Step 4 - Set Up System And Calibrate - Carrier 45J Installation, Start-Up And Service Instructions Manual

LEGEND
AFS — Airflow Switch
CAP — Capacitor
SCR — Silicone Control Rectifier
Fig. 8 — Typical Power Connections for Fan Powered Units with 3-Stage Electric Heat
Step 4 — Set Up System and Calibrate
GENERAL
The parallel fan powered terminals (45M, 45N, and 45R) are
designed to provide varying quantities of cold primary air to a
space in response to a thermostat demand for cooling. For a
heating demand, the fan will operate to supply ceiling plenum
air to the space. For units equipped with a heating coil, the heater
will operate as required to meet a heating demand.
The series fan powered terminals (45J, 45K, and 45Q) are de-
signed to provide a constant airflow to the space. The air sup-
plied to the space is a mixture of primary air and ceiling plenum
air. The fan speed is adjusted to provide the required airflow to
the space. In response to a cooling demand from a thermostat,
the damper will increase the amount of cold primary air while re-
ducing the amount of ceiling plenum air to decrease the tempera-
ture of the air being delivered to the space.
Most terminal control packages provide pressure compensa-
tion to allow pressure independent operation of the primary air
damper, regardless of changes to the available static pressure in
the supply ductwork. To balance the unit it is necessary to set
both the minimum and maximum airflow set points of the con-
troller. The many types of control options available each have
specific procedures required for balancing. Refer to the submittal
information for these requirements.
SET POINTS
Maximum and minimum airflow set points are normally
specified for the job and specific for each unit on the job. Default
set point values are provided by the factory and can be reset to
the specific requirements in the field. The fan speed must be
field adjusted after all discharge ductwork and diffusers have
been installed.
Field Adjustment of the Maximum and Minimum Airflow
Set Points
Each fan powered terminals unit is equipped with a flow
probe installed in the primary air inlet which measures a differ-
ential pressure. The relationship between the airflow probe pres-
sure and the corresponding airflow is shown in the Flow Probe
Graph. See Fig. 9. This chart is attached to each unit.
SYSTEM CALIBRATION OF THE INLET AIRFLOW
SENSOR
To achieve efficient pressure independent operation, the ve-
locity sensor and linear averaging flow probe must be calibrated
to the controller. This will ensure that airflow will be accurate for
all terminals at system start-up.
System calibration is accomplished by calculating a flow co-
efficient that adjusts the pressure fpm characteristics. The flow
coefficient is determined by dividing the flow for a given unit
(design air volume in cfm), at a different velocity pressure of 1.0
in. wg, by the standard pitot tube coefficient of 4005. This ratio
is the same for all sizes, if the standard averaging probe is used.
Carrier inlet areas are shown in Table 8. The design air vol-
ume is shown. It can be determined from Table 7 that the aver-
age design air velocity for units is equal to 0.2660 fpm at 1.0-in.
wg.
Determine the design air velocity by dividing the design air
volume (the flow at 1.0 in. wg) by the nominal inlet area (sq ft).
This factor is the K factor.
7