quantity is placed in the formula V = √2gh, so as to exert its influence through the height of flue H, the condition may be expressed:

V= √2g T-t / 491 H

The factor g, representing the acceleration of gravity, is constant and equal to 32 feet per second. The quantity 2g may be removed from under the radical and the formula becomes: