McQuay AGS 225D, Operating Manual

Page: 31 / 82

OM AGSD-1 31
Pump Selection
The pump output used will be determined by the Evap Pump Control setpoint. This setting
allows the following configurations:
#1 only – Pump 1 will always be used
#2 only – Pump 2 will always be used
Auto – The pump used will be the one with the least run hours
Auto Pump Control
Auto control compares the run hours of each pump before there is a need to start a pump,
and the pump with the least hours will be designated as the one to start. The selected pump
output will be ON if the Evap State is set to START or RUN. Both outputs will be OFF if
the Evap State is set to OFF.
Leaving Water Temperature (LWT) Reset
The Active Leaving Water setpoint is set to the current Leaving Water Temperature (LWT)
setpoint, unless the unit is in COOL or COOL w/Glycol 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 to all circuits for capacity control after
the applicable reset is applied. Reset is not available in the ICE mode.
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.
Start Reset Delta T
LWT set Point+Max Reset
(54)
LWT Set Point
(44)
Return Reset
0
Max Reset
(10)
Evap Delta T (
o
F)
Active
LWT
(
o
F)
The active setpoint is reset using the following parameters:
1.
Cool LWT setpoint
2. Max Reset setpoint
3. Start Reset Delta T setpoint
4. Evap Delta T
Reset is accomplished by changing the Active Leaving Water variable from the Cool LWT
setpoint to the Cool LWT set-point + Max Reset setpoint as the Evaporator EWT – LWT
(Evap delta t) varies from the Start Reset Delta T set-point to 0.