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American Society of Heating and Ventilating Engineers Guide, 1929 Temreeature or Water in Plow Riser Chapter IV--Hot Water Heating Systems and Piping Fig. 7. Friction Heads, in Pipes and Elbows, for a 20 deg. Temperature Difference of the Water in the Flow and Return Lines the friction heads in the pipe and in the elbows can be calculated by known formulae, or, they can be determined more readily from the diagrams of Figs. 7 and 8. It will be noted from Fig. 7, that for a 1-in. pipe, for 12,000 B.t.u. per hour, and for a temperature drop of 20 deg., the friction head is 18 milinches per foot of-pipe and 35 milinches. 125 124