Type Of Sensors For Economizer - Honeywell Design and Application Technical Reference Manual

The C7400A enthalpy sensor combines
temperature and humidity measurements into
a single device. The output is 4 to 20 mA. All
sensing elements are solid state electronics
which have been determined to be durable
over more than fifteen years of usage. There
are no setpoints on the sensor since all
adjustments are done on the control
module.The C7400A sensors are used with
the legacy economizers, e.g., W7459 and
W7212.
In some dry, arid climates it may be sufficient
to use a dry bulb sensor in place of a C7400
enthalpy sensor to achieve energy savings.
The output of the C7650 sensor has large
dead bands and should only be used in
differential temperature changeover
applications where there is a C7650 sensor in
the return air and one in the outdoor air and
the economizer compares the two
temperatures for the lower temperature. Using
the A, B, C, or D settings on the economizer in
single dry bulb mode results in a 10 degree
"dead band" with the C7650 dry bulb sensor.
See chart. The economizer will allow free
cooling when the temperature is rising through
the dead band until the temperature rises
above the highest temperature on the dead
band. On temperature decline, the economizer
will not reset to OK for economize until the
outdoor temperature goes below the lower
temperature. This only happens if power is to
the economizer during the decline through the
deadband.
During the temperature decline if the
economizer power is cycled off when the
temperature is in the 10F deadband, the
economizer will determine the outdoor air
temperature is good for free cooling and will
reset the OK to economize.

Type of Sensors for Economizer

Section 12 - Sensors for Economizer Modules
C7400 Enthalpy
C7650 Temperature
mA
OUTPUT
19
18 LIGHT ON
17
LIGHT OFF
16
15
14
B SETPOINT
13
DEADBAND
12
A SETPOINT
11
DEADBAND
10
9
40 45 50
DEG. F
DEG. C
4 7 10
C7650 SETPOINT CHART
THE SETPOINT LINES ON THE CHART ARE AN INDICATION OF OUTSIDE DRY
BULB TEMPERATURES AT WHICH SWITCHING OF THE DAMPERS FROM
RETURN AIR TO OUTSIDE AIR OCCURS. IT IS INDICATED ON THE W7459
MODULE BY THE FREE COOLING LIGHT.
159
D SETPOINT
DEADBAND
LIGHT ON
C SETPOINT
DEADBAND
LIGHT OFF
LIGHT ON
LIGHT OFF
LIGHT OFF
55 60 65 70 75 80 85
13 16 18 21 24 27 29
Honeywell Economizers 63-8594-02
LIGHT ON
90 95 100
32 35 38
M25284A