This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The required size of the warm-air flue between the heater and the register, depends first upon the difference in temperature between the air in the flue and that of the room, and second, upon the height of the flue. In dwellings, hospitals, etc., where the conditions are practically constant, it is customary to allow 2 square inches area for each square foot of radiation when the room is on the first floor, and 1 1/2 square inches when it is on the second floor.
In schoolhouse work it is more usual to calculate the size of flue from an assumed velocity of air flow through it. This will vary greatly according to the outside temperature and the prevailing wind conditions. The following figures may be taken as average velocities obtained in practice, and may be used as a basis for calculating the required flue areas for the different stories of a school building.
1st floor 280 feet per minute. 2nd " 340 " " " 3rd " 400 " " "
These velocities will be increased somewhat in windy weather and will be reduced when the atmosphere is damp.
Having assumed these velocities, and knowing the number of cubic feet of air to be delivered to the room per minute, we have only to divide this quantity by the assumed velocity, to obtain the required flue area in square feet.
Example - A schoolroom on the second floor is to have an air supply of 2,000 cubic feet per minute; what will be the required flue area?
2000/ 340 = 5.8 + sq. feet. Ans. The velocities would be higher in the coldest weather and dampers should be placed in the flues for throttling the air supply when necessary.