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Fan Coil Water Distribution

Chilled and hot water must run to the fan-coil units. The piping arrangement determines the quality of performance, ease of operation, and initial cost of the system.

Two-Pipe Changeover Without Central Ventilation. In this system either hot or cold water is supplied through the same piping. The fan-coil unit has a single coil. The simplest system with the lowest initial cost is the two-pipe changeover fan coil with (1) outside air introduced through building apertures, (2) manual three speed fan control, and (3) hot and cold water temperatures scheduled by outdoor temperatures. This system is generally used in residential buildings with operable windows and relies on the occupant to control fan speed and open or close windows. The changeover temperature is set at some predetermined set point. If a thermostat is used to control water flow, it must reverse its action depending on whether hot or cold water is available.

The two-pipe system can not simultaneously heat or cool, which is required for most projects during intermediate seasons when some rooms need cooling and others need heat. This problem can be especially troublesome if a single piping zone supplies the entire building. This deficiency may be partly overcome by dividing the piping into zones based on solar exposure. Then each zone may be operated to heat or cool, independent of the others. However, one room may still require cooling while another room on the same solar exposure requires heating—particularly if the building is partially shaded by an adjacent building.

Another deficiency of the two-pipe changeover system is the need for frequent changeover from heating to cooling, which complicates the operation and increases energy consumption to the extent that it may become impractical. For example, two-pipe changeover system hydraulics must consider the water expansion (and relief) that occurs during the cycling from cooling to heating.

For these reasons, the designer should consider the disadvantages of the two-pipe system carefully; many installations of this type waste energy and have been unsatisfactory in climates where frequent changeover is required and where interior loads require cooling simultaneously as exterior spaces require heat.

Two-Pipe Changeover With Partial Electric Strip Heat. This
arrangement provides simultaneous heating and cooling in intermediate seasons by using a small electric strip heater in the fan-coil < unit. The unit can handle heating requirements in mild weather, typically down to 40°F, while continuing to circulate chilled water to handle any cooling requirements. When the outdoor temperature drops sufficiently to require heating in excess of the electric strip heater capacity, the water system must be changed over to hot water.

Two-Pipe Nonchangeover with Full Electric Strip Heat. This system is not recommended for energy conservation, but it may be practical in areas with a small heating requirement.

Four-Pipe Distribution

The four-pipe distribution of secondary water has cold water supply, cold water return, warm water supply, and warm water return pipes. The four-pipe system generally has the highest initial cost, but it has the best fan-coil system performance. It provides (1) all-season availability of heating and cooling at each unit, (2) no summer/winter changeover requirement, (3) simpler operation, and use of any heating fuel, heat recovery, or solar heat. In addition, it can be controlled to maintain a “dead band” between heating and cooling so simultaneous heating and cooling cannot occur. For discussion of two, three, and four pipe systems with central ventilation see later sections of this chapter.



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