30 November 1998
STANDARD CONTROL LOOPS
The standard control loops described in this chapter consist of control system equipment and
devices, arranged to perform specific control system functions. In the ensuing discussions of the
different types of control loops, all loop devices (including transmitters) are shown, and their associated
indicators (such as thermometers) are included where required. The sensing elements are included with
the transmitters and are not shown separately. Signals from the transmitters represent the changing
conditions at the sensing elements. Also, the required panel-mounted and field-mounted pneumatic
indicators are shown. Modulating control signals from controllers are converted from 4-20 milliamperes to
21-103 kPa (3-15 psig) by a current-to-pneumatic transducer (IP) connected to a positive positioner (PP)
of a valve or damper actuator as applicable.
2. COOLING COIL TEMPERATURE CONTROL LOOP.
a. The cooling coil temperature control loop is a constant temperature control loop and is shown in
figure 3-1. Temperature sensing element and transmitter TT sends a temperature signal to controller TC
(or to the DDC panel), which modulates an IP. The pneumatic signal from the IP is connected to
positive-positioner PP, which operates cooling coil valve VLV. The conditions that must be operative for
the control valve to be controlled are: the supply fan is on and the control system is in the occupied
b. A relay contact between TC and IP is open when either constraint is operative.
Figure 3-1. Cooling coil temperature control loop.
3. OUTSIDE AIR PREHEAT COIL TEMPERATURE CONTROL LOOP.
a. When the mixed air temperature of the outside air and the return air is too low, a preheat coil will
be used to heat the outside air. This modulating control loop will be used only with hot water or hot glycol
heating units. A variation of the preheat coil control loop for use with steam preheat coils is shown in
chapter 5. The purpose of raising the mixed air temperature is to prevent freezing of chilled water coils
and hot water coils downstream of the mixed air plenum. The coil is sized to raise the temperature of the
maximum design quantity of outside air just high enough to bring the mixed air temperature within the
range of 7 to 10 degrees C (45 to 50 degrees F). The loop controls the temperature of the air leaving the
preheat coil before the air mixes with return air. The setpoint of the controller of this loop is the HVAC
system designer's calculation of the coil discharge air temperature required to maintain a minimum
temperature in the mixed air plenum when the outside air is at the coldest temperature. This setpoint
assures an adequate minimum temperature entering the cooling coil of an HVAC system. The outside air
preheat coil temperature control loop is shown in figure 3-2.
Figure 3-2. Outside air preheat coil temperature control loop.