All Air Systems

Air is used as the media in an all air system. Air transports thermal energy from the conditioned space to the HVAC plant. As shown in Figure , In these systems air is processed in the A/C plant namely AHU (Air Handling Unit). AHU consists of Dampers, Mixing chambers, Filters, Cooling/ Heating coils, Humidifiers, Fans/ Blowers etc. in a packaged cabinet.This processed air is then supplied to the conditioned spaces through Air Distribution system. Air Distribution system consists of Ducts, Dampers and Diffusers. This air extracts (or supplies in

This air extracts (or supplies in case of winter) the required amount of sensible and latent heat from the conditioned space. The duct that supplies the air to spaces is called Supply Air Duct (SAD). The return air from the conditioned space is conveyed back to the plant, where it again undergoes the required processing thus completing the cycle. The duct that returns the air from spaces to A/C plant is called Return Air Duct (RAD). Adequate Fresh air is always supplied by AHU to maintain Ventilation and Indoor Air Quality (IAQ).ASHRAE 62.1 standards are followed for ventilation, mostly[4].Outside Fresh air and Return air are balanced in proportion by rule ‘Supply Air = Return Air + Fresh Air’.Since Fully treated air is supplied by AHU,No further processing of air is required by terminal units in the conditioned space. All air systems can be further classified into :

  1. Single duct, constant volume, single zone system
  2. Single duct, constant volume, multi zone system
  3. Single duct, Variable volume (VAV) system
  4. Dual duct, constant volume system
  5. Dual duct, variable volume (VAV) system

Advantages of All Air systems:

  • Effective Room air distribution and Ventilation is possible under widely varying load conditions
  • All air systems present great opportunities for Energy conservation such as heat recovery wheels.
  • Precise control is possible by use of high class controls. Close range of Temperature (± 0.15ºC) and Humidity (± 0.5 %) can be achieved .
  • Simultaneous cooling and heating can be provided by dual duct system. Also, switching between summer and winter mode is quite easy in All air systems.
  • AHU which is a complete package can be remotely located, well away from conditioned spaces. It helps to reduce noise levels in occupied spaces.
  • Stairwell Pressurization, kitchen ventilation etc. can be easily achieved.

Disadvantages of All Air systems:

  • Since systems are quite large in size, require separate spaces like AHU Room. It reduces use of effective floor space. Particularly in high rise buildings, it is difficult to provide long runs of ducts, as AHUs are located either on roof or basement.
  • Difficult to install in existing buildings, as large space is required in false ceiling to lay the ducts.
  • Testing, Adjusting, Balancing (TAB) may be very difficult task in case of VAV systems

Applications of All Air systems:

  • Precision HVAC applications such as IT Data centers, Research Labs, Process industries, Clean rooms, Operation Theatres, Hospitals etc. where accurate and precise control of space environment is required
  • Theatres, Auditorium, Functional halls, Retail stores etc. where loads are uniform with small external loads.
  • Large corporate buildings with varied requirements of heating and cooling which can be served by Dual duct systems with simultaneous cooling and heating.
  • Multi zone All air systems are suitable for Offices, classrooms, hotels, ships etc. where individual control is required.

[1] Edward G. Pita, “Air conditioning Principles and systems”, Prentice Hall India publication. 4th Edition, 2012
[2] McQuiston, Parker, Spitler, “Heating, ventilation and Air conditioning – Analysis and Design”, Wiley student publication. 6th Edition,
[3] ASHRAE Handbook, “2008 HVAC Systems and Equipments”, 2008 Revision.
[4] ASHRAE Handbook, “2007 HVAC Applications”, 2007 Revision.
[5] Carrier Air Conditioning Company, “Handbook of Air conditioning system design”, McGraw Hill New York. 2000 Edition


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