Copyright Goodheart-Willcox Co., Inc. 258 Hydronic Heating: Systems and Applications Total loop length: The length of tubing for each zone is measured from the outlet of the manifold station to its inlet. To determine the amount of tubing that should be installed for each individual zone, use the following calculations: ■ ■ For tubing to be spaced 12″ on center, multiply the square footage of the zone by 1.0. ■ ■ For tubing to be spaced 10″ on center, multiply the square footage of the zone by 1.2. ■ ■ For tubing to be spaced 9″ on center, multiply the square footage of the zone by 1.33. ■ ■ For tubing to be spaced 8″ on center, multiply the square footage of the zone by 1.5. ■ ■ For tubing to be spaced 6″ on center, multiply the square footage of the zone by 2.0. 50 45 40 35 30 25 20 15 10 5 0 75 85 95 105 115 125 135 Supply water temperature at manifold (°F) Room heat loss (Btu/hr/sq. ft.) 145 155 165 175 185 195 205 65°F room temperature 3/4”, 5/8”, 1/2”, and 3/8” PEX tubing—9” on center 20-degree supply/return temperature differential Rx = 0 Rx = 0.5 Rx = 1.0 Rx = 1.5 Rx = 2.0 Rx = 2.5 Rx = 3.0 Rx = 3.5 Rx = 4.0 Water Supply Temperatures for 4-Inch Concrete Slab Goodheart-Willcox Publisher Figure 12-39. This table is used to determine the proper supply water temperature for the heating loop.