It is important that a good chimney flue, straight and smooth inside and proportioned to the area of the grate, be provided for each water heater. No other smoke pipe should be permitted to connect to this flue, nor should other openings to it be permitted, as thev would spoil the chimney draft.

Fig. No. 120

Smoke flues should be cased with flue linings to give them a smooth interior surface. The best form for flue linings is round or oval, as smoke and hot gases pass up with less frictional resistance in a round flue than in a square one.

Square flues are much more efficient than rectangular ones, on account of the less surface exposed for a given area of flue; for instance, a flue 12X12 inches has an area of 144 square inches and a perimeter of only 48 inches, while a flue 8 X 18 inches having an equal area, has a perimeter of

52 inches, thus presenting 4 additional inches to offer resistance. No satisfactory formula was ever devised to calculate the area of smoke flues under varying conditions.

A simple empirical rule that will be found satisfactory for determining the area of flues for water heaters follows:

Rule

Allow for smoke flue one-eighth the sectional area of heater grate.

Example

What size of smoke flue will be required for a water heater containing 4 square feet of grate?

Solution

4 square feet=576 square inches. 1/8 of 576=72 square inches=area of smoke flue. The nearest sizes of commercial flue linings are: Square, 8 1/2 X 81/2 inches=72.25 square inches; round, 102 times .7584=78.54 square inches.