Single-Step Demand Limit — The single step de-
mand limit provides a means to limit the capacity of the VAV
unit using an external switch. Single step demand limit will
limit the compressor displacement based on the ratio of the
wiper arm to the full scale resistance. The exact percentage
of capacity reduction differs depending on the number of ca-
pacity steps.
A 3-wire, 5 to 20 Kohm, field-supplied potentiometer is
required for this option. The potentiometer should be wired
to the processor J3 connections. In order to control the de-
mand limit, the wiper arm of the potentiometer should be
switched open and closed based on the demand limit
requirement.
If the wiper arm wire is open, all capacity stages can be
used. When the wiper arm wire is closed, the capacity is
reduced by the amount set on potentiometer P4.
Demand Limit Control Module (DLCM) — The
DLCM provides a 2-step demand limit control using an ex-
ternal switch. The first step is between 50% and 100% of the
maximum compressor displacement. See Fig. 20. The sec-
ond step is between 0% and 49% of the maximum compres-
sor displacement. The exact percentage differs depending on
the number of capacity steps.
Two adjustable potentiometers are used to set the 2 de-
mand limit points. Potentiometer P1 is used to set a demand
limit between 50% and 100% of the unit capacity. Potenti-
ometer P2 is used to set a demand limit between 0% and
49% of unit capacity.
If no power is supplied to the demand limit control mod-
ule, all capacity stages can be used. When power is supplied
to terminal IN1 only, the first step of the demand limit con-
trol is energized and the capacity is reduced by the amount
set on potentiometer P1. When power is supplied to terminal
IN2 only, or to both IN1 and IN2, the capacity is reduced by
the amount set on potentiometer P2.
OPERATING INFORMATION
Digital Display —
The VAV control system uses a 2-digit
LED display located on the display board to display opera-
tional information and diagnostic codes.
CODES 0 THROUGH 6, CAPACITY STEPS — These codes
indicate the number of cooling stages active at the time the
display button is pressed. The highest code indicated on the
display will be 6 for the 024-034 and 054-074 units, and 4
for the 044 units.
Capacity steps are directly related to pin terminal connec-
tor J6 output. At step zero, the unit has no mechanical cool-
ing on, and the economizer may or may not be operating
(depending on the outdoor air conditions). Once a cooling
load is detected (T1 thermistor reads above the supply-air
set point), the economizer will begin modulating to meet the
load if the outdoor enthalpy is good. As long as the outdoor-
air enthalpy is acceptable, no mechanical cooling will take
place until the economizer dampers are fully open. The rest
of the steps and the operational sequence vary due to the
number of compressors and unloaders. Refer to Operating
Sequence section on page 13 for the unit stages of operation.
CODES 20 THROUGH 30 AND 88, OPERATIONAL STA-
TUS — These codes indicate special operational modes, such
as initialization, morning warm-up, temperature reset, de-
mand limit, or an internal failure of the board. Codes 23-25
and 27-29 are not used on these units.
Initialization — When the control is turned on, the display
20
shows a for approximately 2 minutes to indicate that the
control is in the initialization mode. During this time, the
economizer dampers open and close to determine the resis-
tance range of the economizer position potentiometer (P2)
for full economizer operation. The processor loads the nec-
essary constants for proper unit operation and checks the ther-
mistors and other potentiometers for their values and valid-
ity. After the initialization period, the display screen goes
blank until the display button is pressed. If the display but-
ton is pressed during the 2-minute initialization period, the
control goes into the quick test mode.
LEGEND
IC — Integrated Circuit
IN — Input
P—Potentiometer
RTN — Return
Fig. 20 — Two-Step Demand Limit Module
LEGEND
ECON — Economizer
MIN — Minimum
P—Potentiometer
VAV — Variable-Air Volume
Fig. 19 — Accessory Relay Board
(Standard; Factory Supplied)
12
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