Copyright Goodheart-Willcox Co., Inc. 238 Hydronic Heating: Systems and Applications ceilings—completely out of sight. One additional principal difference is that radi- ant heating systems typically circulate the heated water at lower temperatures than conventional systems. With a conventional hydronic heating system, the hot water supply temperature typically ranges between 140°F and 180°F. With radiant heating, this temperature range is usually between 100°F and 140°F. 12.1.1 How Radiant Heating Relates to Human Comfort By definition, comfort is controlling the rate at which a body loses heat. The human body operates at a core temperature of approximately 98.6°F and gener- ates 300–400 Btu/hr depending on the level of activity. By controlling the rate at which our bodies absorb or reject heat, we can better control our individual comfort level. As stated in previous chapters, most individuals are comfortable at a room temperature of 70°F for heating applications. By maintaining a heating output source close to 70°F, there is less fluctuation in space temperature and less chance for heat stratification between floors and ceilings. Radiant heating has the capability to deliver this temperature into the conditioned space more evenly—and at a lower supply temperature—than a forced-air system. It accom- plishes this by first delivering the heat at a lower supply temperature than forced air, and then by using heat from radiation to warm objects rather than air—thus keeping the space at a more comfortable temperature. Figure 12-1 illustrates how this is accomplished. Notice the ideal heating curve. This curve represents the optimum comfort level in degrees Fahrenheit from floor to ceiling for an average person at rest. Compare this curve to the radiant floor heating curve. Notice that there is minimal difference between the two curves. Figure 12-2 1 2 3 4 5 6 7 8 9 60 70 80 90 0 Height above floor (ft) Air temperature (°F) Ideal heating curve Radiant floor heating curve Goodheart-Willcox Publisher Figure 12-1. This illustration compares the radiant floor heating curve to that of the ideal heating curve based on human body temperature. 1 2 3 4 5 6 7 8 9 60 70 80 90 0 Height above floor (ft) Air temperature (°F) Ideal heating curve Forced-air heating curve Goodheart-Willcox Publisher Figure 12-2. This illustration compares a heating curve for a forced-air heating system to that of the ideal heating curve based on human body temperature.