Which type of air conditioning system uses control valves to select the pipe serving as the heat exchanger?

The three-pipe system is designed to utilize control valves to select the appropriate pipe for serving as the heat exchanger. This type of system typically has one pipe for the supply of chilled water, one for hot water, and a third pipe that is either a return line or a common line that can switch between the two, allowing for both heating and cooling capabilities simultaneously.

In this setup, the control valves play a crucial role, enabling the system to efficiently manage the flow of either hot or chilled water to the building's heat exchangers based on the current heating or cooling demand. This flexibility allows for greater efficiency and comfort in temperature regulation within the space being conditioned.

In contrast, the two-pipe system has only one pipe dedicated to either heating or cooling at any time, which limits its operational flexibility. The single-pipe system does not facilitate multiple temperature control methods, and the four-pipe system, although having more complexity, uses dedicated pipes for both heating and cooling, thus not requiring the selection feature that characterizes the three-pipe system.