Two pipes: in the fresh air-fan coil system or fan coil system, when the refrigerant and heat medium pipes run in winter and summer, they all use the same pipe system, that is, hot water and return water in winter, while chilled water (and return water) are transported by the same pipe in summer. The switching of cold and hot sources is carried out in chilled water pump room and heat exchange station, which is called double pipe system. Double-pipe system can save investment cost and reduce maintenance, which is a common control method in air-conditioning system, but it can not meet the needs of annual air-conditioning system.

Three pipes: two pipe systems are set for cold water and hot water respectively, while the return water shares the same pipe system. Four pipes: separate cold and hot water supply pipes and return pipe system are adopted (Figure 4 pipe is connected with air conditioning equipment), and the inlet and outlet pipes are equipped with changeover control valves, which are mainly used for the air conditioning system used all year round, that is, cold sources and heat sources are used at any time, and the quantity of cold and hot water is regulated by temperature regulating valves. Three-pipe and four-pipe control systems are generally used in air-conditioning systems with special needs because of their high investment and operation costs.

Divisional two-control water system: When some air-conditioning areas in the building need to supply cold water all year round, and some air-conditioning areas need to supply cold water and hot water alternately at regular intervals, the divisional two-control water system should be adopted. This system has some characteristics of two-control and four-control, and its regulation performance is between four-control and two-control. Because from the adjustment range. Four-control system is independent adjustment of each terminal equipment, while two-control system can only carry out cold and heat conversion of the whole system together, while two-control system can realize independent control in different areas.
The key to the design of the partitioned two-control system lies in the reasonable partition: if the partition is proper, it can better meet the air requirements of different regions, and its regulation performance can approach that of the four-control system. With regard to the number of partitions, the more partitions, the more areas that can be controlled independently, but the more complex the pipeline system is, which not only increases the investment correspondingly, but also complicates the management. Therefore, it is necessary to carefully analyze the characteristics of load changes in design, and generally, two partitions can meet the needs. If the load differences caused by the inner and outer zones and orientation are obvious in a building, three zones can also be considered. Compared with the current two-control system, the initial investment and occupied building space of the two-control system are similar to those of the four-control system, and the regulation performance is similar to that of the four-control system when the partition is reasonable. It is a scheme that can effectively improve the air-conditioning standard without significantly increasing the investment, and its design combined with relevant new air-conditioning technologies can make the air-conditioning system more economical and reasonable.