HVAC systems are given broad classifications based on the medium which is used to transfer heat within the system. There are many variations and combinations of these types. It is helpful to understand the basic system classification scheme.

The basic system types are:

  1. All-Air
  2. Air-Water
  3. All-Water
  4. Packaged Terminal.

1.All-Air Systems :- 
All-Air systems perform all the conditioning processes with air. The processes are cooling and dehumidification, heating and humidification, along with air cleaning and air distribution. Conditioning of air is usually done in central station equipment located re-motely from the space. An all-air system supplies only conditioned air to the space. No other cooling or heating medium crosses the boundary into the conditioned space.

2.Air-Water Systems :- 
Air-Water systems use both air and water for cooling and heating. Conditioning of air and water is performed in a remote central plant, then distributed to terminal units in the conditioned space where they are used to satisfy the space cooling and heating loads and the ventilation requirement. The chilled and heated water may be delivered to the building from the central plant in 2-pipe changeover type or 3-pipe or 4-pipe simultaneous type piping systems. Fan-coil units with central ventilating air, terminal reheat units, induction reheat terminals, under-window induction terminals, and fanpowered induction terminals are examples of air-water systems.

3.All-Water Systems :- 
All-Water systems use heated or chilled water circulated through a terminal unit situated within the conditioned space, and the terminal unit provides cooling and dehumidification or heating according to the zone load requirements. No conditioned air is brought to the room from a central air handling system. Outside air for ventilation is introduced either by normal infiltration through window and door cracks or through wall intakes located behind each unit. The terminal units may be fan-coil units or unit ventilators. Heating-only systems serving heated water terminals, such as reheat coils and convectors, are often referred to as “hydronic” systems.

4.Packaged Systems :- 
Packaged systems are similar to All-Air systems in that they perform the conditioning processes of cooling and dehumidification, heating, and ventilating with air but the apparatus is located in the conditioned space. Air distribution may be ducted or from integral grilles. Conditioning of heating water for hydronic coils and of loop water for water-source heat pumps is usually done in central station equipment located remotely from the space. No other cooling or heating medium crosses the boundary into the conditioned space.

Basic components of an HVAC system :-
The basic components or equipment of an HVAC system that delivers conditioned air to satisfy thermal comfort of space and occupants and the achieve the indoor air quality are listed below.

  • Air filter
  • Supply fan
  • Ducts
  • Terminal devices
  • Return air system
  • Water chiller
  • Humidification and dehumidification equipment
  • Heating and cooling coils
  • Self-contained heating or cooling unit
  • Mixed-air plenum and outdoor air control
  • Cooling tower
  • Outdoor air intake
  • Boiler
  • Control
  • Exhaust or relief fans and an air outlet

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