*2.1.2.5 Dual-duct systems*

Dual-duct systems allow separate control of temperatures in conditioned places and zones. Temperature control is achieved by supplying the mixing box with air from hot air duct and cold air duct; that is hot air and cold air are mixed in mixing box in proper required proportion based on the zone thermostat, and then air can be supplied to the zone to maintain required zone temperatures. These systems are commonly used in multiroom buildings such as offices, hotels, apartments, hospitals, schools, and large laboratories. **Figure 12** shows a dual-duct system that supplies different zones [1–5].

Advantages of dual-duct systems


**Figure 12.** *A schematic diagram of dual-duct system. Redrawn with modification from [1].*


*2.2.1 Fan coil unit*

**Figure 13.**

of a fan coil unit [1–5].

2.Do not need ducts

4.Easy to install

**Figure 14.**

**13**

1.Low cost

**Advantages of fan coil unit**s

3.Does not occupy much space

*A schematic diagram of a fan coil unit. Readapted from [5].*

Fan coil units are used in all-water central air conditioning. Cold and/or hot water from central chillers and boilers flows through the unit coils. The air temperature is controlled by controlling the flow rate of water through the coil via control valves, e.g., solenoid valves. Fan coil units are cheap in price and widely used in hotels, office buildings, and medical centers. **Figure 14** shows a schematic diagram

*Pictures of some common variable air volume terminal units. Readapted from [2].*

*Central Air Conditioning: Systems and Applications DOI: http://dx.doi.org/10.5772/intechopen.89455*

10.These systems have flexible air duct system designs. The choice of medium and high air speeds is possible on an economic basis and according to the requirements of the building.

Disadvantages of the dual-duct systems


#### *2.1.2.6 Variable air volume (VAV) systems*

Variable air volume systems can vary the thermal loads according to zone thermal load variations. One advantage of these systems is that both the initial cost and the operating cost are low because the air volume requires simple control within 20% of the air outlets. These systems are used with fixed thermal loads throughout the year. Applications of these systems can be found in commercial stores, office buildings, hotels, hospitals, housing, and schools. **Figure 13** shows various common terminal units of variable air volume [1–5].
