Copyright Goodheart-Willcox Co., Inc. Chapter 12 Radiant Heating Systems 257 Tubing spacing: The tubing spacing is a function of the type of floor construction, water temperature, and overall desired comfort. Decreasing the tubing spacing lowers the required supply water temperature and produces a more even surface temperature, but it increases the amount of tubing required. For slab-on-grade and thin-slab applica- tions, the maximum tubing spacing is 9″ on center. Anything wider than this will cause uneven floor temperatures. Spac- ing tends to narrow along perimeter walls—such as in base- ments—as seen in Figure 12-38. Required supply water temperature: The factors that affect the supply water temperature are: ■ ■■ Installation method ■■ Btu per square foot load ■■ Room temperature set point ■■ R-value of the floor covering ■ Temperature differential between supply and return water The chart shown in Figure 12-39 outlines these param- eters and is one method to determine the required supply water temperature for the heating loop. Example: A zone has a heat loss of 22 Btu per square foot, a desired room temperature set point of 68°F, a 20°F temperature differential, and no appre- ciable R-value for the floor covering. By using these parameters and the chart in Figure 12-39, the required supply water temperature for this zone would be approximately 98°F. Goodheart-Willcox Publisher Figure 12-37. This chart shows the R-values for various types of finished floor coverings. R-Value for Select Carpets and Carpet Pads Thickness (in.) Carpeting 1/4 3/8 1/2 5/8 3/4 Commercial glue down 0.60 0.90 1.20 1.50 1.80 Acrylic level loop 1.04 1.56 2.08 2.60 3.12 Acrylic plush 0.83 1.25 1.66 2.08 2.49 Polyester plush 0.96 1.44 1.92 2.40 2.88 Nylon saxony 0.88 1.32 1.76 2.20 2.64 Nylon shag 0.54 0.81 1.08 1.35 1.62 Wool plush 1.10 1.65 2.20 2.75 3.30 Carpet Pads Rubber (solid) 0.31 0.47 0.62 0.78 0.93 Rubber (waffled) 0.62 0.93 1.24 1.55 1.86 Hair and jute 0.98 1.47 1.96 2.45 2.94 Prime urethane (2-lb. density) 1.08 1.62 2.16 2.70 3.24 Bonded urethane (4-lb. density) 1.04 1.56 2.08 2.60 3.12 Bonded urethane (8-lb. density) 1.10 1.65 2.20 2.75 3.30 Michael Helsel Figure 12-38. Tube spacing will be narrower along basement perimeter walls.