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Air conditioning systems for buildings may rely either on air or on a liquid such as water as a distribution medium. Conditioned air is distributed by ducts, while chilled water is distributed by pipes. Chilled water distribution systems are most commonly used for air conditioning large buildings because it is more efficient and economical to transfer heat by water rather than by air. This is due to water’s higher heat capacity, coupled with the fact that water-carrying pipes are smaller and take up far less space than air-carrying ducts. A third advantage of chilled water over air is its flexibility for zone control.
When chilled water is used as a distribution medium, it will eventually cool the air in the living or working space through air-handling units or fan-coil units. Air handlers are typically larger than fan-coil units and have more sophisticated zone controls. Air handlers are often found in large office buildings, hospitals, etc. Fan-coil units are typically smaller and serve smaller spaces. These smaller units are typically controlled by simple thermostats and are often found in residential buildings, hotels, etc.
Water is typically chilled using a system that features cooling towers and chillers. The operation of chillers and cooling towers is a subject for future discussion. Chilled water distribution systems can use the same piping system as hot water heating systems. A water-source heat pump system would also provide heat, using the same water loop.Two-pipe vs. four-pipe systems
When using chilled water with air handlers or fan coils to
cool (and heat with hot water) a building, there is a distinction between two-pipe and four-pipe systems. In a two-pipe system, there is only one water loop, and thus two pipes (supply pipe and return pipe) at each air handler or fan coil. The building can either be in cooling mode or in heating mode. In these situations, there often is a ‘switch-over date’ when the chiller is shut down for the season and the boiler is started, or vice-versa. Control valves are used to switch the system from heating to cooling and back. See illustration.
After the switch-over date, if there is a heat wave in the fall, it likely will not be possible to cool the building because the chiller has been shut down and the water loop has been directed through the boiler.
In a four-pipe system, there are two separate water loops used: one for heated water and one for chilled water. This allows both systems to operate simultaneously for those times when some parts of the building need to be heated and other parts need to be cooled. Additionally, there is no seasonal switchover needed. It is easier to switch between heating and cooling when the outdoor temperature is unstable. Four-pipe systems are obviously more expensive because of the additional water loop, controls and more expensive coils in the air handlers. See illustration.
Heat pumps may be either liquid or air source. Liquid or
water source heat pump systems typically consist of several heat pump units above the ceiling tiles or in a closet on each floor of a high-rise building. In a liquid source heat pump system, there is a single water loop.
When air conditioning, the water loop
is gathering heat from the refrigerant at the heat pumps. This takes place in the heat pump’s condensing coil. The water loop must then reject the heat; this is typically accomplished through a cooling tower outside the building. See illustration.
When heating, the water loop is providing heat to the refrigerant at the heat pumps. The water loop must then gather more heat; this is typically accomplished through a boiler inside the building.
In some heat pump systems, heat is moved from one side of the building to the other: the heat that comes into the loop from spaces on the south side is absorbed by spaces on the north side.Carson Dunlop Weldon & Associates is a leading provider of commercial inspections and commercial inspection training, author of the Technical Reference Guide and the CommQuotTM Commercial Fee Quoting and Proposal Writing System. This article and accompanying diagrams have been taken from a new, not-yet-released Commercial Building Inspection Training Module written by Carson Dunlop Weldon & Associates Ltd. Visit www.cdwengineering.com.