Question

What is the difference between dual duct and multizone air-conditioning systems? Which one provides more temperature variation across the zones and which one is economical to operate?

Answer 1

Multizone and dual duct are All air type Central HVAC systems.

Multizone :

1)In a multi-zone all-air system, individual supply air ducts are provided for each zone in a building.

2)Cold air and hot air are mixed at the air handling unit to achieve the thermal requirement of each zone.

3)A particular zone has its conditioned air that cannot be mixed with that of other zones, and all multiple zones with different thermal requirement demand separate supply ducts.

4)Multi-zone all-air system consists of an air handling unit with parallel flow paths through cooling coils and heating coils and internal mixing dampers.

5) It is recommended that one multi-zone serve a maximum of 12 zones because of physical restrictions on duct connections and damper size. If more zones are required, additional air handlers may be used.

6)The advantage of the multi-zone system is to adequately condition several zones without energy waste associated with a terminal reheat system.

7) However, leakage between the decks of air handler may reduce energy efficiency. The main disadvantage is the need for multiple supply air ducts to serve multiple zones.

Dual Duct :

1) It is a terminal-controlled modification of the multi-zone concept. A central air handling unit provides two conditioned air streams such as a cold deck and a hot deck.

2)These air streams are distributed throughout the area served by the air handling unit in separate and parallel ducts.

3)Each zone has a terminal mixing box controlled by zone thermostat to adjust the supply air temperature by mixing the supply cold and hot air.

4)This type of system will minimize the disadvantages of previous system and become more flexible by using terminal control.

Since, temperature at each zone can be different, the multizone HVAC system provides more variation of temperatures than double duct systems.

The multizone HVAC will have a huge setup cost. Also, if there are mistakes in setting up the system, the effeciency will go down. But a well designed multizone HVAC system will be more economical to use than double duct HVAC system