Carrier 48TCA04---A12 User Manual

Page 75

When in the occupied mode and the indoor relative

humidity is greater then the Occupied High Humidity

setpoint, then the H3_EX_RV output point will be

energized. When in the unoccupied mode and indoor

relative humidity is greater then the Unoccupied High

Humidity setpoint, then the H3_EX_RV output point and

supply fan output will be energized. There is a fixed 5%

hysteresis that the indoor relative humidity must drop

below the active setpoint to end the dehumidification

mode and deenergize the H3_EX_RV output. If the

PremierLink controller is in the unoccupied mode, then

the fan relay will deenergize if there is no other mode

requiring to the fan to be on. This function will not

energize mechanical cooling as a result of the indoor

relative humidity exceeding either setpoint.

A high humidity alarm will be generated if the indoor

relative humidity exceeds the high humidity setpoint by

the amount configured in the Control Humidity Hysteresis

in the ALARMS table for 20 minutes. The alarm will

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 CO

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:

Indoor fan has been on for at least 30 seconds

Enthalpy is low

SAT reading is available

OAT reading is available

SPT reading is available

OAT ≤ SPT

OAT < OATMAX (OATMAX default is 75_F)

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:

Indoor fan has been ON for at least 30 seconds.

Cool mode is not active and the time guard between

modes equals zero.

Mode is occupied or the Temperature Compensated Start

or Heat mode is active.

48TC