Copyright Goodheart-Willcox Co., Inc. 256 Hydronic Heating: Systems and Applications flooring is necessary to select the appropriate supply water temperature. The chart shown in Figure 12-37 displays the correlation between the type of floor covering and R-value. For instance, nylon saxony carpeting that is 1/4″ thick will have an approximate R-value of 0.88. Supply and return water temperature differential: This is the temperature drop between the inlet and outlet of the manifold station. A differential tempera- ture of 20°F is common for most radiant floor projects, although a 10°F differ- ence can be used. This calculation is also used when sizing the circulator. Keep in mind that a 10°F temperature differential will double the fluid flow requirement in GPM, compared to a 20°F temperature differential. TECH TIP Floor Covering Temperature Limitations Certain floor coverings have temperature limitations. For instance, solid hardwood flooring can shrink and expand with fluctuating temperatures, leaving unsightly gaps and cracks. For this reason, special considerations are necessary when using various types of flooring with radiant heating systems. Most hardwood floors have a maximum floor surface temperature of 80°F. All other flooring types have a maximum floor surface temperature of approximately 87.5°F. Be sure to consult the flooring manufacturer for their recommendations when choosing a particular floor covering. Goodheart-Willcox Publisher Figure 12-36. Radiant floor surface temperatures can be estimated by knowing the Btu per square foot of heated space and the desired room temperature set point. Radiant Floor Surface Temperatures (°F) Set Point Temperature (°F) Heat Loss (Btu/hr per ft2) 10 15 20 25 30 35 40 45 50 55 60 75 80.0 82.5 85.0 87.5 90.0 92.5 95.0 97.5 100.0 102.5 105.0 74 79.0 81.5 84.0 86.5 89.0 91.5 94.0 96.5 99.0 101.5 104.0 73 78.0 80.5 83.0 85.5 88.0 90.5 93.0 95.5 98.0 100.5 103.0 72 77.0 79.5 82.0 84.5 87.0 89.5 92.0 94.5 97.0 99.5 102.0 71 76.0 78.5 81.0 83.5 86.0 88.5 91.0 93.5 96.0 98.5 101.0 70 75.0 77.5 80.0 82.5 85.0 87.5 90.0 92.5 95.0 97.5 100.0 69 74.0 76.5 79.0 81.5 84.5 86.5 89.0 91.5 94.0 96.5 99.0 68 73.0 75.5 78.0 80.5 83.0 85.5 88.0 90.5 93.0 95.5 98.0 67 72.0 74.5 77.0 79.5 82.0 84.5 87.0 89.5 92.0 94.5 97.0 66 71.0 73.5 76.0 78.5 81.0 83.5 86.0 88.5 91.0 93.5 96.0 65 70.0 72.5 75.0 77.5 80.0 82.5 85.0 87.5 90.0 92.5 95.0 64 69.0 71.5 74.0 76.5 79.0 81.5 84.0 86.5 89.0 91.5 94.0 63 68.0 70.5 73.0 75.5 78.0 80.5 83.0 85.5 88.0 90.5 93.0 62 67.0 69.5 72.0 74.5 77.0 79.5 82.0 84.5 87.0 89.5 92.0 61 66.0 68.5 71.0 73.5 76.0 78.5 81.0 83.5 86.0 88.5 91.0 60 65.0 67.5 70.0 72.5 75.0 77.5 80.0 82.5 85.0 87.5 90.0