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b. A temperature sensing element and transmitter TT, in the discharge air stream from the preheat
coil, sends a temperature signal to preheat coil temperature controller TC. Controller TC operates
transducer IP to maintain the setpoint of the controller by modulating a valve VLV. Since the TC setpoint
is normally in the range of 4.5 to 13 degrees C (40 to 55 degrees F), the valve is controlled during the
heating season when the outside-air temperature is below the TC setpoint. When the outside air
temperature is at or above the TC setpoint, VLV is closed.
c. In this control loop, TC is direct acting (DIR), and VLV is normally open (NO) and fails open under
the pressure of the valve actuator's return spring upon loss of electric signal, pneumatic signal, or
positive-positioner air supply. The purpose of this is to avoid freezing of the preheat coil and other coils in
the HVAC system should such an event occur in cold weather.
d. The preheat coil control loop functions continuously, without regard to the operating condition of
the HVAC system. This has the advantage of maintaining a minimum temperature in the ductwork when
the HVAC system supply fan is off.
e. For DDC applications, the DDC panel takes the place of controller TC.
4. HEATING COIL TEMPERATURE CONTROL LOOP.
a. Heating coils in HVAC systems are usually controlled by either of the following methods:
(1) Coil discharge air temperature setpoint is fixed.
(2) Coil discharge air temperature setpoint is based on the outside air
temperature. This is usually referred to as "set point reset" of the coil discharge
air temperature according to a "reset schedule". Although set point reset can
help to conserve energy through reduced piping system heat loss, its primary
advantage is improved control system performance and occupant comfort (in
multizone and dual duct systems) through improved temperature regulation as
the downstream air dampers tend to modulate freely as system capacity better
matches the load.
b. Figure 3-3 shows an example of set point reset of a heating coil (HC) temperature controller. The
output of the reverse acting OA controller (TC) is the input to the control point adjustment (CPA) of the
direct acting HC discharge air temperature controller (TC). As the outside air temperature increases, the
output of the OA TC decreases, which causes the setpoint of the HC TC to decrease linearly.
Alternatively, as the outside temperature decreases, the output of the OA TC increases, which causes
the setpoint of HC TC to increase linearly.
Figure 3-3. Heating coil temperature control loop (scheduled from outside air temperature).
c. Figure 3-4 and Table 3-1 define an example setpoint reset schedule. The schedule requires that
the discharge air set point not rise above 49 C (120 F) or below 32 C (90 F) and that there is a linear