Carrier 48TC*D08 User Manual

Page 81

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:

IAQ Pre--Occupancy Purge option is enabled in the

CONFIG configuration table

Unit is in the unoccupied state

Current Time is valid

Next Occupied Time is valid

Time is within 2 hours of next Occupied period

Time is within Purge Duration (user--defined 5 to 60

minutes in the CONFIG configuration table)

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:

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%.

If the OAT < NTLO then PURGEMP = LTMP (Low

Temperature Minimum Position -- defaults to 10%)

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:

NTEN option is enabled in the CONFIG configuration

table

Unit is in unoccupied state

Current time of day is valid

Temperature Compensated Start mode is not active

COOL mode is not active

HEAT mode is not active

SPT reading is available

OAT reading is available

Enthalpy is low

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