30 November 1998
signal at its CPA input corresponds to a 49 C (120 F) setpoint. With these configuration parameters
defined, the controller scales, in a linear fashion, CPA input signals between 4 and 20 mA. These CPA
input low and high range configuration parameter selections are extracted directly from the reset
schedule shown in Figure 3-4 and Table 3-1.
g. The discharge air temperature controller minimum and maximum setpoints can also be defined as
32 C (90 F) and 49 C (120 )F, but are optional since the full range of the CPA input has already been
defined by the low and high range scale configuration parameters.
h. The outside air reset controller (TC REV) proportional band instructs the controller to change its
output in proportion to its input. In our example, the outside air controller proportional band must be
selected so that as the outside air temperature changes from -18 C (0 F) to 16 C (60 F) (also called the
"throttling range", TR), the controller output changes full-scale from 20 to 4 mA. This signal is applied to
the CPA input of the discharge air temperature controller, changing its setpoint from 49 C (120 F) to
32 C (90 F). The proportional band calculation was previously defined in equation 2-1 and in this
( TR x 100 )
(eq. 2-1, repeated)
( 16 ( 16 ) C ) x 100
( 60 0 F ) x 100
55 ( 35 ) C
130 ( 30 ) F
i. The outside air reset controller (TC REV) setpoint is (arbitrarily) selected to be at the midpoint
(50%) of the controller throttling range (TR). In our example, the throttling range is -18 C (0 F) to 16 C
(60 F) as dictated by the reset schedule. Therefore the setpoint is -1 C (30 F).
j. The outside air reset controller (TC REV) manual reset value is selected to be 50% because the
setpoint was selected to be at the midpoint (50%) of the throttling range. By definition of proportional
control, to achieve the desired result, the outside air controller setpoint and manual reset values must be
set to corresponding values, where in this case 50% was selected.
k. The outside air reset controller minimum and maximum output ranges must be selected to be 4
and 20 mA, respectively. Note that with some controllers this may be defined as 0 and 100% output,
respectively. The minimum and maximum output range parameters ensure that the outside air controller
output will change full range to correspondingly adjust the discharge air temperature controller setpoint
over its full 32 C (90 F) to 49 C (120 F) range as defined in in the reset schedule.
l. Table 3-3 shows the final controller configuration parameter settings. These parameters and the
reset schedule are to be shown on the control system Equipment Schedule drawing.