Carrier 48TC*D08 - Manual (Page 81)

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There is also a SAT tempering routine that will act as SAT
low limit safety to prevent the SAT from becoming too
cold should the economizer fail to close. One stage of
heating will be energized if it is not in the Cooling or Free
Cooling mode and the OAT is below 55_F and the SAT is
below 40_F. It will deenergize when the SAT > (SPT +
10_F).
Indoor Air Quality — If the optional indoor air quality
(IAQI) sensor is installed, the PremierLink controller will
maintain indoor air quality within the space at the user
configured differential setpoint (IAQD) in the CONFIG
configuration table. The setpoint is the difference between
the IAQI and an optional outdoor air quality sensor
(OAQ). If the OAQ is not present then a fixed value of
400 ppm is used. The actual space IAQ setpoint (IAQS) is
calculated as follows:
IAQS = IAQD + OAQ (OAQ = 400 ppm if not present)
As air quality within the space changes, the minimum
position of the economizer damper will be changed also
thus allowing more or less outdoor air into the space
depending on the relationship of the IAQI to the IAQS.
The IAQ algorithm runs every 30 seconds and calculates
IAQ minimum position value using a PID loop on the
IAQI deviation from the IAQS. The IAQ minimum
position is then compared against the user configured
minimum position (MDP) and the greatest value becomes
the final minimum damper position (IQMP). If the
calculated IAQ minimum position is greater than the IAQ
maximum damper position (IAQMAXP) decision in the
SERVICE configuration table, then it will be clamped to
IAQMAXP value.
If IAQ is configured for low priority, the positioning of
the economizer damper can be overridden by comfort
requirements. If the SPT > OCSP + 2.5 or the SPT <
OHSP -- 2.5 then IAQ minimum position becomes 0 and
the IQMP = MDP. The IAQ mode will resume when the
SPT ≤ OCSP + 1.0 and SPT ≥ OHSP -- 1.0.
If IAQ is configured for high priority and the OAT < 55_F
and the SAT < (SPT -- 10_F), the algorithm will enable the
heat stages to maintain the SAT between the SPT and the
SPT + 10_F.
IAQ Pre--Occupancy Purge — This function is designed to
purge the space of airborne contaminants that may have
accumulated 2 hours prior to the beginning of the next
occupied period. The maximum damper position that will
be used is temperature compensated for cold whether
conditions and can be pre--empted by Temperature
Compensated Start function. For pre--occupancy to occur,
the following conditions must be met:
S IAQ Pre--Occupancy Purge option is enabled in the
CONFIG configuration table
S Unit is in the unoccupied state
S Current Time is valid
S Next Occupied Time is valid
S Time is within 2 hours of next Occupied period
S Time is within Purge Duration (user--defined 5 to 60
minutes in the CONFIG configuration table)
S OAT Reading is available
If all of the above conditions are met, the economizer
damper IQMP is temporarily overridden by the
pre--occupancy damper position (PURGEMP). The
PURGEMP will be set to one of the following conditions
based on atmospheric conditions and the space
temperature:
S If the OAT ≥ NTLO (Unoccupied OAT Lockout
Temperature) and OAT < 65_F and OAT is less than or
equal to OCSP and Enthalpy = Low then PURGEMP =
100%.
S If the OAT < NTLO then PURGEMP = LTMP (Low
Temperature Minimum Position -- defaults to 10%)
S If the OAT > 65_F or (OAT ≥ NTLO and OAT > OCSP)
or Enthalpy = High then PURGEMP = HTMP (High
Temperature Minimum Position defaults to 35%).
The LTMP and HTMP are user adjustable values from 0
to 100% in the SETPOINT table. Whenever PURGEMP
results in a number greater than 0%, the IAQ
pre--occupancy purge mode will be enabled turning on the
Indoor Fan Relay and setting the economizer IQMP to the
PURGEMP value. When IAQ pre--occupancy mode is not
active PURGEMP = 0%.
Unoccupied Free Cooling — Unoccupied free cool
function will start the indoor fan during unoccupied times
in order to cool the space with outside air. This function is
performed to delay the need for mechanical cooling when
the system enters the occupied period. Depending on how
Unoccupied Free Cooling is configured, unoccupied mode
can occur at any time in the unoccupied time period or 2
to 6 hours prior to the next occupied time. Once the space
has been sufficiently cooled during this cycle, the fan will
be stopped. In order to perform unoccupied free cooling
all of the following conditions must be met:
S NTEN option is enabled in the CONFIG configuration
table
S Unit is in unoccupied state
S Current time of day is valid
S Temperature Compensated Start mode is not active
S COOL mode is not active
S HEAT mode is not active
S SPT reading is available
S OAT reading is available
S Enthalpy is low
S OAT > NTLO (with 1_F hysteresis) and < Max Free Cool
setpoint
If any of the above conditions are not met, Unoccupied
Free Cool mode will be stopped, otherwise, the mode will
be controlled as follows:
The NTFC setpoint (NTSP) is determined as NTSP =
(OCSP + OHSP) / 2
The Unoccupied Free Cool mode will be started when:
SPT > (NTSP + 2_F) and SPT > (OAT + 8_F)
The Unoccupied Free Cool mode will be stopped when:
SPT < NTSP or SPT < (OAT + 3_F)
48TC