END OF SECTION EXERCISE SOLUTIONS 1. Why does ducting in forced-air heating systems have to be so much larger than the tubing in a hydronic heating system of equal heating capacity? Solution Due to higher heat capacity compared to air, the volume of water that must be moved through a building to deliver a certain amount of heat is only about 0.03% that of air. Since flow rate of water to deliver the same amount of heat is lower, the distribution system size can be reduced. 2. Why is it better to surround a person with warm surfaces as opposed to just warm air? Solution People’s bodies lose thermal energy via radiant heat transfer from their skin surfaces to nearby colder surfaces. For this reason, most people will not be comfortable in a room containing several cool surfaces such as large windows or cold floors, even if the room’s temperature is 70 F. 3. What type of heat transfer creates the wind chill effect we experience during winter? Solution Forced Convection 4. A certain block of material conducts heat at a rate of 100 Btu/h. One side is maintained at 80 F and the other at 70 F. Describe what happens to the rate of heat transfer when the: a. The 80 F side is raised to 130 F b. The thickness of the block is doubled c. The 80 F side is raised to 130 F, and the thickness is cut in half