Copyright Goodheart-Willcox Co., Inc. Chapter 12 Radiant Heating Systems 255 calculation is necessary. Once this is accomplished, determine the following parameters:■ ■■ Btu/hr heat loss per square foot for each zone ■■ Radiant floor surface temperature ■■ Tubing size ■■ R-value of the finished flooring ■■ Supply and return water temperature differential ■■ Tubing spacing ■■ Required supply water temperature ■■ Total loop length ■■ Fluid flow in GPM ■ Feet of head loss Btu/hr heat loss per square foot for each zone: Divide the heat loss of the room or zone to be heated by the total square foot area of that zone. Remember to subtract those areas where radiant tubing will not be installed—such as under counters or vanities. Radiant floor surface temperature: Once the Btu per square foot is calcu- lated, the designer can determine the floor surface temperature. Use the chart shown in Figure 12-36 to estimate this factor. Example: A calculation of 25 Btu per square foot and a room temperature set point of 68°F would result in a radiant floor surface temperature of 80.5°F. This estimate helps determine the comfort level for the customer and can be adjusted by moving the room temperature set point either up or down. Tubing size: Most radiant flooring systems use either 3/8″ or 1/2″ PEX or PE-RT tubing. Both sizes are fairly equal with regard to heat output per square foot when used in a high-mass radiant application. However, tubing size does affect pressure loss and GPM flow rates. Smaller tubing results in higher head loss than larger tubing. On the other hand, 1/2″ tubing provides nearly twice the volume of fluid flow per cubic inch compared to 3/8″ tubing. Both factors can affect circulator sizing. Smaller diameter tubing is suggested for shorter loop lengths, and larger diameter tubing is recommended for larger areas. R-value of the finished flooring: The R-value of the finished floor covering will differ depending on the type of material. Knowing the R-value of the finished Photo by Warren Gretz, NREL 11480 Figure 12-34. When installing radiant wall panels, use aluminum plates between the wall and tubing to increase the amount of heat transfer. Alexander Raths/Shutterstock.com Figure 12-35. The final step when installing radiant wall panels is to run the tubing to the manifold station and terminate in the same manner as with radiant floor systems.