Carrier 38AQ034 - Manual (Page 10)

OPERATING SEQUENCE
(Refer to wiring diagrams in 38AQ wiring
booklet)
Standby (OFF) Mode — During the standby or OFF
mode, crankcase heater is energized. Reversing valve
may Or may not be energized depending on mode of
operation (heating or cooling) when thermostat is
satisfied.
COOLING — When thermostat calls for first-stage
cooling (TCI closed), indoor fan motor starts imme­
diately. Compressor and outdoor fans start between 3
seconds and 5 minutes depending on length of time unit
is off after thermostat is satisfied, due to 5-minute Time
Guard® II circuit. Outdoor fan motors nos. 2 and 3 may
or may not operate depending on position of fan cycling
pressure
switch (FCPS) and air temperature switch
(ATS). The reversing valve solenoid (RVS) becomes de­
energized
causing reversing valve to shift to cooling
position. Crankcase heater is off. Liquid line and accu­
mulator oil return solenoid valves open, allowing refrig­
erant and oil flow.
Defrost Timer does not operate during cooling mode.
When thermostat calls for second-stage cooling (TC2
closed), unloader is de-energized causing compressor to
run fully loaded.
If a malfunction occurs, causing high-pressure switch
(HPS) discharge gas thermostat (DGT) or loss-of-charge
switch (LCS) to open, compressor and outdoor fans
stop and are locked out by a Signal-Loc circuit, and a
warning light comes on at the thermostat. RVS remains
de-energized
so reversing valve does not shift. These
safeties reset by adjusting thermostat up to open TCI and
TC2, or by momentarily switching subbase to OFF posi­
tion. When thermostat is satisfied (TCI open), com­
pressor, indoor fan motors shut off. Liquid line and
accumulator oil return solenoid valves close. Reversing
valve does not shift, but remains in cooling position until
there is a call for heating. If compressor oil pressure is
lost, or if oil pressure fails to build on start-up, an oil
pressure safety switch shuts down unit. Switch must be
manually reset at the unit. DO NOT RESET MORE
THAN ONCE! If oil pressure switch trips, determine
cause and correct. DO NOT JUMPER OIL PRESSURE
SAFETY SWITCH!
Crankcase Heater is in a lockout circuit. If crankcase
heater is defective, compressor is locked off. The heat
pump remains off until corrective action is taken. This
lockout circuit cannot be reset by adjusting the thermostat.
Unit is equipped with a no-dump reversing valve
circuit. When unit is in cooling mode, reversing valve
remains
in cooling mode position until thermostat calls
for heating. When unit is in heating mode, reversing valve
remains in heating mode position until thermostat calls
for cooling.
HEATING — When thermostat calls for heating (THl
closed), indoor fan motor starts. Compressor and out­
door fans start between 3 seconds and 5 minutes depend­
ing on length of time unit is off after thermostat is satis­
fied, due to 5-minute Time Guard II circuit. Liquid line
and
accumulator oil return solenoid valves open when
compressor operates, and close when compressor is OFF.
Compressor always runs fully loaded in the heating mode.
All 3 outdoor fans also run during heating mode. Revers­
ing
Valve Solenoid (RVS) shifts for heating operation
at start-up.
Defrost Timer runs continuously during heating mode.
Crankcase heater is off whenever compressor is on. If a
malfunction
occurs, causing high-pressure switch (HPS),
discharge gas thermostat (DGT) or loss-of-charge switch
(LCS) to open, compressor and outdoor fan motors stop
and
are locked out by a Signal-LOC eircuit, and a
warning light appears on the thermostat. RVS remains
energized so reversing valve does not shift. Reset lockout
system by adjusting thermostat down to open THl and
TH2, or by momentarily switching subbase to the OFF
position. If compressor oil pressure is lost or if oil pres­
sure fails to build on start-up, an oil pressure safety
switch shuts down unit. Switch must be manually reset at
the unit. DO NOT RESET MORE THAN ONCE! If oil
pressure switch trips, determine cause and correct. DO
NOT JUMPER OIL PRESSURE SAFETY SWITCH!
Crankcase Heater is in a lockout circuit. If crankcase
heater is defective, compressor is locked off. The heat
pump remains off until corrective action is taken. This
lockout circuit cannot be reset by adjusting the thermo­
stat.
When thermostat is satisfied (THl opens), com­
pressor, indoor fan motor and outdoor fan motors shut
off. Reversing valve remains in heating mode until
thermostat calls for cooling.
If outdoor ambient temperature is above approxi­
mately 45 F, defrost thermostat (DFT) senses outdoor
coll temperature is above 28 F, and prevents start of any
defrost
cycle. When outdoor ambient temperature is
below approximately 45 F, DFT senses outdoor coil
temperature is below 28 F and allows defrost cireuitry to
be energized every 60 minutes. When this occurs, revers­
ing valve switches back to cooling position and outdoor
fans shut off. Defrost is terminated when liquid refrig­
erant becomes warm enough to open defrost thermostat.
Defrost timer limits length of each defrost cycle to a
maximum of 10 minutes.
Two defrost interlock relays are provided for use on
systems having more than one 38AQ unit. When any
38AQ unit is in defrost mode, up to 2 additional 38AQ
units
can be prevented from defrosting by temporarily
stopping their defrost timer motors until defrost on
another unit is completed.
Compressor and outdoor fan motor overcurrent pro­
tection is achieved with circuit breakers (heating and
cooling modes). These require manual reset at the out­
door unit control box.
Refer to Fig. 12 for typical 38 AQ heat pump refrigerant
circuit operation.
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