High Mixed Air Temperature Alarm; Low Mixed Air Temperature Alarm; Economizer Operation; Figure 41 Mixed Air Temperature Sensor - Bard MEGA-TEC W090A Service Instructions Manual

7. To apply an offset, press ENTER key to scroll to
Offset.
8. Press UP or DOWN keys to change to the desired
value.
9. Press ENTER key to save the value.
FIGURE 41
Mixed Air Temperature Sensor

High Mixed Air Temperature Alarm

To keep the economizer from bringing in air that is too
warm, an alarm will be generated when the mixed air is
2° above the return air temperature for 300 seconds.
This alarm will not be generated if the mixed air
temperature sensor has failed. The alarm can only be
activated during a free cooling call. The alarm is a user
reset and must be cleared by the end user.

Low Mixed Air Temperature Alarm

To keep the economizer from bringing in air that is too
cold, an alarm will be generated when the mixed air
temperature is 5°F below the mixed air temperature
control setpoint for 300 seconds. If this alarm is active,
the economizer will be disabled for the current cooling
call. This alarm will reference the mixed air temperature
control setpoint to ensure that when the mixed air setpoint
changes, the alarm dynamically changes with it. This
alarm will not be generated if the mixed air temperature
sensor has failed. The alarm can only be activated during
a free cooling call and requires a user reset.

Economizer Operation

See Figure 67 on page 56 for ventilation airflow paths.
The economizer has four types of operation. The
first mode is "None" where the economizer is never
utilized, except for emergency purposes. The second
mode is "Dry Bulb" where the outdoor temperature is
the only consideration for economizer use on a free
cooling call. The third mode is "TempHum" where the
outdoor temperature and humidity are considered
for economizer use on a free cooling call. The fourth
mode is "Enthalpy" where the outdoor temperature,
humidity and calculated dew point are considered for
economizer operation on a free cooling call.
Manual 2100-671C
Page 30 of 59
To change the economizer type:
1. Press MENU key to go to the Main Menu screen.
2. Press UP or DOWN keys and ENTER key to enter
USER password 2000.
3. Press UP or DOWN keys to scroll to System Config;
press ENTER key.
4. Press UP or DOWN keys to scroll to Economizer
Setup A2; press ENTER key.
5. Press ENTER key to scroll to Type (see Figure 42).
6. Press UP or DOWN keys to change the Type desired
value to None, Dry Bulb, TempHum or Enthalpy.
7. Press ENTER key to save the value and scroll to
the next parameter.
FIGURE 42
Economizer Setup
NOTE: The following parameters are for the
temperature consideration for economizer use.
Applies to Dry Bulb, TempHum and Enthalpy
type.
8. The cursor should now be on the Mixed Air Tamp
parameter.
9. Press UP or DOWN keys to change the parameter
to the desired value.
10. Press ENTER key to save the value and scroll to
the next parameter.
11. The cursor should now be on Delay parameter.
12. Press UP or DOWN keys to change the parameter
to the desired value.
13. Press ENTER key to save the value and move to
the next parameter.
14. The cursor is now on the Min Position parameter.
15. Press UP or DOWN keys to change the parameter
to the desired value (see Economizer Operation −
Minimum Position on page 32 for more information).
16. Press ENTER key to save the value.
17. Press the DOWN key to navigate to the A3 screen.
NOTE: The A3 screen will not display if economizer
mode is set to None. Also, the contents of the
screen will change depending on which type is