JOHNSON CONTROLS
111
SECTION 5 – SEQUENCE OF OPERATION
FORM 145.05-NOM7
ISSUE DATE: 10/31/2019
5
SAT HIGH SETPOINT
(Default = 65.0 °F)
SAT LOW SETPOINT
(Default = 55.0 °F)
Return Air Temp
(RAT)
RAT SETPOINT
FOR HIGH SAT
RAT SETPOINT
FOR LOW SAT
Active SAT
Setpoint
Increasing RAT
FIGURE 68 -
ACTIVE SAT SETPOINT VS. RETURN
AIR TEMP
LD10307A
COMPRESSOR CONTROL AND
SEQUENCING
Whenever a change in the unit's cooling demand oc-
curs (compressor turned ON, compressor turned OFF,
etc.), a 3-1/2 minute Interstage Delay Timer is initiat
-
ed. During the countdown of the timer, a compressor is
not staged ON or OFF until the timer has expired. The
only exception is if the compressor protection circuit
experiences a fault.
On CONSTANT VOLUME and VARIABLE AIR
VOLUME, when the Unit Controller enters an active
COOLING mode, the Unit Controller sets the “COOL-
ING CONTROL OFFSET” to 2.0°F. The Unit Control-
ler compares the SUPPLY AIR TEMP CURRENT to
the “SUPPLY AIR TEMP ACTIVE SP” plus or minus
the “COOLING CONTROL OFFSET.” If the SUPPLY
AIR TEMP CURRENT is greater than the “SUPPLY
AIR TEMP ACTIVE SP” plus the “COOLING CON-
TROL OFFSET” and all of the compressors are not
ON, the Unit Controller initiates a call for compres-
sor or additional compressor operation based on the
“NEXT STAGE TO ENABLE.”
If the SUPPLY AIR TEMP CURRENT is less than the
“SUPPLY AIR TEMP ACTIVE SP” minus the “COOL-
ING CONTROL OFFSET” and all of the compressors
are not OFF, the Unit Controller turns a compressor
OFF based on the “NEXT STAGE TO DISABLE.”
Compressor Staging Sequence
Compressors are staged up and down to control the
SUPPLY AIR TEMP CURRENT to the “SUPPLY
AIR TEMP ACTIVE SP,” but only if the unit goes into
COOLING mode when the Return Air Setpoint RAT is
0.5°F higher than its setpoint found in the COOLING
key on the unit's display.
The compressor staging sequence is determined by the
unit size, compressor staging sequence, and the num-
ber of starts. Two compressors cannot be brought on at
the same time, and there is always at least a 15 second
time delay between starting compressors.
System A Compressor (25–60 Ton Units
Only)
The compressor used in System A on 25–60 ton units
is a Copeland scroll compressor with digital unloading.
The digital unloading feature allows the compressor to
operate at approximately 1/3 of capacity, 2/3 of capac-
ity, and full capacity. The compressor requires an ad-
ditional control to monitor and control the unloading of
the compressor. To energize the compressor, a binary
signal originates at the J16-1. A 1–5VDC analog output
is also required at J12-2 and J12-3. The + output is con-
tained at J12-2 and the common at J12-3.
shows the relationship between the voltage
output and the compressor capacity.
TABLE 21 -
SYSTEM A COMPRESSOR CAPACITY
(25–60 TON)
VDC OUTPUT
COMPRESSOR CAPACITY
1.0 VDC
0 Capacity
2.32 VDC
1/3 Capacity
3.64 VDC
2/3 Capacity
5 VDC
Full Capacity
System A Compressor Unloading
Operation (25–60 Ton Units Only)
The compressor unloads by allowing the scrolls to sepa-
rate. This is accomplished by bypassing the discharge gas
so that it does not put pressure on the top of the scrolls.
When the pressure is removed, the scrolls open. The Unit
Controller uses a 15 second duty cycle to maintain the
capacity. If the compressor is at 1/3 (33%) capacity, the
compressor is unloaded for 10 seconds and loaded for 5
seconds (see
. If the compressor
is at 2/3 (67%) capacity, the compressor is unloaded for
5 seconds and loaded for 10 seconds (see
. In the 15-second cycle, the loaded portion of
the cycle occurs first before it unloads.
During operation, the solenoid is not energized during
the loaded state of the compressor. When the compres-
sor is unloaded, the solenoid is energized via the con-
troller with 24VAC. If the solenoid fails, the compres-
sor defaults to 100% loaded capacity.