Which pipe is common to both the heating and cooling function of a three-pipe system?

In a three-pipe system, which is typically used in heating, ventilation, and air conditioning (HVAC), the common pipe that serves both the heating and cooling functions is the return piping. This is because the return piping is responsible for carrying water back to the central system, whether it is heated or chilled, allowing for temperature regulation across spaces served by the system.

Return piping effectively connects the output of the various terminals (like fan coils or units in the system) back to the boiler or chiller. Since it serves both functions, the return piping is indeed integral to the flow of both hot and cold water. On the other hand, the supply piping may carry either the heated or chilled water but is not common to both functions simultaneously in a three-pipe configuration.

Recognizing the role of return piping in maintaining system efficiency is crucial for understanding how heating and cooling demand is balanced in HVAC systems. The design allows for a simplified approach to managing temperature control within the spaces serviced by the system, making return piping indispensable.