McQuay WGS130AA: WGS140AW, Operating Manual

Page: 45 / 98

OM WGS-5 WGS 130A to 190A 45
T3 – Transition from Run to Off
Requires any of the following

Unit state = Off AND LWT > Freeze setpoint

Low OAT Lockout is active AND No compressors running AND LWT > 70°F
T4 – Transition from Start to Off
Requires any of the following

Unit state = Off AND LWT > Freeze setpoint

Low OAT Lockout is active AND No compressors running AND LWT > 70°F
T5 – Transition from Run to Start
Requires all of the following

Evaporator water flow loss for time > flow proof
Pump Selection
The pump output used will be determined by the Evap Pump Control set point. This setting
allows the following configurations:
#1 only – Pump 1 will always be used
#2 only – Pump 2 will always be used
Auto – The primary pump is the one with the least run hours, the other is used as a backup
#1 Primary – Pump 1 is used normally, with pump 2 as a backup
#2 Primary – Pump 2 is used normally, with pump 1 as a backup
Primary/Standby Pump Staging
The pump designated as primary will start first. If the evaporator state is start for a time
greater than the recirculate timeout set point and there is no flow, then the primary pump
will shut off and the standby pump will start. When the evaporator is in the run state, if
flow is lost for more than half of the flow proof set point, the primary pump will shut off
and the standby pump will start. Once the standby pump is started, the flow loss alarm logic
will apply if flow cannot be established in the evaporator start state, or if flow is lost in the
evaporator run state.
Auto Control
If auto pump control is selected, the primary/standby logic above is still used. When the
evaporator is not in the run state, the run hours of the pumps will be compared. The pump
with the least hours will be designated as the primary at this time.
Leaving Water Temperature (LWT) Reset
The Active Leaving Water variable shall be set to the current Leaving Water Temperature
(LWT) setpoint unless the unit is in COOL mode and any of the reset methods below are
selected. The type of reset in effect is determined by the LWT Reset Type setpoint. The
Active Leaving Water variable is sent from the unit controller to all circuits for capacity
control after the applicable reset is applied.
Reset Type – NONE
The Active Leaving Water variable is set equal to the current LWT setpoint.
Reset Type – RETURN
The Active Leaving Water variable is adjusted by the return water temperature.