Honeywell M8405 - Manual (Page 12)

SOLID STATE ECONOMIZER SYSTEM
63-2484—2 12
Fig. 16. Mounting the W7212 module on sheet metal.
C7400 Enthalpy Sensor and C7650A
Temperature Sensor Location and Mounting
Outdoor air sensing: Mount the C7400 Enthalpy Sensor or
C7650A Temperature Sensor in any orientation where it is
exposed to freely circulating air but protected from rain, snow
and direct sunlight.
Return air sensing: For differential enthalpy or temperature
control, a second C7400 Enthalpy Sensor or C7650A
Temperature Sensor is connected to the W7459 or W7212.
Mount the second sensor in the return air duct as far as
possible from the outdoor air sensor.
Wiring
CAUTION
Can Cause Electrical Shock or Equipment Damage.
Disconnect power supply before connecting wiring.
Disconnect the power supply before connecting wiring to
prevent electrical shock or equipment damage. All wiring must
comply with applicable local codes, ordinances and
regulations.
OPERATION AND CHECKOUT
C7046C (Discharge) and C7150B (Mixed) Air Sensors
The C7046C discharge air sensor is applied following the coils
in the supply air to the conditioned space. The C7150B mixed
air sensor is mounted in the mixing box, where the “best”
combination of outdoor and return air temperature can be
measured. Use either a discharge or a mixed air sensor to
provide input to the economizer module through T and T1
Terminals.
Fig. 17. C7046C and C7150B Air Temperature Sensors
resistance versus temperature.
C7400A Solid State Enthalpy Sensor
The C7400A Solid State Enthalpy Sensor is used with a solid
state economizer control and damper actuator to proportion an
outdoor air damper in a ventilation system.
Figure 18 is a partial psychometric chart with single C7400A
Sensor and W7459A Economizer Logic Module performance
curves. The curves illustrate the reset in the temperature
control point due to changes in relative humidity.
The enthalpy control setpoint A,B,C or D combines
temperature and humidity conditions, resulting in the control
curve shown in Fig. 18. When the enthalpy or outdoor air is
below (left of) the appropriate curve, the outdoor air damper
can proportion open on a call for cooling. If outdoor air enthalpy
rises above (to the right of) the control curve, the outdoor air
damper closes to the minimum position.
For differential enthalpy, turn the control setpoint to D (fully
clockwise). If outdoor air enthalpy is lower than return air enthalpy,
the outdoor air damper proportions open on a call for cooling.
If outdoor air is higher enthalpy than return air enthalpy, the
outdoor air damper closes to minimum position. Differential
enthalpy control provides energy savings and increased
comfort by using the air with the lowest enthalpy.
If outdoor air enthalpy and return air enthalpy are equal, the
outdoor air damper proportions open on a call for cooling.
The relationship between the C7400A Sensor output current
and relative humidity is shown in Fig. 19.
C7650A Solid State Temperature Sensor
The C7650A Solid State Temperature Sensor is used with a
solid state economizer control and damper actuator to
proportion an outdoor air damper in a ventilation system.
When outdoor air temperature is higher than return air
temperature, the outdoor air damper closes to the minimum
position. When the outdoor air temperature and the return air
temperature are equal, the outdoor air damper proportions
open on a call for cooling.
The relationship between the C7650A Sensor output current
and the air temperature is shown in Fig. 20.
M20601
4500
4000
3500
3000
2500
2000
1500
1000
55 60 65 70 75 80 85 90 95 100
15 20 25 30 35
RESIST
ANCE (OHMS)
TEMPERATURE (DEGREES) M9099
F
C
3,000 OHMS AT
77F (25C)