Carrier 38AUM Series Installation, Operation & Maintenance Manual page 21

OPERATING SEQUENCE
Base Unit Controls
Indoor (Supply) Fan— The indoor fan contactor (IFC) is remotely located at the fan coil or fan section. If the
thermostat fan operation is selected as Continuous, the IFC is energized and the indoor (supply) fan motor runs
continuously. If the thermostat fan operation is selected as Automatic, the IFC will be energized on a call for Cooling;
indoor (supply) fan motor runs. When thermostat call for Cooling is satisfied, the IFC is de-energized and indoor
(supply) fan motor stops.
Cooling Unit — 38AUMZ (Single Circuit)
On a thermostat call for Cooling, IFC will be energized and indoor (supply) fan motor runs. Thermostat output Y1
is energized; terminal Y1 at 38AUMZ unit receives 24-v. 24-v received at CADM1 terminal Y. If anti-recycle time
delay period has not expired, CADM1 relay will remain open, de-energizing Solenoid Valve Relay (SVR) and
preventing compressor start. When safety pressure switches are closed and CADM1 time delay expires, CADM1 relay
closes, SVR and compressor contactor C1 are energized; liquid line solenoid valve LLSV opens, all outdoor fan
motors start and Compressor 1 starts.
On a thermostat calling for Stage 2 Cooling, thermostat output Y2 is energized; terminal Y2 at 38AUZ unit receives
24-v. 24-v received at CADM2 terminal Y. If anti-recycle time delay period has not expired, CADM2 relay will remain
open, preventing Compressor 2 start. When safety pressure switches are closed and CADM2 time delay expires,
CADM2 relay closes, compressor contactor C2 is energized; Compressor 2 starts.
As space cooling load is satisfied, thermostat outputs Y2 and Y1 are sequentially de--energized, removing 24-v at
38AUMZ terminals Y2 and Y1. Compressor 2 stops on Y2 opening. CADM2 begins its three-minute anti-recycle time
delay. On Y1 opening, Compressor 1 stops, all outdoor fan motors stop and SVR relay is de-energized. Liquid line
solenoid valve is de-energized and valve closes. CADM1 begins its three-minute anti-recycle time delay.
Cooling Unit — 38AUMD (Two Circuit)
On a thermostat call for Cooling, IFC will be energized and indoor (supply) fan motor runs. Thermostat output Y1
is energized; terminal Y1 at 38AUD unit receives 24-v. 24-v received at CADM1 terminal Y. If anti-recycle time delay
period has not expired, CADM1 relay will remain open, de-energizing Solenoid Valve Relay 1 (SVR1) and preventing
compressor start. When safety pressure switches are closed and CADM1 time delay expires, CADM1 relay closes,
SVR1 and compressor contactor C1 are energized; liquid line solenoid valve LLSV1 opens, all outdoor fan motors
start and Circuit 1 compressor starts.
On a thermostat calling for Stage 2 Cooling, thermostat output Y2 is energized; terminal Y2 at 38AUMD unit receives
24-v. 24-v received at CADM2 terminal Y. If anti-recycle time delay period has not expired, CADM2 relay will remain
open, de-energizing Solenoid Valve Relay 2 (SVR2) and preventing compressor start. When safety pressure switches
are closed and CADM2 time delay expires, CADM2 relay closes, SVR2 and compressor contactor C2 are energized;
liquid line solenoid valve LLSV2 opens and Circuit 2 compressor starts.
As space cooling load is satisfied, thermostat outputs Y2 and Y1 are de-energized, removing 24-v at 38AUD terminals
Y2 and Y1. Circuit 2 compressor stops on Y2 opening; SVR2 is de-energized and LLSV2 closes. CADM2 begins its
three-minute anti-recycle time delay. On Y1 opening, Circuit 1 compressor stops, all outdoor fan motors stop and
SVR1 relay is de-energized. Liquid line solenoid valve LLSV1 is de-energized and valve closes. CADM1 begins its
three-minute anti-recycle time delay.
All Units
If either the Low Pressure Switch or High Pressure Switch opens while thermostat output Y1 or Y2 remain energized,
the compressor contactor is de-energized, the compressor stops and liquid line solenoid is de-energized (valve
closes). CADM initiates a TRIP event (cooling demand sensed at CADM terminal Y but no current is measured at T1,
T2, T3 motor sensors); CADM relay opens and RED LED is illuminated. TRIP condition maintains lockout of
compressor operation until CADM is manually reset. Reset CADM by cycling unit main power.
Complete system shutdown may be caused by loss of main power, open compressor internal overload, open
low-pressure or high-pressure switch, or a fault detected by the CADM logic. Compressor operation without cooling
may indicate the compressor's ASTP feature is active; disconnect unit power and allow compressor to cool. See
Service section for further details.
Heating — Refer to fan coil unit installation instructions and accessory heating device installation instructions for
operating sequences in heating mode.
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