Carrier 38AH024-034 - Manual (Page 17)

SEQUENCE OF OPERATION
Single Air Handler, 2-Stage Cool Thermostat
— At start-up, the thermostat calls for first-stage cooling
(TCI closes) and with all safety devices satisfied, circuit
no. 1 control relay (CRl) closes, deenergizing the crank­
case heater at compressor no. 1. The indoor-fan motor starts
immediately, and the liquid line solenoid valve (LLSV) for
solenoid drop refrigeration control on circuit no. 1 opens.
Outdoor-fan motor no.l and compressor no. 1 start after
one second. Fan no. 2 starts if the outdoor ambient is above
60 F (15.6 C).
On standard units, compressor no. 1 operates either fully
loaded or at one step of unloading depending on the suction
pressure, which in turn is dependent on the evaporator load
conditions. As cooling demand increases, TC2 of the 2-stage
thermostat energizes control relay no. 2. CR2 deenergizes
the crankcase heater at compressor no. 2 and brings on com­
pressor no. 2 after one second. The LLSV for solenoid drop
refrigerant control (circuit no. 2) opens as soon as CR2 is
energized. On standard units, compressor no. 2 operates
fully loaded.
As cooling demand is satisfied, thermostat contacts TC2
break and deenergize CR2 which then deenergizes compres­
sor no. 2 and energizes its crankcase heater. The LLSV on
circuit no. 2 closes, preventing refrigerant migration back
to compressor no. 2 during the off cycle. If the space tem­
perature continues to decrease, stage 1 thermostat contacts
TCI break and deenergize CRl. Immediately after CRl is
deenergized, compressor no. 1 shuts off and its crankcase
heater is energized. The LLSV on circuit no. 1 closes, pre­
venting refrigerant migration back to compressor no. 1
during the off cycle. Outdoor-fan motors no. 1 and 2 de­
energize as soon as CRl is deenergized.
NOTE: If thermostat fan switch is in the auto position, the
indoor fan will cycle on and off as the thermostat calls for
cooling (or heating). If the fan switch is in the continuous
position, the fan will run as long as the outdoor unit is pow­
ered up.
Two Air Handlers — Two 2-Stage Cool Ther­
mostats (One Per Air Handler) — At start-up of
circuit no. 1, the thermostat controlling circuit no. 1 calls
for first stage of cooling when thermostat contact TCI closes.
If all safety devices are satisfied, circuit no. 1 control relay
CRl energizes which deenergizes the crankcase heater in­
side compressor no. 1. The indoor-fan motor on the circuit
no. 1 fan coil starts immediately, and the main liquid line
solenoid valve (LLSV) for solenoid drop refrigerant control
on circuit no. 1 opens. Outdoor-fan motor no. 1 and com­
pressor no. 1 start after one second. Fan no. 2 starts if the
outdoor ambient is above 60 F (15.6 C).
On standard units, compressor no. 1 operates either fully
loaded or at one step of unloading depending on the suction
pressure, which in turn is dependent on the evaporator load
conditions. As cooling demand increases, TC2 of the 2-stage
thermostat energizes the capacity control LLSV resulting in
full activity of the evaporator surface. The fully active
evaporator surface will raise the suction pressure until the
pressure-operated capacity control valve setting is ex­
ceeded, so that the compressor becomes fully loaded.
The thermostat controlling circuit no. 2 calls for first stage
of cooling when thermostat contact TC2 closes. Assuming
all safety devices are satisfied, circuit no.2 control relay
CR2 energizes, which deenergizes the crankcase heater in­
side compressor no. 2. The indoor-fan motor on circuit
no. 2 fan coil starts immediately, and the main LLSV for
solenoid drop refrigerant control on circuit no. 2 opens. Com­
pressor no. 2 starts after one second. The outdoor fans are
already running if circuit no. 1 is running. If circuit no. 1 is
not running when thermostat no. 2 calls for cooling, the
outdoor fans are off. In this situation, outdoor-fan motor
no. 1 starts when compressor no. 2 starts. Outdoor-fan mo­
tor no. 2 comes on if the outdoor ambient is above 60 F
(15.6 C).
As cooling demand is satisfied at thermostat no. 1 or 2
(whichever comes first), contacts TC2 break, and the ca­
pacity control LLSV on that circuit is deenergized. This
prevents refrigerant flow through the upper (circpit no. 1 or
2) evaporator section causing the suction pressure to drop
and the compressor on that circuit to unload.
NOTE; Circuit no. 2 does not have a factory-installed un­
loader. If desired, one must be added in the field.
As the space temperature drops further, the space ther­
mostat contacts TCI open which will deenergize control re­
lay CRl or CR2. Immediately after CRl or CR2 is deen­
ergized, the compressor CRl or CR2 control is shut off and
its crankcase heater is energized. The LLSV on that circuit
closes, preventing refrigeration migration back to the com­
pressor during the off cycle. Outdoor-fan motors no. 1 and
2 are deenergized only if the space thermostats for both cir­
cuits no. 1 and 2 are not calling for cooling.
NOTE; If thermostat fan switch is in the auto position, the
indoor fan will cycle on and off as the thermostat calls for
cooling (or heating). If the fan switch is in the continuous
position, the fan will run as long as the outdoor unit is pow­
ered up.
Restart — Manual reset of the 24-v control circuit is nec­
essary if unit shutdown is caused by automatic reset de­
vices (including COTP [compressor overcurrent protection]
and HPS [high-pressure switch]) or if shutdown is caused
by manual reset devices (including OPS [oil pressure switch]
and compressor circuit breaker protection). To restart the
unit when COTP or HPS has tripped {after COTP and HPS
have reset automatically), open and then close the thermo­
stat contacts. Opening and then closing thermostat contacts
interrupt and reapply 24-v power to the compressor lockout
(CLOl and CL02) which resets the devices.
It is necessary to manually reset the compressor circuit
breaker and OPS at the unit if either of these safeties should
shut down the unit.
IMPORTANT: If either OPS trips, it must be reset
first before making and breaking the thermostat con­
tacts to reset CLOl and CL02. If this procedure is
not followed, the CLOl and CL02 cannot reset.
If the LPS (low-pressure switch) is not closed, the call
for cooling is interrupted and the Time Guard® device is
activated. This deenergizes control relay(s) (CRl or CR2)
and deenergizes the compressor for a 5-minute time period.
At the end of this 5-minute period, if the LPS is closed, the
compressor restarts. If the system is low on charge, LPS
cannot reset and the unit will stay off on this safety.
Causes of Complete Unit Shutdown:
• interruption of supplied power
• open compressor overtemperature protection (COTP)
• compressor electrical overload protection (CB1 or CB2)
• open high-pressure or low-pressure safety switches
• open oil pressure switch
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