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18
Demand Limit —
The demand limit function provides a
means to limit the cooling capacity of the VAV unit using an
external discrete switch function. When enabled by the closure
of the external switch, the control will limit the available com-
pressor staging capacity according to user set points established
at demand limit potentiometer(s).
The unit controls support two types of demand limit: single-
step and 2-step control.
SINGLE-STEP DEMAND LIMIT — This function will limit
the total compressor staging based on the ratio of the set point
potentiometer’s wiper arm position to the full scale resistance
of the potentiometer. The exact percentage of capacity reduc-
tion differs depending on the number of capacity steps.
A field-supplied potentiometer and control switch are re-
quired for this function. See installation section for specifica-
tion on potentiometer and field wiring.
TWO-STEP DEMAND LIMIT — Two-step demand limit is
provided with the installation of the accessory Demand Limit
Control Module kit plus installation of 2 field-supplied control
switches (SPST-NO each). This accessory control provides for
a first step reduction of 50% to 100% of the maximum com-
pressor staging; the second step provides for reduction
between 0% and 49%. The exact percentage of capacity reduc-
tion differs depending on the number of capacity steps.
When demand limit is active, the LED display will show
code 22 .
INSTALLATION
Single-Step Demand Limit
— A 3-wire 5 to 20 K-ohm poten-
tiometer must be field-supplied and installed. A single-
pole normally open switch is also required (field-supplied
and -installed). Locate the potentiometer (designated P4) and
the switch in a suitable location (external from the unit or in the
unit control box).
SENSOR 1 SENSOR 2 SENSOR 3 SENSOR 4
RED
BLK
RED
BLK
RED
BLK
RED
BLK
TO ACCESSORY SPACE
TEMPERATURE RESET
CONTROL BOARD
RED
BLK
12
TB3
15
SIZES 034-048
12
TB4
15
SIZES 054-104
RED
BLK
RED
BLK
RED
BLK
SENSOR 1
SENSOR 2
SENSOR 3
RED
BLK
SENSOR 6SENSOR 5
RED
BLK
SENSOR 4
RED
BLK
RED
BLK
RED
BLK
SENSOR 8SENSOR 7 SENSOR 9
TO ACCESSORY SPACE
TEMPERATURE RESET
CONTROL BOARD
RED
BLK
12
15
TB3
SIZES 034-048
12
15
TB4
SIZES 054-104
SPACE TEMPERATURE RESET — 4 SENSOR AVERAGING APPLICATION
SPACE TEMPERATURE RESET — 9 SENSOR AVERAGING APPLICATION
Fig. 26 — Space Temperature Sensor Averaging
NOTE: Sensor part number is HH79NZ014.