Dry Bulb Changeover - Honeywell Design and Application Technical Reference Manual

Section 9 - W7212, W7213 and W7214 Economizer Modules
5. Connect 24 Vac across terminals TR and
TR1.
6. Carefully adjust the potentiometer on the
face of the device with a small
screwdriver until the mixed air
temperature reaches the calculated
value.
NOTE: Ensure that the sensed air is well
mixed.
7. If all minimum and maximum position
adjustments are complete, remove the
T-T1 jumper and reconnect the mixed air
sensor.
Formula to aid minimum position
adjustment
(T × OA) + (T × RA)= T
O R
Where:
T = Outdoor air temperature
O
OA = Percent of outdoor air
T = Return air temperature
R
RA = Percent of return air
T = Resulting mixed air temperature
M
NOTE: The following sample calculation uses
only Fahrenheit temperature.
63-8594-02 Honeywell Economizers
M
86
EXAMPLE: Assume local codes require 10%
outdoor air during occupied conditions,
outdoor air is 60°F and return air is 75°F.
Under these conditions, what is the
temperature of the mixed air?
(0.1 × 60°F) + (0.9 × 75°F) =
6.0°F + 67.5°F = 73.5°F
Mixed air will be 73.5°F when OA is 60°F and
RA is 75°F with 10% outdoor air.

Dry bulb changeover

Single or differential economizing can be
accomplished using temperature instead of
enthalpy. With any of the analog economizers
you need to use a C7660 dry bulb sensor with
a 4mA or 20 mA output to the economizers for
single (referential) changeover. The sensor
determines if the outdoor air is above or below
the selected setpoint and either sends a 4 mA
signal (NOT OK to economize) or a 20mA
signal (OK to economize) to the logic module.
For differential changeover you need to use a
C7650 sensor in the outdoor air and a C7650
sensor in the return air. CAUTION: You cannot
use a C7760 sensor in the outdoor air and one
in the return air for differential control, the
system will not work.
See the sensor section of this manual for
additional explanation of the sensor operation.