Specifications

63
The set points that are used for tempering are listed below.
• Tempering in Cool Supply Air Set Point (SASP) (T.CL) —
This is used when the unit is in cooling mode and the
economizer is at the minimum position. Tempering will add
heat if the temperature is below the set point.
• Tempering Vent OCC SASP (T.VO) — This is the tempera-
ture set point that will be used for tempering during the Vent
mode while in occupied mode.
• Tempering Vent Unocc SASP (T.VU) — This is the temper-
ature set point that will be used for tempering during the
Vent mode while in unoccupied mode.
• Tempering Purge SASP (T.PRG) — This is the temperature
set point that is used when a pre-occupancy purge is
running.
MECHANICAL COOLING CONTROL — When mechani-
cal cooling is required by the run mode selection routine, the
ComfortLink™ control has the capability to control the staging
of the compressors in several different ways. Three scroll
compressors are used on sizes 020 to 030 and four on sizes
035 to 060. In addition, the ComfortLink control supports the
use of an optional Minimum Load Hot Gas Bypass Valve
(MLV) that is directly controlled by the ComfortLink Control,
and provides an additional stage of capacity as well as low load
coil freeze protection. The control also integrates the use of an
economizer with the use of mechanical cooling to allow for the
greatest use of free cooling. When both mechanical cooling
and the economizer are being used, the control will use the
economizer to provide better temperature control and limit the
cycling of the compressors.
The control also checks on various other operation parame-
ters in the units to make sure that safeties are not exceeded and
the compressors are reliably operated.
High Saturated Condensing Temperature Override
— If,
while compressors are running, the Saturated Condensing
Temperature (SCT.A or SCT.B), is within 5 F of the maximum
condensing temperature for 15 seconds or more, then the
control will request a reduction in capacity will occur for the
circuit. Additionally, the control will allow capacity to be added
to the circuit for 10 minutes if needed.
Low Saturated Suction Temperature Override
— If, while
compressors are running, the Saturated Suction Temperature
(SST.A or SST.B), is less than 28 F for two minutes or more,
then the control will turn off a compressor on the circuit. Addi-
tionally, the control will not allow capacity to be added to that
circuit for 10 minutes. If the unit is equipped with the optional
minimum load valve in circuit A, then the control will use the
MLV valve to prevent low saturated suction conditions in
circuit A. If an economizer is being used it will also be closed
to try to prevent a low SST condition.
Mechanical Cooling Allowed
— If unoccupied free cooling
is in effect or the active economizer is open less than 98%, then
mechanical cooling is not allowed. If the economizer is open to
98% or more, then mechanical cooling will be allowed.
Low Ambient Compressor Lockout
— If the Outside Air
temperature (OAT) is below the Compressor Lockout tempera-
ture (CLO.A) then the compressors will not be allowed to
run. The Compressor Lockout temperature can be set in the
CONFIGURATION-COOL submenu.
React to Demand Limit
— If the percentage of capacity
currently running is greater than the allowed amount as
determined by the demand limiting routine then the capacity
will be limited or reduced.
Stage Control — The type of staging control will be de-
termined based on the CTL.T configuration variable that can
be found in the CONFIGURATION menu under the UNIT
submenu. There are 6 options to select from but these will only
use 2 basic sequencing routines.
• TSTAT 2STAGE — uses 2 stage control
• SPT 2STAGE — uses 2 stage control
• TSTAT MULTI — uses multi-stage control
• SPT MULTI — uses multi-stage control
• VAV RAT — uses multi-stage control
• VAV SPT — uses multi-stage control
TWO-STAGE CONTROL — When two-stage control is be-
ing used with a conventional thermostat, the compressor on-off
operation will be mapped to the Y1, Y2 input signals from
the thermostat. Staging of compressors will be as shown in
Tables 23 and 24.
If a T55, T56, or T58 sensor is being used with 2-Stage
Control, then the space temperature demand routine will be
used to establish the low and high cooling demand. The
demand will then be mapped to stage 1 and stage 2. See
Tables 23 and 24. See the HVACSPT2 Routine for more
information.
The logic will first check for the availability of the economiz-
er. If free cooling can be used, then the control will first attempt to
use the free cooling. When in free cooling mode, the
HVACSPT2 demand driven routine will always be used. If no
mechanical cooling is active, the economizer will first attempt to
control to a cooling control point of either the supply air set point
high (SA.HI) or supply air set point low (SA.LO) depending on
whether high cool or low cool mode is in effect. The mode is
determined by the status of the Y1 and Y2 inputs for thermostat
equipped units or the HVACSPT routine for space sensor
equipped units. If the set point cannot be satisfied, then cooling
will be brought on one circuit at a time when the supply air tem-
perature is greater the 1.5 F above the current cooling control
point. A start-up time delay of 10 minutes and steady state delay
after a compressor is energized of 5 minutes is enforced.
If one circuit of mechanical cooling is on, then the econo-
mizer will be attempt to control to 53 F. If low cool mode is in
effect, the second circuit of mechanical cooling will be locked
off. If both circuits of mechanical cooling are running, then the
economizer will attempt to control to 48 F.
If the economizer is active and the outside air temperature
(OAT) is less than the cooling control point + 0.5 F, the com-
pressors will be locked off.
When mechanical cooling is on, the control will also use the
economizer to trim the leaving-air temperature to prevent un-
necessary cycles of the compressor stages.
MULTI-STAGE CONTROL — Multiple stage control types
(CTL.T= SPT MULTI, TSTAT, MULTI, VAV RAT AND VAV
SPT) will use Carrier’s patented temperature control algorithm,
based on leaving-air temperature with mixed air temperature
compensation to determine when to stage up or down. See
Tables 25-28.
The unit has an Outdoor Air Temperature sensor (OAT),
Return Air Temperature sensor (RAT) and Evaporator
Discharge Air Temperature sensor (EDT). These can be read
using the TEMPERATURE-UNIT submenu. Because the
control is sent the position of the economizer and the above
three temperatures, the control can calibrate the economizer
damper and the actual amount of outside air and return air at
any given position. Over a few days of operation the control
will develop a table of data that allows for calculation of the
Mixed Air Temperature (MAT). This can be configured using
the Estimate MAT? (MAT.S) setting. If there is a need to reset
the table, the Reset MAT Table (R.MAT) command can be
used. Both variables are located in the CONFIGURATION-
UNIT submenu. This calculated MAT will then be used by the
staging multi-stage routine to estimate the amount of tempera-
ture change when a stage is added or removed.
STAGING ALGORITHM — The control will use the
Carrier Temperature Control Algorithm. The algorithm has
self-adjusting gains that (aside from an overall system gain)
eliminate the need for user gains for each of the proportional,
integral and differential terms. The algorithm attempts to