Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 54

Where:
Z.GN = configuration used to modify the threshold levels used
for staging (Configuration  COOL  Z.GN)
ADD.R = R.PCT * (C.CAP – capacity after adding a cooling
stage)
SUB.R = R.PCT * (C.CAP – capacity after subtracting a cool-
ing stage)
Both of these terms, Z.PLU and Z.MIN, represent a threshold
both positive and negative, upon which the "SUM" calculation
must build up to in order to cause the compressor to stage up or
down.
Comparing SUM and Z — The "SUM" calculation is com-
pared against Z.PLU and Z.MIN.
• If "SUM" rises above Z.PLU, a cooling stage is added.
• If "SUM" falls below Z.MIN, a cooling stage is subtracted.
There is a variable called SMZ that can simplify the task of
watching the demand build up or down over time. It is calculat-
ed as follows:
If SUM is positive: SMZ = 100*(SUM/Z.PLU)
If SUM is negative: SMZ = 100*(SUM/Z.MIN)
Mixed Air Temperature Calculation (MAT)
The mixed-air temperature is calculated and is a function of the
economizer position. Additionally, there are some calculations
in the control which can zero in over time on the relationship of
return and outside air as a function of economizer position.
There are 2 configurations which relate to the calculation of
"MAT." These configurations can be located at the local dis-
play under Configuration  UNIT.
ITEM EXPANSION
UNIT UNIT CONFIGURATION
MAT.S MAT Calc Config
MAT.R Reset MAT Table
Entries?
MAT Calc Config (MAT.S) — This configuration gives the
user 3 options in the processing of the mixed-air temperature
(MAT) calculation:
• MAT.S = 0
There will be no MAT calculation.
• MAT.S = 1
The control will attempt to learn MAT over time. Any time
the system is in a vent mode and the economizer stays at a
particular position for long enough, MAT = EDT. Using
this, the control has an internal table whereby it can more
closely determine the true MAT value.
• MAT.S = 2
The control will stop learning and use whatever the control
has already learned. Using this setting infers that the con-
trol has spent some time set to MAT.S = 1.
First set MAT.S = 1. Then go into the Service Test mode, turn
on the fan, and open the economizer to a static position for 5
minutes. Move to several positions (20%,40%,60%,80%). It is
important that the difference between return and outside tem-
perature be greater than 5 degrees. (The greater the delta, the
better). When done, set MAT.S = 2, and the system has been
commissioned.
Reset MAT Table Entries? (MAT.R) — This configuration al-
lows the user to reset the internally stored MAT learned config-
uration data back to the default values. The defaults are set to a
linear relationship between the economizer damper position
and OAT and RAT in the calculation of MAT.
SumZ Overrides
There are a number of overrides to the SumZ algorithm which
may add or subtract stages of cooling.
CCN
RANGE DEFAULTS
POINT
0 to 2 MAT_SEL 1
No/Yes MATRESET No
• High Temp Cap Override (H.TMP)
• Low Temp Cap Override (L.TMP)
• Pull Down Cap Override (PULL)
• Slow Change Cap Override (SLOW)
Economizer Trim Override
The unit may drop stages of cooling when the economizer is per-
forming free cooling and the configuration Configura-
tion  ECON  E.TRM is set to Yes. The economizer controls to
the same supply air set point as mechanical cooling does for SumZ
when E.TRM = Yes. This allows for much tighter temperature
control as well as cutting down on the cycling of compressors.
For a long cooling session where the outside-air temperature
may drop over time, there may be a point at which the econo-
mizer has closed down far enough where the unit could remove
a cooling stage and open up the economizer further to make up
the difference.
Mechanical Cooling Lockout (Configuration  COOL 
MC.LO)
This configuration allows a configurable outside-air tempera-
ture set point below which mechanical cooling will be com-
pletely locked out.
DEMAND LIMIT CONTROL
Demand Limit Control may override the cooling algorithm and
clamp or shed cooling capacity during run time. The term De-
mand Limit Control refers to the restriction of the machine ca-
pacity to control the amount of power that a machine will use.
Demand limit control is intended to interface with an external
Loadshed Device either through CCN communications, exter-
nal switches, or 4 to 20 mA input.
The control has the capability of loadshedding and limiting in 3
ways:
• Two discrete inputs tied to configurable demand limit set
point percentages.
• An external 4 to 20 mA input that can reset capacity back
linearly to a set point percentage.
• CCN loadshed functionality.
NOTE: It is also possible to force the demand limit variable
(Run Status  COOL  DEM.L).
To use Demand Limiting, select the type of demand limiting to
use. This is done with the Demand Limit Select configuration
(Configuration  BP  DMD.L  DM.L.S).
To view the current demand limiting currently in effect, look at
Run Status  COOL  DEM.L.
The configurations associated with demand limiting can be
viewed at the local display at Configuration  BP  DMD.L.
See Table 33.
Demand Limit Select (DM.L.S)
This configuration determines the type of demand limiting.
• 0 = NONE — Demand Limiting not configured.
• 1 = 2 SWITCHES — This will enable switch input de-
mand limiting using the switch inputs connected to the
CEM board. Connections should be made to TB202 termi-
nals 1, 2, 3, and 4.
• 2 = 4 to 20 mA — This will enable the use of a remote 4 to
20 mA demand limit signal. The CEM module must be
used. The 4 to 20 mA signal must come from an externally
sourced controller and should be connected to TB202 ter-
minals 10 and 11.
• 3 = CCN LOADSHED — This will allow for loadshed and
red lining through CCN communications.
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