iWorx® PSU1
505-012, Effective: August 10, 2012 15
© 2012 Taco Electronic Solutions, Inc.
Four possible operational modes are provided. The first is Single mode where a lead pump is manually selected via the
LCI. If the lead pump does not provide the required flow as determined by the flow/pressure switch input, then an alarm
is generated. Depending on the Lead Pump Sel setting and pump runtimes, the lag pump may also be turned on. The
second mode, Standby mode, is similar to Single mode with the exception that the lag pump is always started when the
lead pump fails to provide the required flow. The third mode, Equal Runtime Mode, allows for standby operation plus
equal runtime rotation of the two pumps based on a runtime setting on the LCI.
These three modes (Single, Standby and Equal Runtime) also allow the option for Delta T temperature modulation via
digital pump enable and modulated pump output signals. The fourth mode -- 2 Stage mode -- allows for standby opera-
tion, plus provides two-stage operation (digital pump control only) of the pumps based on the Delta T of the primary
loop.
NOTE: The Delta T displayed and used for sequencing algorithms is always the absolute value of the Sup-
ply - Return Temperature. The controller is indifferent to the sign of the equation result. This behavior pro-
motes automatic changeover between heating and cooling systems at the point where Delta T is 0 °F.
SEQUENCE OF OPERATION
This section describes the detailed sequence of operation for the PSU1 control strategy.
Operational Modes
The four primary modes of operation for sequencing primary pumps are described below.
Table 1: Operational Mode Features
Single Mode
If only one pump is used, or if one pump is unavailable due to servicing, then this mode may be used to disable standby
failover. Whenever the lead primary pump is turned on by the controller and the Enable Alarm setting is not “Disabled”,
the Pump Flow Proof is monitored. If the Pump Flow Proof switch remains open after a configurable delay, the lead
pump is turned off and disabled, the In Alarm? indicator displays “Yes” and an alarm for the lead pump is communi-
cated to the LCI. If the Lead Pump Sel setting is set to “Lowest Hours” and the lag pump has a lower runtime, or if the
Lead Pump Sel setting is set to “Alternating,” then the lag pump is started. If purely single pump operation is desired,
then Operation Mode must be set to “Single” and the Lead Pump Sel setting must be set to either “Pump1” or “Pump2”.
Operational
Mode Setting
Lead Pump Sel
Setting
Standby Failover?
Pump Alternation
During Operation?
Delta T Pump
Staging?
Delta T Modulated
Outputs?
Single Pump1, Pump2 No No No Yes (Optional)
Lowest Hours Yes, if lag hours >=
lead
Alternating Yes
Standby Any Yes No No Yes (Optional)
Equal Runtime Any Yes Yes No Yes (Optional)
2 Stage Any Yes No Yes No
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