Sequence Of Operation - Honeywell C7400 Manual

1 MINIMUM DAMPER POSITION ADJUSTMENT
IS PRESENT ONLY ON W7459A,D MODELS.
Fig. 25. Location of enthalpy setpoint potentiometer,
minimum position potentiometer and LED.
Differential Enthalpy Changeover
Setting
(USE THIS OPTION ONLY WITH TWO-STAGE COOLING
THERMOSTATS.)
Differential enthalpy control uses two C7400A Enthalpy
Sensors connected to one W7459 Solid State Economizer
Logic Module. Differential dry bulb is not an option for these
economizers.
The setpoint scale markings, located on the W7459, are
A,B,C and D. Turn the setpoint potentiometer fully clockwise
to the D setting. The economizer selects the air that has lower
enthalpy or temperature for cooling; for example, if outdoor air
has lower enthalpy or temperature than return air, the outdoor
air damper is opened to bring in outdoor air for free cooling.
Remote Minimum Position Control
Remote control of outdoor air dampers is desirable when
requiring temporary additional ventilation. The addition of a
S963B1128 Remote Potentiometer allows occupants to open
or close the dampers beyond minimum position for modified
ventilation.

SEQUENCE OF OPERATION

ENTHALPY
CHANGEOVER
SETPOINT
Economizer Sequence of Operation
(WITH AND WITHOUT DIFFERENTIAL ENTHALPY)
The economizer operating sequence for both the modulating
and the three-position control systems are identical when the
outdoor enthalpy temperature is above the mixed air setpoint
of 55°F (13°C). The operating sequence for both, using a
standard two-stage thermostat is:
1. The first stage of the thermostat signals a need for
MINIMUM
DAMPER
POSITION
1
2. The W7459 Solid State Economizer Logic Module
SETTING
LED LIGHTS
WHEN OUTDOOR
3.
AIR IS SUITABLE
FOR FREE COOLING
M9098B
4. If the logic module uses the outside air for cooling, the
5.
6. Refer to Table 4 for further information on outside air
17
SOLID STATE ECONOMIZER SYSTEM
cooling.
begins to make decisions regarding the unit/
economizer operation.
a. On the standard (non-differential) economizers, the
logic module checks the outdoor air C7400 Enthalpy
Sensor or C7660 Temperature Sensor to determine
if the enthalpy or temperature is below the setpoint.
b. On the differential economizers, the logic module
checks to determine if the outdoor air temperature is
below the return air temperature.
mixed air sensor prevents the entering air from going
below 50°F (10°C).
a. On the modulating system, the control closes the
outside air damper and opens the return air damper
to mix the outside air and return air to maintain 50°–
55°F (10°–13°C).
b. On the three-position system, the mixed air sensor
switch opens, closing the fresh air damper until the
mixed air sensor temperature returns above 55°F
(13°C), closing the switch and re-opening the out-
side air damper.
a. If the logic module senses that the outside air is not
suitable for cooling, the air conditioning unit com-
pressor is energized and the space is cooled with
refrigerated air.
b. On the two-stage thermostat, the economizer is the
first stage if the outside air temperature is suitable
for cooling. The compressor on the unit is energized
if the second stage of the thermostat is energized,
thereby creating an integrated economizer.
damper positions.
63-2484—03