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All Water Central Systems

General

all water central systems

The space cooling function is performed by chilled water circulated from a central plant to air handling or terminal units in or adjacent to the conditioned spaces. Heating water is supplied through the same or a separate piping system. The piping systems can vary from a simple direct return primary loop or the more complicated primary-secondary loop arrangements.

All water systems have been widely applied to office buildings, multifamily residences, better hotels and motels, and schools to handle the perimeter spaces. They are not well suited to interior spaces.

All water system advantages include:

  • Water is a more energy and space efficient method of cooling and heating energy distribution
  • Recirculation of air is unnecessary so commingling of odors and contaminants, or concerns over fire and smoke spreading from one zone to another, are minimized
  • First cost is often less than for other central systems.
  • Less building space is required
  • More suitable for retrofit applications
  • Off-hour conditioning does not require a central air system operation
  • Cooling can be easily shut off in unoccupied areas
  • Quieter than unitary systems
  • Minimal space needed for air handling rooms and duct clearances
  • Individual zone temperature control
  • Variable speed secondary pumps can be used (to improve comfort control and reduce operating costs)
  • Can use heat recovery techniques.

The disadvantages include:

  • Some maintenance must be performed in occupied areas
  • Unless dehumidification and latent load is handled with a separate ventilation system (air-water system), a condensate drain pan system is required and terminal air filters must be periodically cleaned
  • Ventilation is usually from a wall aperture and is not easily controlled due to wind and stack effect; otherwise it is often accomplished by opening windows. Factory-packaged guaranteed performance units on the lee side of a building.
  • Problems can occur with rain leakage with wall openings, cold drafts can occur on the floor from excessive flow, and coils can freeze
  • A separate ventilation system is required for quality installations
  • Relative humidity may be high in summer, particularly if chilled water flow is modulated for temperature control.

Terminal equipment is typically fan-coil units, room fan-coil units, and (for schools and meeting rooms) unit ventilators. Unit ventilators are similar to room fan coil units but have a more elaborate system for introducing fresh air from a wall penetration, and an option for free cooling by outdoor air with an economizer cycle control. In some cases, valence terminals are used. Valence terminals depend on natural convection from a water coil in a valence enclosure near the ceiling line, usually along the perimeter wall; filters are not employed and there is no ventilation with this terminal.

The water systems serving the zone coils may be two pipe, with or without seasonal changeover, three pipe or four pipe.

Links to Related Topics

Ventilation
Indoor Air Quality
Filtration
Outside Air Control
Controls
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Dual Duct
All-Air Central Multizone
All-Air Rooftop
Air-Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System