3.7.4.2 Air-Side Economizers | COMNET

90.1-2019

*Economizer Control Type*
Applicability	All systems with an air-side economizer
Definition	An air-side economizer increases outside air ventilation during periods when cooling loads can be reduced from increased outside airflow. The control types include: No economizer. Fixed dry-bulb. The economizer is enabled when the temperature of the outside air is lower than a fixed setpoint. Differential dry-bulb. The economizer is enabled when the temperature of the outside air is lower than the return air temperature. Fixed enthalpy. The economizer is enabled when the enthalpy of the outside air is lower than a fixed setpoint Differential enthalpy. The economizer is enabled when the enthalpy of the outside air is lower than the return air enthalpy. Differential dry-bulb and enthalpy. The economizer is enabled when the outside air dry-bulb is less than the return air dry-bulb AND the outside air enthalpy is less than the return air enthalpy. Fixed dewpoint and dry-bulb. The economizer is enabled when the dewpoint and dry-bulb temperature of the outside air are below the specified setpoints.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Outdoor air economizers shall not be included in baseline HVAC systems 1, 2, 9, and 10. Outside air economizers shall be included in baseline HVAC systems 3 through 8, 11, 12, and 13, in climate zones 2B, 3B, 3C, 4C, 5B, 5C, 6B, 7, and 8. Economizers are not required in other climate zones. Economizer control shall be fixed dry bulb with a temperature setpoint as indicated under Economizer High-Temperature Lockout below. Exceptions: Economizers shall not be included for systems meeting one or more of the exceptions listed below. a. Systems that include gas-phase air cleaning to meet the requirements of Section 6.1.2 in Standard 62.1. In the scenario, where there isn’t a one-to-one mapping between the systems in the proposed and baseline building, all systems serving the zones that qualify for the exception in the proposed building will not have economizers. b. Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems. This exception shall only be used if the system in the proposed design does not use an economizer. c. Systems serving computer rooms which are baseline systems 3-4 shall not have an economizer. Systems that server computer rooms that are baseline system 11 shall include an integrated fluid economizer meeting the requirements of Standard 90.1-2019 Section 6.5.1.2, also summarized below and in Section 3.8.4 of this manual. Fluid Economizers Fluid economizer systems shall be capable of providing up to 100% of the expected system cooling load at outdoor air temperatures of 50°F dry bulb/45°F wet bulb and below. Exceptions: Systems primarily serving computer rooms in which 100% of the expected system cooling load at the dry-bulb and wet-bulb temperatures listed in Table 6.5.1.2.1 of Standard 90.1-2019 is met with water-cooled fluid economizers.. Systems primarily serving computer rooms in which 100% of the expected system cooling load at the dry-bulb temperatures listed in Table 6.5.1.2.1 of Standard 90.1-2019 is met with air-cooled fluid economizers. Systems where dehumidification requirements cannot be met using outdoor air temperatures of 50°F dry-bulb/45°F wet-bulb and where 100% of the expected system cooling load at 45°F dry-bulb/40°F wet-bulb is met with water-cooled fluid economizers.

*Economizer Integration*
Applicability	Airside economizers
Definition	This input specifies whether or not the economizer is integrated with mechanical cooling. It is up to the modeling software to translate this into software-specific inputs to model this feature. The input could take the following values: Non-integrated: The system runs the economizer as the first stage of cooling. When the economizer is unable to meet the load, the economizer returns the outside air damper to the minimum position and the compressor turns on as the second stage of cooling. Integrated: The system can operate with the economizer fully open to outside air and mechanical cooling active (compressor running) simultaneously, even on the lowest cooling stage.
Units	List: Non-integrated, Integrated
Input Restrictions	As designed
Baseline Building	Integrated

*Economizer High-Temperature Lockout*
Applicability	Systems with fixed dry-bulb economizer
Definition	The outside air setpoint temperature above which the economizer will return to minimum position
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed
Baseline Building	Economizer control shall be fixed dry bulb with a temperature setpoint of 70°F in climate zones 5A and 6A and 75^°F in all other climate zones where economizers are required

*Economizer Low-Temperature Lockout*
Applicability	Systems with air-side economizers
Definition	A feature that permits the lockout of economizer operation (return to minimum outside air position) when the outside air temperature is below the lockout setpoint
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed
Baseline Building	None

*Economizer High Enthalpy Lockout*
Applicability	Systems with fixed enthalpy or differential enthalpy economizers
Definition	The outside air enthalpy above which the economizer will return to minimum position
Units	Btu/lb
Input Restrictions	As designed. The default is 28 Btu/lb. (High altitude locations may require different setpoints.) The software shall apply a fixed offset and add 2 Btu/lb to the user-entered value.
Baseline Building	Not applicable since no baseline building system has an enthalpy economizer