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MINUTES LEFT FOR START — This value is displayed
only in the network display tables (using Service Tool,
ComfortVIEW™ or ComfortWORKS® software) and
represents the amount of time to elapse before the unit will start
its initialization routine. This value can be zero without the
machine running in many situations. This can include being
unoccupied, ENABLE/OFF/REMOTE CONTACT switch in
the OFF position, SCN not allowing unit to start, Demand
Limit in effect, no call for cooling due to no load, and alarm or
alert conditions present. If the machine should be running and
none of the above are true, a minimum off time (DELY, see
below) may be in effect. The machine should start normally
once the time limit has expired.
MINUTES OFF TIME (DELY) [Configuration OPT2] —
This user-configurable time period is used by the control to
determine how long unit operation is delayed after power is
applied/restored to the unit. Typically, this time period is con-
figured when multiple machines are located on a single site.
For example, this gives the user the ability to prevent all the
units from restarting at once after a power failure. A value of
zero for this variable does not mean that the unit should be
running.
LEAD/LAG DETERMINATION — This is a configurable
choice and is factory set to be automatic for all units. The value
can be changed to Circuit A or Circuit B leading as desired. Set
at automatic, the control will sum the current number of logged
circuit starts and one-quarter of the current operating hours for
each circuit. The circuit with the lowest sum is started first.
Changes to which circuit is the lead circuit and which is the lag
are also made when total machine capacity is at 100% or when
there is a change in the direction of capacity (increase or
decrease) and each circuit’s capacity is equal.
CAPACITY CONTROL OVERRIDES — The following over-
rides will modify the normal operation of the routine.
Deadband Multiplier
— The user configurable Deadband
Multiplier (Z.GN) [Configuration, SLCT] has a default value
of 1.0. The range is from 1.0 to 4.0. When set to other than 1.0,
this factor is applied to the capacity Load/Unload Factor. The
larger this value is set, the longer the control will delay between
adding or removing stages of capacity. Figure 12 shows how
compressor starts can be reduced over time if the leaving water
temperature is allowed to drift a larger amount above and be-
low the set point. This value should be set in the range of 3.0 to
4.0 for systems with small loop volumes.
First Stage Override
— If the current capacity stage is zero,
the control will modify the routine with a 1.2 factor on adding
the first stage to reduce cycling. This factor is also applied
when the control is attempting to remove the last stage of
capacity.
Slow Change Override
— The control prevents the capacity
stages from being changed when the leaving fluid temperature
is close to the set point (within an adjustable deadband) and
moving towards the set point.
Ramp Loading
(CRMP) [Configuration, SLCT] — Limits the
rate of change of leaving fluid temperature. If the unit is in a
Cooling mode and configured for Ramp Loading, the control
makes 2 comparisons before deciding to change stages of ca-
pacity. The control calculates a temperature difference between
the control point and leaving fluid temperature. If the differ-
ence is greater than 4 °F (2.2 °C) and the rate of change (°F or
°C per minute) is more than the configured Cooling Ramp
Loading value (CRMP), the control does not allow any chang-
es to the current stage of capacity.
Low Entering Fluid Temperature Unloading
— When the
entering fluid temperature is below the control point, the
control will attempt to remove 25% of the current stages being
used. If exactly 25% cannot be removed, the control removes
an amount greater than 25% but no more than necessary. The
lowest stage will not be removed.
Minimum Load Control
— If equipped, the minimum load
control valve is energized only when one compressor in the
circuit is running. If the close control feature is enabled the
minimum load control valve may be used as needed to obtain
leaving fluid temperature close to set point.
Cooler Freeze Protection
— The control will try to prevent
shutting the chiller down on a Cooler Freeze Protection alarm
by removing stages of capacity. If the cooler fluid selected
is Water, the freeze point is 34 F (1.1 C). If the cooler fluid
selected is Brine, the freeze point is the Brine freeze Point
(BR.FZ) [Set Points, FRZ]. This alarm condition (A207) only
references leaving fluid temperature and NOT Brine Freeze
point. If the cooler leaving fluid temperature is less than the
freeze point plus 2.0° F (1.1° C), the control will immediately
remove one stage of capacity. This can be repeated once every
30 seconds.
Low Saturated Suction Protection
— The control will try to
prevent shutting a circuit down due to low saturated suction
conditions by removing stages of capacity. These circuit alert
conditions (T116, T117) compare saturated suction tempera-
ture to the configured Brine Freeze point (BR.FZ) [Set Points,
FRZ]. The Brine Freeze point is a user-configurable value that
must be left at 34 F (1.1 C) for 100% water systems. A lower
value may be entered for systems with brine solutions, but this
value should be set according to the freeze protection level of
the brine mixture. Failure to properly set this brine freeze point
value may permanently damage the brazed plate heat exchang-
er. The control will initiate Mode 7 (Circuit A) or Mode 8 (Cir-
cuit B) to indicate a circuit’s capacity is limited and that even-
tually the circuit may shut down.
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0 200 400 600 800 1000
TIME (SECONDS)
2 STARTS
3 STARTS
DEADBAND EXAMPLE
LWT (F)
MODIFIED
DEADBAND
STANDARD
DEADBAND
8
7
6
5
LWT (C)
LEGEND
LWT —
Leaving Water Temperature
Fig. 12 — Deadband Multiplier