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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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