As an illustration of the application of the formula, suppose that the temperature in the house and in the flue is 70°F. and that the outside temperature is 20°F. The height of the chimney is 30 feet. The area of the flue is 1 square foot. Volume = 14,400 √(((_T_ - _t_)/491)_H_) = 14,400√(((70 - 20)/491) × 30) = 25,140 cubic feet per hour. Such a ventilating flue would be sufficient in size, under the conditions given, to furnish air at the rate of 25,140 cubic feet per hour or 30 cubic feet per minute to 13 persons, provided of course that the air could enter the building at the rate demanded. Where no provision is made for the air to enter the building it must find its way by the accidental openings. A common illustration of this effect may be noticed in the rate at which the fire of a stove will burn in a tightly closed room. The opening of a door or window causes an immediate increase of combustion, because of the extra air supply. It is evident that in well-constructed houses other means should be provided for admitting air than that of accidental opening. The following table calculated by the above formula gives the quantity of air in cubic feet per hour discharged through a flue of 1 square foot cross-section. The table shows the calculated discharge from flues of heights varying from 15 to 40 feet, and with temperature differences from 10° to 100° between the outside air and that of the house. --------+------------------------------------------------------------- Height | Temperature of air in the flue above that of external air of flue +--------+--------+--------+--------+--------+--------+------- in feet | 10 | 15 | 20 | 25 | 30 | 50 | 100 --------+--------+--------+--------+--------+--------+--------+------- 15 | 7,980 | 9,720 | 11,280 | 12,550 | 13,800 | 17,820 | 25,140 --------+--------+--------+--------+--------+--------+--------+------- 20 | 9,180 | 11,180 | 13,080 | 14,520 | 15,900 | 20,520 | 29,040 --------+--------+--------+--------+--------+--------+--------+------- 25 | 10,260 | 12,600 | 14,520 | 16,260 | 17,820 | 22,980 | 32,460 --------+--------+--------+--------+--------+--------+--------+------- 30 | 11,280 | 13,800 | 15,900 | 17,825 | 19,500 | 25,140 | 35,580 --------+--------+--------+--------+--------+--------+--------+------- 35 | 12,180 | 14,880 | 17,160 | 19,200 | 21,060 | 27,180 | 38,400 --------+--------+--------+--------+--------+--------+--------+------- 40 | 13,020 | 15,900 | 18,360 | 20,520 | 22,500 | 29,040 | 40,980 --------+--------+--------+--------+--------+--------+--------+-------