Honeywell M8405 - Manual (Page 16)

SOLID STATE ECONOMIZER SYSTEM
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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
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
cooling.
2. The W7459 Solid State Economizer Logic Module
begins to make decisions regarding the unit/
economizer operation.
3.
a. On the standard (non-differential) economizers, the
logic module checks the outdoor air C7400 Enthalpy
Sensor or C7650 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.
4. If the logic module uses the outside air for cooling, the
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 outside
air damper.
5.
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.
6. Refer to Table 4 for further information on outside air
damper positions.
1 Standard economizer position based on enthalpy control set on the A setting and 50% relative humidity.
2 Closed position is either the minimum position or fully closed, depending on the job setting.
3 Opening/closing is dependent on the mixed air temperature.
Table 4. Outdoor air damper positions.
Outside
Temperature 1
Standard Economizer Damper Position
Return
Temperature 1
Differential Enthalpy Damper Position
Modulating 2 3-Position 2 Modulating 2,3 3-Position 2,3
80 Closed Closed 80 Closed Closed
85 Open Open
75 Closed Closed 75 Closed Closed
80 Open Open
70 Open Open 75 Open Open
65 Open Open
60 Open Open 75 Open Open
55 Open Open 75 Open Open
54 and down Modulating Opening/Closing 75 Modulating Opening/Closing