Table of Contents
Advertisement
Stage 4 Relays K1, K2, K3, and K5 are energized. Com-
pressor no. 1 is running at
pressor no. 2 is running at full capacity. The crankcase
heater for compressor no. 2 has been deenergized.
Stage 5 Relays K1, K3, and K5 are energized. Compressor
no. 1 is running at
2 is running at full capacity.
Stage 6 Relays K1 and K5 are energized. Both compres-
sors are running fully loaded.
The 024-030 units have 2 condenser fans and use a fixed
resistor in place of the T3 and T4 thermistors. The resistor
is set at 1200 ohms (a saturated temperature of approxi-
mately 142 F). This temperature will ensure that the K11
relay will remain energized during compressor operation. The
OFM2 will be cycled on an outdoor-ambient temperature of
60 F. At outdoor temperatures below 60 F, OFM2 will be
locked out.
The 034 unit has 2 condenser fans, one of which is con-
trolled by the microprocessor. The OFM1 is energized with
compressor no. 1. The OFM2 is controlled by the processor
and is cycled based on input from circuit thermistor T3 or
T4.
On the 054,064 units, the first 2 condenser fans energize
with the compressors; compressor no. 1 controls OFM1, and
compressor no. 2 controls OFM2. The OFM3 and OFM4 are
staged by the microprocessor based on the condensing tem-
perature input from thermistor T3 or T4.
On the 074 units, the first 3 condenser fans energize with
the compressors; compressor no. 1 controls OFM1, and
compressor no. 2 controls OFM2 and OFM3. The OFM4
and OFM5 are staged by the microprocessor based on con-
densing temperature input from either circuit's T3 or T4
thermistor.
SIZE 044 UNITS — These units have 2 compressors and 1
unloader on compressor no. 1. See Fig. 22 for compressor
and condenser fan motor locations. The 044 unit operating
sequence is as follows:
Stage 1 Relays K1 and K3 are energized. Compressor
no. 1 starts with the unloader energized. Compres-
sor no. 1 is running at
heater on compressor no. 1 has been deenergized,
and the first stage condenser fan has been ener-
gized. Outdoor (condenser) fan motor no. 1 (OFM1)
has started.
Stage 2 Relay K1 is energized. Compressor no. 1 is fully
loaded.
Stage 3 Relays K1, K3, and K5 are energized. Compressor
no. 1 is running at
2 is running at full capacity. The crankcase heater
for compressor no. 2 is deenergized. The second
stage condenser fan has been energized. Both OFM1
and OFM3 are operating.
Stage 4 Relays K1 and K5 are energized. Both compres-
sors are running fully loaded.
The 044 unit has one fan that can be controlled by the
processor. The other 2 are controlled by the compressors.
The OFM1 is energized by compressor no. 1, and OFM3 is
energized by compressor no. 2. The OFM2 is cycled by the
processor based on input from either circuit (thermistors T3
and T4).
Head Pressure Control —
a basic head pressure control function which allows the units
to operate in cooling down to 45 F. If cooling is required at
outdoor ambient temperatures lower than 45 F, refer to ac-
cessory head pressure control literature for details.
⁄
capacity, and com-
1
3
2
⁄
capacity, and compressor no.
3
1
⁄
capacity. The crankcase
2
⁄
capacity, and compressor no.
1
2
All units have as standard
SIZE 024-030 UNITS — All units have a fan cycling ther-
mostat which cycles OFM2 (located over the control box).
The thermostat switch opens at 60 F ± 3 F and closes at
70 F ± 3 F.
SIZE 034-074 UNITS — Head pressure control is handled
by the processor. The processor attempts to maintain the head
pressure by cycling the condenser fan motors. No condenser
fans will be running without a call for mechanical cooling.
Thermistors T3 and T4 provide the condensing temperature
information to the processor. These VAV rooftop units have
dual refrigeration circuits, and the higher circuit temperature
will govern unit operation. If the condensing temperature is
above 133 F (236 psig), a condenser fan stage will be added.
If the condensing temperature is 78 F (142 psig) or less, the
number of condenser fans operating will be decreased. After
each fan stage, the processor will wait one minute for the
head pressures to stabilize before changing again, unless ther-
mistor T3 or T4 senses a temperature greater than 125 F
(278 psig), in which case all condenser fans are started.
During start-up, if the outdoor ambient is above 70 F (as
sensed by thermistor T3 or T4), the first-stage, processor-
controlled fans are turned on to prevent excessive discharge
pressures.
Inlet Guide Vanes —
fan static pressure is controlled by inlet guide vanes. The
inlet guide vanes operate independently from the micro-
processor. The supply static pressure is controlled by a dif-
ferential pressure switch. If the unit is equipped with a re-
turn fan, building pressure is controlled by another differential
pressure switch.
Temperature Reset —
lows the unit to reset the leaving-air set point to a higher
value once most of the cooling load has been met. When the
space conditions are satisfied, the VAV terminals will begin
to throttle down. The unit senses this as an increase in the
supply static pressure, and if equipped, the inlet guide vanes
will begin to close in response to the increasing pressure.
Allowing the supply temperature to be reset to a higher value
maintains the air circulation in the space. The accessory re-
set package is required for temperature reset, and includes:
• a thermistor (T10)
• a reset temperature potentiometer (P7)
• a reset limit potentiometer (P3)
NOTE: In order for temperature reset to operate, DIP switch
2 must be set to the ON position.
If multiple sensors are required to average the space tem-
perature to determine when to start reset, see Fig. 24. Only
Carrier part no. HH79NZ014 sensors are to be used, in ar-
rangements of 4 or 9 sensors, with total sensor wiring not to
exceed 1000 ft.
NOTE: The modified leaving-air set point can be reset no
higher than the potentiometer P1 limit of 70 F. If the space
temperature (as sensed by thermistor T10) is greater than the
reset temperature (P7), no reset occurs.
When the space temperature is equal to the reset tempera-
ture setting of potentiometer P7, the LED display will show
the code
21
, at which point reset is initiated. The leaving-
air set point will be modified by the following formula:
MSP = SP + [(P7 - T10) x P3 x
Where:
MSP — Modified Leaving-Air Set Point
SP
— Leaving-Air Set Point
P7
— Temperature to Start Reset
T10 — Space Temperature
P3
— Reset Limit
14
In most VAV units, the supply
Accessory temperature reset al-
1
⁄
]
3
Hide quick links:
Advertisement
Table of Contents
