9
CONTROL SETUP
General
The 35E single-duct VAV (variable air volume) terminal is
designed to supply a varying quantity of cold primary air to a
space in response to a thermostat demand. Some units have
reheat options to provide heating demand requirements as
well. Most VAV terminals are equipped with pressure com-
pensating controls to regulate the response to the thermostat
independent of the pressure in the supply ductwork.
To balance the unit, it is necessary to set both the maximum
and minimum set points of the controller. The many types of
control options available each have specific procedures re-
quired for balancing the unit.
Set Points
Maximum and minimum airflow set points are normally
specified for the job and specific for each unit on the job.
Where maximum and minimum airflow levels are not speci-
fied on the order, default values are noted on unit ID label.
Field Adjustment of Minimum and Maximum
Airflow Set Points
Each 35E unit is equipped with a flow probe that measures a
differential pressure proportional to the airflow. The relation-
ship between flow probe pressures and cfm is shown in the
Linear Averaging Probe Chart (Fig. 7). This chart is attached
to each unit. There are several conventions (and no universal-
ly accepted method) in use for representing this flow factor:
MAGNIFICATION FACTOR
The magnification factor may be expressed as the ratio of ei-
ther velocity or pressure of the output of the sensor to that of
a pilot tube.
For example, a velocity magnification may be used. All
probes develop an average signal of 1-in. wg at 2625 fpm.
This gives a velocity magnification of 4005 / 2625 or 1.52.
It may be a pressure magnification factor. In this case, the ra-
tio of pressures at a given air velocity is presented. For a ve-
locity constant of 2626, at 1000 fpm, this is 0.1451 / 0.0623 =
2.33.
K-FACTOR
The K-factor may be represented in two ways:
It may be a velocity K-factor, which is the velocity factor
times the inlet area, (which for all probes is 2625 fpm/in. wg).
Alternatively, it may be the airflow K-Factor, which is the ve-
locity factor times the inlet area. For an 8-inch unit, therefore,
this would be 2625 * 0.349, or 916. A separate airflow factor
is required for each size. All Carrier VAV terminals have
round inlets. Table 5 is a K-Factor table for all Carrier VAV
terminal inlets.
System Calibration of the Linear Averaging
Flow Probe
To achieve accurate pressure independent operation, the ve-
locity sensor and flow probe must be calibrated to the con-
troller. This will ensure that airflow measurements will be ac-
curate 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 giv-
en unit (design air volume in cfm), at a pressure of 1.0-in. wg
differential pressure, by the standard pitot tube coefficient of
4005. This ratio is the same for all sizes regardless of which
probe is used.
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.
Carrier inlet areas are shown in the table below. The design
air volume is also shown in this table. It can be determined
from this table that the average design air velocity for 35E
units is equal to 2656 fpm at 1.0-in. wg.
Record the information on a performance sheet (see Fig. 8). This
will provide a permanent record of the balancing information.
• installation location information
•boxsize
• cooling minimum airflow (cfm) limit
• cooling maximum airflow (cfm) limit
• reheat (cfm) limit (if applicable)
• heating minimum airflow (cfm) limit (if applicable)
• heating maximum airflow (cfm) limit (if applicable)
• calibration gain (after balancing)
• set points
Table 5 — Inlet Probe Area and K Factor
UNIT SIZE 35E 04 05 06 07 0809 10 12 14 16 22
INLET DIAMETER 4.0 5.0 6.0 7.0 8.0 9.0 10.0 12.0 14.0 16.0 22.0
VELOCITY MAGNIFICATION 1.52 1.52 1.52 1.52 1.52 1.52 1.52 1.52 1.52 1.52 1.52
VELOCITY CONSTANT 2625 2625 2625 2625 2625 2625 2625 2625 2625 2625 2625
CFM K FACTOR 229 358 515 702 916 1160 1432 2062 2806 3665 7000
INLET AREA (SQ FT) 0.087 0.136 0.196 0.267 0.349 0.442 0.545 0.785 1.069 1.396 2.667
RECOMMENDED MIN CFM 40 62 89 122 159 201 248 357 486 635 1212
