Honeywell m8405 User Manual

SOLID STATE ECONOMIZER SYSTEM

63-2484—2

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.

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.

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.

Standard economizer position based on enthalpy control set on the A setting and 50% relative humidity.

Closed position is either the minimum position or fully closed, depending on the job setting.

Opening/closing is dependent on the mixed air temperature.

Table 4. Outdoor air damper positions.

Outside

Temperature

Standard Economizer Damper Position

Return

Temperature

Differential Enthalpy Damper Position

Modulating

3-Position

Modulating

2,3

3-Position

2,3

Closed

Open

Closed

Open

54 and down

Modulating

Opening/Closing

Modulating

Opening/Closing