An undersized heating main will cause which of the following conditions?

An undersized heating main results in high velocity and poor heat transfer due to the restricted flow of water or steam through the system. When the diameter of the heating main is too small for the volume of fluid that needs to circulate, the fluid must travel at an increased speed to maintain the same flow rate. This high velocity can lead to turbulence, which hinders the effective transfer of heat from the fluid to the surrounding environment. Additionally, as the velocity increases, the pressure drop across the system can rise, causing inefficiencies and insufficient heating performance.

The other options describe conditions that are not typically associated with undersized heating mains. Low velocity and high efficiency usually occur in systems with appropriately sized mains that allow for smooth flow and effective heat transfer. Reduced noise levels could be a result of proper sizing or other factors but would not likely be characterized by an undersized heating main, which often produces noise due to high velocities and turbulence. Lastly, lower pressure drops are not a characteristic of undersized mains; in fact, reduced pipe diameter tends to increase pressure drops rather than decrease them.