Cooling Mode; Discharge Air Control With Modulating; Discharge Air Temperature (Dat) Sensor; High Discharge Air Temperature Condition - Carrier Omnizone 50BVC Installation, Start-Up, Service And Controls Operation And Troubleshooting

For discharge air control applications the minimum on-times and
off-times in Table 26 are applicable:
Table 26 — Discharge Air Control
COMPR 1 COMPR 2
Min Min Min Min
ON OFF ON OFF
10 5 10 5
If for any reason the compressor alarms reset, the unit compres-
sors will start within 10 seconds of each other.

COOLING

Cooling will be enabled whenever:
• Unit is in occupied mode
• The fan output is on
• The loop valve is open

COOLING MODE

When commanded into cooling mode, the unit will energize the
condenser water valve and wait for its valve end switch to be
made prior to energizing the compressors.
Once the valve has been proved open the unit will command the
compressor to stage according to the cooling percentage required.
This value is provided via a reverse acting PID loop which com-
pares the supply air temperature (SAT) value and the SAT cooling
set point (AV:66).
The unit monitors return air temperature to assure air entering the
unit is greater than 60°F (adjustable) prior to running in the cool-
ing with modulating hot gas reheat, when the controller is set to
operate with multiple reset points. If the controller is set to operate
with single return temperature reset the factory default value will
be the free cooling value 50°F (adjustable).
If at any time the cooling set point is greater than the return air
temperature (RAT) the unit will enter into economizer assist
mode.
Compressors will be staged as follows:
Compressor 1 will run:
When the fan is running
AND the condenser valve is proved.
AND the cooling demand is greater than 25%
Compressor 2 will run:
When compressor one has run for 10 minutes
AND the cooling demand is greater than 50%
Compressor 3 will run:
When compressor two has run for 10 minutes
AND the cooling demand is greater than 75%
Compressor 4 will run:
When compressor 2 has run for 10 minutes
AND the cooling demand is greater than 90%
When the unit runs in cooling mode the hot gas re-heat valve will
be enabled and modulated to maintain supply air temperature set
point, factory default is 55°F (AV:93, adjustable) ± 4°F.
DISCHARGE AIR CONTROL WITH MODULATING RE-
HEAT
When in cooling mode, if the unit is equipped with modulating hot
gas re-heat, the hot gas re-heat will be enabled and modulated to
maintain supply air temperature set point, factory default is 55°F
(adjustable) ± 4°F.
COMPR 3 COMPR 4
Min Min Min Min
ON OFF ON OFF
10 5 7 5
The cooling stages can be reset based on a single point or multiple
return air temperature (RAT) values as indicated in Table 27:
Table 27 — Cooling Stage Reset Values
POINT
Single:
Free Cooling
Mechanical Cooling
Multiple:
Free Cooling
Mechanical Cooling
(Comp 1)
Mechanical Cooling
(Comp 2)
Mechanical Cooling
(Comp 3)
Mechanical Cooling
(Comp 4)
All values have a hysteresis of 2.0°F.
If discharge air set point reset is required, when the value of the
discharge air set point is greater than the value of the return air re-
set for a particular compressor, the return limit will have to be ad-
justed via the BACnet tool to compensate for the demand changes
and release. The particular compressor stage or the unit must be
set to a single reset point and the discharge set point reset can be
adjusted as needed.
Any of the following alarms will immediately shut down all com-
pressor stages. Refer to the sensor section or the integration points
list for default values:
• Leaving water high
• Leaving water low
• Entering later low
• Fan alarms
• Low static pressure
• Water differential pressure switch (DPS)
• Smoke

DISCHARGE AIR TEMPERATURE (DAT) SENSOR

The DAT sensor is shipped loose in the electrical controls box
compartment and is to be field installed in the supply duct work
and terminated on UI-6 of the I/O Flex 6126 controller.
The sensor should be installed where the air flow pattern is lami-
nar to avoid temperature stratification. If supplemental heating is
to be installed then the DAT sensor should be mounted down-
stream of the discharge side of the heating coil.

HIGH DISCHARGE AIR TEMPERATURE CONDITION

(COOLING)
DAT measurements are tested for a high limit trip above 70°F. An
alarm is asserted for high discharge air temperature under the fol-
lowing conditions:
• DAT is above the high limit for 5 minutes
• Fan operation asserted
• Cooling mode
• Valid DAT sensor measurement

HIGH STATIC LOCK

The controller will monitor the static pressure high limit switch (if
installed) and will increment a counter every time the static pres-
sure switch trips. A High Static Alarm will be generated, and will
reset automatically. Upon receiving the alarm 3 times, the unit will
lock out to protect the ductwork and prevent cycling of major unit
components.
If the lock state is reached in addition to the high static alarm, code
100 will be set and broadcast over the BACnet network via the
Systems Status point (AV: 16).
49
VALUE
50°F < RAT
RAT > 50°F (enabled)
50°F <RAT < 59°F
60°F <RAT < 69°F (adj.)
70°F <RAT < 77°F (adj.)
78°F <RAT < 83°F (adj.)
RAT > 83°F (adj.)