York YD A, Operating & Maintenance

JOHNSON CONTROLS

14

FORM 160.69-O2 

ISSUE DATE: 9/30/2020 

SECTION 1 – DESCRIPTION OF SYSTEM AND FUNDAMENTALS OF OPERATION

Normal or Single Compressor Mode 

(Software version C.OPT.11.03.01.004 (or later))
The COMPRESSOR MODE Setpoint, available on the 

Capacity Compressor Cycling Screen, is  used to se-

lect either NORMAL Mode or SINGLE COMPRES-

SOR Mode. In Normal Mode, the second compressor 

is brought on line to meet the load requirement as de-

scribed previously. In Single Compressor Mode, op-

eration is restricted to one compressor. The Lag com-

pressor will not be brought on line, regardless of the 

load requirement. 
When starting the chiller in Single Compressor mode, 

the operator does not designate which compressor will 

run. Rather, the “Lead” compressor is automatically 

selected by the software per the present software op-

eration. The software to select the “Lead” compressor 

has not changed.  
If Single Compressor Mode is selected while both 

compressors are running, a soft shutdown is performed 

on the “Lag” compressor. All faults for this inhibited 

compressor are continued to be handled.  
If the chiller is shutdown while running in Single Com-

pressor mode, the software selects the “Lead” com-

pressor as it normally does on a subsequent restart.   

COMPRESSOR SHUTDOWN

There are two types of shutdowns performed on one 

or both compressors; Soft Shutdowns and Fault shut-

downs. 
During a Soft Shutdown, first the Pre-rotation Vanes 

are driven closed. When the Vanes have fully closed 

(or 210 seconds have elapsed, whichever occurs first), 

the Discharge Valve is then driven closed. When the 

Discharge Valve has fully closed (or 40 seconds have 

elapsed, whichever occurs first), the run signal to the 

compressor motor starter is de-energized and the com-

pressor enters “Coastdown”. If any Discharge Valve 

does not close within 40 seconds, a cycling shutdown 

is performed and “Discharge #X – Valve Not Closed” 

is displayed.
If the Lag compressor is taken off line due to a Low 

Load condition while both compressors are running (as 

explained above), a Soft Shutdown is performed on the 

Lag compressor. 
If one of the following shutdowns is initiated, a Soft 

Shutdown is performed simultaneously on all running 

compressors: 

•  Operator Initiated Soft Shutdown at Keypad
•  “Remote Stop”
•  “Leaving Chilled Liquid temperature – Low Tem-

perature”

•  “Discharge Valve X – Valve Not Closed”
•  “Multiunit – Contacts Open”
•  “System Cycling (1 & 2) – Contacts Open”
•  “Control panel – Schedule”

If any faults other than those listed above occur during 

a Soft shutdown, the Soft Shutdown is aborted, the run 

signal to the compressor motor starter is de-energized 

and the compressor(s) immediately enters “Coast-

down”.
If an Operator initiates a “Local Stop” with the Keypad 

Rocker Switch or any fault occurs other than those list-

ed immediately above, a Fault Shutdown is performed 

on all running compressors. Simultaneously, the run 

signal to the compressor motor starter(s) is de-ener-

gized, the compressor(s) enters “Coastdown”, and the 

Pre-rotation Vanes and Discharge Valve(s) are driven 

closed.