all water systems

water is used as working fluid that transports thermal energy between conditioned space and A/C plant. Chilled water is circulated for cooling while Hot water is circulated through coils for space heating. As only water is supplied by HVAC plant to the space, Fresh air has to be supplied by another unit to maintain Indoor air quality and ventilation. These systems are also known as Chilled water systems for cooling applications. Based on number of pipes used in system, they are classified as 2-pipe system and 4-pipe system.

A 2-pipe system can provide either cooling or heating only, but not both at a time. As shown in Figure , chiller supplies cold water or heater supplies hot water to different spaces. Out of 2 pipes, one is for supply and the other is for return of water to central plant. Chilled water/ hot water circulates through Fan coil units (FCU), or convectors or Radiators located in the each space. Thermal energy is exchanged between water and air in the conditioned space. Flow control valve controls the water flow rate as per load variation. Flow control valve is driven by zone thermostat which gives signal to adjust flow rate as per load. Since The same inside air is circulated repeatedly by terminal units, Fresh air ventilation must be supplied by external units.

A 4-pipe system consists of two supply pipelines – one for cold water and one for hot water; and two return water pipelines. The cold and hot water are mixed in a certain proportion as per the zone load, and the mixed water is supplied to the terminal units in the conditioned space. The return water is split into two streams, one stream flows to the heating coil while the other flows to the cooling coil.Terminal units used in all water system are of basically three types namely Fan coil units (FCU), Convector and Radiator

Advantages of All Water systems:

  • All water system needs very less space as compared to all air systems for same capacity. Because, water is used as transport media, which has higher specific heat capacity and density than air. Hence, pipe sizes are very small as compared to large sized ducts. Therefore, system consumes very less space due to absence of big duct works and fans .
  • Less expensive Central A/C plant solution is offered by all water system .
  • It is more suitable for Individual room/ zone control.
  • Simultaneous cooling and heating is possible with 4-pipe system
  • It can be easily installed in the old buildings without making much changes in the existing space setting

Disadvantages of All water systems:

  • Multiplicity of fan coil units means a great deal of maintenance work and cost .
  • Control of space humidity is limited, as only cooling and dehumidification occurs and FCUs do not have humidifiers or reheat coils.
  • Separate ventilation system must be used to supply fresh air, which adds to cost. Also control of ventilation air is not precise.
  • Condensate may cause problems of dampness in ceiling or walls if condensate drain fails.

Applications of All water systems:

  • All water systems are suitable for use as low cost central HVAC systems in multi zone High Rise commercial applications
  • All water system is best solution to replace All air system in High Rise Tall buildings, as it is much easier to carry water through small sized pipes than air through very big sized ducts across farther heights.
  • It is very effective system in certain applications where space extremely limited, such as narrow ceiling and shafts.i.e. Buildings which were originally not designed for HVAC systems.
  • In case of Individual room control as required in hotels, multi storey apartments, offices etc. all water systems are more famous.

[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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