This feature ensures that the common register heats, cools or shuts down the room according to the energy needs of the room and the available medium in the piping. The main advantage of a two-pipe connection is the saving of investment costs: instead of separate pipes for heating and cooling water, there are only one pipes in the building that distribute either hot or cold water, and there is only one exchanger or other heat exchange surface in the rooms. However, the system has several limitations that can fundamentally affect the comfort of the building. In addition, there is a risk that, if incorrectly designed or installed, the entire system will be functional only to a limited extent and will become a source of annoyance and disputes between professions. Let's look at the basic principles of the connection function with change-over and the most common mistakes in the project and implementation.