Rule 13

For rooms on the first floor, add together the total glass surface and 1/4 of the area of the exposed walls in square feet, and multiply the total by 1.5; the product is the proper area of the pipe in square inches. For second-story rooms, multiply by 1 to 1.25, according to the exposure; and for the third story, by . 75 to 1.

242. The stacks, or wall flues, are usually flattened in form, and present more surface for friction than the leader pipes, which are usually round. When a stack is connected to a leader of considerable length, the area of the latter should exceed that of the former by 20 to 30 per cent., or even more in extreme cases.

In computing the size of a stack, some allowance must be made if its length in proportion to its area is unusually great. Allowance must also be made for all elbows and offsets.

If the quantity of air required per minute is known, the size of the pipe to carry it may be computed by dividing the required volume by the velocity of the air-current. In all ordinary cases the velocity of the air-current may be safely assumed at 4 feet per second at the first floor, 5 feet at the second floor, and 6 feet per second at the third floor.

Another method, equally good, is to reckon that one square inch of stack, or flue, area will supply 100 cubic feet of air per hour at the first floor, 125 at the second, and 150 at the third floor.

It is assumed in all of the foregoing rules that the average temperature of the hot air in the flues is about 120°, and that the air is moved solely by natural draft.

243. The following table shows the sizes allowed in common practice for pipes and registers in ordinary dwellings:

Table 22. Size Of Hot-Air Pipes And Registers