Knowledge Centre – Passive Strategies

Cooling Equipment
Direct Expansion System
Central Air Conditioners

Direct Expansion System

Window Units and Split ACs
Window units and split ACs are small sized equipment suitable for small spaces measuring no more than 40 m2. Window AC is a compact system where all three components, evaporator, condenser, and compressor are packed together to complete the cycle of refrigerant flow and cooling. However, in a split system, the evaporator is located in a cabinet inside the space where as the condenser and compressor are enclosed in an outdoor cabinet. Such systems cool a space rather than a building. These systems have low capital cost but high operational cost due to low efficiencies.

winndow ac

Window air conditioners are packaged DX systems consisting of a vapor compression refrigeration cycle components (evaporator, condenser, compressor, and expansion device), fans and controls. Units are placed on the boundary between zone and external space because they carry both the exterior heat exchange element (condenser) and interior heat exchanging element (evaporator) inside the same frame.


A typical split system contains an external unit housing the condenser and the compressor, and an internal air handler unit with the evaporator, expansion device and fan. Working principle is same as for other DX systems i.e. supply air for the zone is cooled by making it pass directly over the refrigerant carrying cooling coil. Refrigerant is circulated between the units through insulated tubing. Split systems can serve two or three zones by using ductwork.

Ducted Splits and Packaged Air Conditioners
Ducted Splits and Packaged ACs are medium sized equipment which could be used for  medium-size spaces, ranging from 50 m2 to 1,000 m2. In such systems, the cooling component (evaporator, condenser, and compressor) are located at one location, separate from the targeted space, and the cooled air is distributed to the targeted space through ducts. .  These systems have higher capital cost compared to window or split units but operational cost are lower due to better system energy efficiency. Other advantages of the system are the flexibility in operating systems for different zones at different time, low maintenance, easy to install and better life.

Central Air Conditioners

central hvac system

Schematic diagram of a central system with main components. In central systems, the refrigerant is not circulated inside or near the zones. Rather it transfers the heat or cool to a ‘chilled medium’, usually water, which then circulates insides the zones.

Central air conditioner
Central air conditioners are  air conditioning systems designed for large buildings with complex operational requirement and varying schedules. The system has chillers to cool the water which is then transported to all air handling units which operates as per the space requirement and varies its speed & output accordingly.  They are serviced by outdoor condensers that may be air or water cooled (cooling towers). Central air conditioners are more efficient compared to its peers. Moreover, they are quiet and secluded.  These systems have higher capital costs compared to others but operational costs are significantly lower due to better system energy efficiency. However, these are complex systems that need plenty of installation space as well as regular and dedicated maintenance.

High efficiency chillers
Chillers are the key component for cooling in a central system to work as a packaged vapour compression cooling machine. It is designed to either reject heat to condenser water (water cooled system) or to ambient air (air cooled system). Based on the technologies, 4 major categories of chillers are available in the market – screw, scroll, reciprocating, and centrifugal. Selection is made based on the initial cost, functional use, size of the cooling requirement, and energy efficiency potential. For a net zero energy building, efforts should be made to select equipment that exceeds the efficiency requirements of ECBC.

Chilled water system supporting components
Chilled water system is a complex system with many supporting components like primary and secondary, pumps, cooling tower, fans, variable frequency drives, controls etc. The overall efficiency of a chilled water system is a resultant of a combined performance of all these equipment, commonly known as system efficiency. Detailed discussion on the advantages and disadvantages of each component should be done for best selection, optimized size, and maximum energy efficiency.

Heat generation through internal sources like equipment, occupants, lighting etc adds to the internal heat gains and increases the cooling requirement. In an event when the external air is cooler than the internal air, the external air could directly be used to meet the cooling requirement of the space. Many climatic zones like cold climate, composite climate, and temperate climate support the use economizers for a significant amount of days in a year. Designer should analyze the weather file and minimize the energy use with the application of an economizer. Two types of economizers are available in the market – air side and water side. Detailed discussion on the advantages and disadvantages of each component should be done for best selection, optimized size, and maximum energy efficiency.

econmiser working

Settings of an air economizer. Load on HVAC systems can be reduced by modifying the system to include air economizers. These vary the volume of cool air added from outside depending on the temperature difference between outdoor and indoor air.