34
IOM 1206-1
Unit Controller Operation
Freeze Protection
To protect the evaporator from freezing, the evaporator pump
will start if all of the following are true:
• LWT equal to or less than the Evap Freeze set point for
at least three seconds
• LWT sensor fault isn’t active
• manual reset flow loss alarm is not active
Freeze protection will end when any of the following are true:
• LWT is at least 1.11°C (2°F) above the Evap Freeze set
point and pump has been in run state for at least as long
as the evaporator recirculation time
• LWT sensor fault is active
• manual reset flow loss alarm is active
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 value, 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.
LWT Target
The LWT Target varies based on settings and inputs.
The base LWT Target is selected as follows:
Leaving Water Temperature (LWT) Reset
The base LWT target may be reset if the unit is in Cool mode
and LWT reset is enabled via the set point.
The reset amount is adjusted based on the 4 to 20 mA reset
input. Reset is 0° if the reset signal is less than or equal to 4
mA. Reset is 5.56°C (10.0°F) if the reset signal equals or
exceeds 20 mA. The amount of reset will vary linearly
between these extremes if the reset signal is between 4 mA and
20 mA.
When the reset amount increases, the Active LWT Target is
changed at a rate of 0.1°C every 10 seconds. When the active
reset decreases, the Active LWT Target is changed all at once.
After the reset is applied, the LWT target can never exceed a
value of 15.56°C (60°F).
Unit Capacity Control
Unit capacity control will be performed as described in this
section. All unit capacity limits described in following
sections must be applied as described.
Compressor Staging in Cool Mode
The first compressor on the unit should be started when
evaporator LWT is higher than the Startup Temperature.
Control Source
Set Point
Mode Input
BAS
Request
Available Modes
Set Point
Base LWT Target
Local
OFF
COOL
Cool Set Point 1
Local
ON
COOL
Cool Set Point 2
Network
COOL
BAS Cool Set Point
Local
OFF
COOL w/Glycol
Cool Set Point 1
Local
ON
COOL w/Glycol
Cool Set Point 2
Network
COOL w/Glycol
BAS Cool Set Point
Local
OFF
COOL/ICE w/Glycol
Cool Set Point 1
Local
ON
COOL/ICE w/Glycol
Ice Set Point
Network
COOL
COOL/ICE w/Glycol
BAS Cool Set Point
Network
ICE
COOL/ICE w/Glycol
BAS Ice Set Point
Local
ICE w/Glycol
Ice Set Point
Network
ICE w/Glycol
BAS Ice Set Point
