Honeywell W7750A Indoor AIR Quality IAQ Override, Freeze Stat, Discharge AIR LOW Limit Control

EXCEL 10 W7750A,B,C CONSTANT VOLUME AHU CONTROLLER

INDOOR AIR QUALITY (IAQ) OVERRIDE

The Excel 10 W7750 Controller supports an IAQ override feature that upon detection of poor air quality in the space, allows the economizer dampers to be opened above the standard minimum position setting to a value set in EconIAQPos. Two different methods of detecting poor air quality are supported, The first is by using an IAQ switch device connected to a digital input on the W7750 Controller, where a contact closure indicates poor air quality and initiates the IAQ override mode. The second, which is only available on the W7750B,C is through an analog input that connects to a CO2 Sensor (0 to 10V). The measured value of CO2 from this sensor (0 to 2000 ppm) is compared to the setpoint IAQSetpt. When the CO2 level is higher than the setpoint (800 PPM), the IAQ override is initiated. The IAQSetpt hysteresis is 50 PPM, IAQ override is deactivated at a CO2 level less than 750 PPM.

When the W7750 Controller is in the COOL mode during an IAQ override, it is possible for the heating outputs to be activated. This can occur if the outdoor air temperature is cold enough to cause the discharge air temperature to drop below the DaTempLoLim setpoint when the dampers open to the EconIAQPos position, and the IaqUseHeat flag is set. If this situation occurs, the heating is controlled to maintain the discharge air temperature at 1°F (0.65°C) above the DaTempLoLim setpoint.

FREEZE STAT

Upon receiving a contact closure, the W7750 control algorithm will close the outdoor air damper and open the hot water and chilled water valves (if available) to the full open position as a safety precaution. If manual-resetoperation is desired, the Freeze Stat device must provide the physical pushbutton, which the operator presses, to reset the system after a freeze condition has occurred.

DISCHARGE AIR LOW LIMIT CONTROL

If the discharge air temperature falls below the user-settable discharge air low limit setpoint (DaTempLoLim), an alarm is issued, and the outdoor air damper is driven below the minimum position setting until the discharge temperature is up to the low limit. If necessary, the damper can go completely closed even during Occupied mode operation. As the discharge temperature warms up, the economizer modulates open until the minimum position setting is reached. At this point, the low limit alarm is cleared.

ECONOMIZER ENABLE/DISABLE CONTROL

The W7750 Controller has inputs to determine if the outdoor air is suitable to augment cooling. The economizer dampers can be enabled/disabled for using outdoor air as the first stage of cooling based on one of ten allowable strategies:

1.Digital Input Enable/Disable—contact closure enables economizer.

2.Outdoor Temperature—when the outdoor temperature is less than OaEconEnTemp, then the outdoor air is suitable to augment cooling.

3.Outdoor Enthalpy, Type A—when the outdoor enthalpy meets the H205 type A requirements, the outdoor air is suitable to augment cooling.

4.Outdoor Enthalpy, Type B—when the outdoor enthalpy meets the H205 type B requirements, the outdoor air is suitable to augment cooling.

5.Outdoor Enthalpy, Type C—when the outdoor enthalpy meets the H205 type C requirements, the outdoor air is suitable to augment cooling.

6.Outdoor Enthalpy, Type D—when the outdoor enthalpy meets the H205 type D requirements, the outdoor air is suitable to augment cooling.

7.Differential Temperature—the difference between outdoor temperature and return air temperature is compared to DiffEconEnTemp to determine whether outdoor air or return air is more suitable for use to augment mechanical cooling.

8.Single Calculated Enthalpy—the calculated outdoor enthalpy in btu/lb is compared to the enthalpy setpoint (OaEnthEn) in btu/lb, and the outdoor temperature is compared to the outdoor temperature limit setpoint (OaEconEnTemp) for a high limit. The compared difference determines whether outdoor air is suitable for use to augment mechanical cooling.

9.Differential Enthalpy, Either Sensed or Calculated— the difference between outdoor enthalpy and return air enthalpy determines whether outdoor air or return air is more suitable to augment mechanical cooling. When enthalpy sensors are configured, they are used for comparing enthalpy. If no enthalpy sensors are available, then enthalpy is calculated using outdoor and return air humidity and temperature sensors. The switching differential is fixed at 1.0 mA for enthalpy sensors, and 0.25 btu/lb for calculated enthalpy.

NOTE: If no return temperature sensor is configured, space temperature is used to calculate return air enthalpy.

10.Network Enabled—the network input DestEconEnable controls the enabling and disabling of the economizer. When using the network input, select Econo Enable Type: No Economizer in E-Vision. The network input has priority over the other nine economizer control selections.

Appendix C. Complete List of Excel 10 W7750 Controller User Addresses.

See Table 18 for W7750 Controller User Address table numbers and point types.

The User Address Index following Table 18 lists the User Addresses alphabetically and gives the page number where the Address is located in each Table Number/Point Type.

After Table 18 there is an alphabetical list of Mappable User Addresses and Table Numbers. Following this is an alphabetical list of Failure Detect User Addresses and Table Numbers.

Table 18. Excel 10 W7750 Controller User

Address Point Types.

Table Number	Point Types
Table 20	Input/Output
Table 21	Control Parameters
Table 22	Energy Management Points
Table 23	Status Points
Table 24	Calibration Points
Table 25	Configuration Parameters
Table 26	LONMARK/Open System Points
Table 27	Direct Access and Special Manual Points
Table 28	Data Share Points

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