Carrier 50TCA04-A07 - Manual (Page 46)

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return to normal when the indoor relative humidity drops
3% below the active humidity setpoint.
Economizer — The economizer dampers are used to
provide free cooling and indoor air quality if optional
CO2 sensor is installed and when the outside conditions
are suitable. Temperature control is accomplished by
controlling the SAT to a certain level determined by the
Economizer PID Loop by calculating a submaster
reference (ECONSR) value. This algorithm will calculate
the submaster reference temperature (ECONSR) based on
OAT and enthalpy conditions and cooling requirements.
The ECONSR value is then passed to the Economizer
Submaster Loop, which will modulate dampers to
maintain SAT at ECONSR level.
The following conditions are required to determine if
economizer cooling is possible:
S Indoor fan has been on for at least 30 seconds
S Enthalpy is low
S SAT reading is available
S OAT reading is available
S SPT reading is available
S OAT ≤ SPT
S OAT < OATMAX (OATMAX default is 75_F)
S Economizer position is NOT forced
If any of the above conditions are not met, the ECONSR
will be set to its MAX limit of 120_F and the damper will
go to its configured minimum position. The minimum
damper position can be overridden by the IAQ routine
described later in this section.
The calculation for ECONSR is as follows:
ECONSR = PID function on (setpoint -- SPT), where:
setpoint = ((OCSP+STO) + (OHSP+STO))/2 when
NTLO (Unoccupied Free Cool OAT Lockout) < OAT <
68_F
setpoint = (OCSP+STO) -- 1 when OAT ≤ NTLO
setpoint = (OHSP+STO) + 1 when OAT ≥ 68_F
The actual damper position (ECONPOS) is the result of
the following calculation. Values represented in the right
side of the equation can be found in the SERVICE
configuration table descriptions in this manual. Note that
that the OAT is taken into consideration to avoid large
changes in damper position when the OAT is cold:
ECONPOS = SubGain x (ECONSR--SAT) + CTRVAL
where SubGain = (OAT -- TEMPBAND) / (ESG + 1)
If the OAT < DXLOCK (DX Cool Lockout setpoint) then
the damper will be modulated to maintain the SAT at the
ECONSR value.
If the OAT is between DXLOCK and 68_F (DXLOCK <
OAT < 68_F) and additional cooling is required, the
economizer will close the to minimum position for three
minutes, the economizer integrator will then be reset to 0
and begin modulating to maintain the SASP after the stage
has been energized for about 90 seconds. This will allow
the economizer to calculate a new ECONSR that takes
into account the cooling effect that has just been turned on
and not return to the value require before the cooling was
added. This will prevent the economizer from causing
premature off cycles of compressors while maintaining the
low SAT temperature setpoint for the number of stages
active. In addition to preventing compressor short cycling,
by using return air across the evaporator coil just after the
compressor has started allows for increased refrigerant
flow rates providing for better oil return of any oil washed
out during compressor start--up.
If the OAT > 68_F and OAT < SPT and the number of DX
stages requested is > 0 by the staging algorithm, then
ECONSR is set to its minimum value 48_F and the
damper will go to 100% open.
If the Auxiliary Relay is configured for exhaust fan
(AUXOUT = 1) in the CONFIG configuration table and
Continuous Power Exhaust (MODPE) is Enable in the
SERVICE configuration table, then the AUXO output
(HS3) will be energized whenever the PremierLink
controller is in the occupied mode. If the MODPE is
disabled then AUXO output will be energized based on
the Power Exhaust Setpoint (PES) in the SETPOINT
table.
Heating — The heat stages are controlled by the Heating
Control Loop, which is used to calculate the desired SAT
needed to satisfy the space. It will compare the SPT to the
Occupied Heat Setpoint (OHSP) + the T56 slider offset
(STO) when occupied and the Unoccupied Heat Setpoint
(UHSP -- Unoccupied Heating Deadband) if unoccupied to
calculate a Staged Heat Submaster Reference (SHSR).
The heat staging algorithm compares the SHSR to the
actual SAT to calculate the required number of heating
stages to satisfy the load. This loop runs every 40 seconds.
The following conditions must be met in order for this
algorithm to run:
S Indoor fan has been ON for at least 30 seconds.
S Cool mode is not active and the time guard between
modes equals zero.
S Mode is occupied or the Temperature Compensated Start
or Heat mode is active.
S SPT reading is available and < (OHSP + STO).
S If it is unoccupied and the SPT < (UHSP -- Unoccupied
Heating Deadband). The indoor fan will be turn on by the
staging algorithm.
When all of the above conditions are met, the SHSR is
calculated and up to 3 stages of heat will turned on and off
to satisfy to maintain the SAT = SHSR. If any of the
above conditions are not met, the SHSR is set to its
minimum value of 35_F.
The Staged Heat Submaster Reference (SHSR) is
calculated as follows:
SHSR = Heating PID function on (error) where error =
(OHSP + STO) -- Space Temperature
The Maximum SHSR is determined by the SATHI
configuration. If the supply--air temperature exceeds the
SATHI configuration value, then the heat stages will turn
50TC