3.7.4.2 Air-Side Economizers | COMNET

*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

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	G3.2 New Construction/Major Alterations 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 90.1-2022 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 90.1-2022 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 90.1-2022 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.
	G3.3 Minor Alterations Economizer control type shall be modeled as minimally compliant with Standard 90.1-2022 Section 6.5.1 with the same type as the proposed. Where the proposed does not include an economizer, it is recommended that an air economizer be modeled in the baseline. However, according to Standard 90.1 Section 6.5.1.5, when applicable based on the scope of the retrofit and the language in Standard 90.1 Section 6.1.4, if the proposed design includes hydronic cooling and humidification systems designed to maintain inside humidity at a dew-point temperature greater than 35°F, then a fluid economizer is specifically required to be modeled in the baseline to meet 90.1-2022 Section 6.5.1. Exceptions to 90.1 Section 6.5.1: Individual fan-cooling units with a supply capacity less than the minimum listed in 90.1 Table 6.5.1-1 Chilled-water cooling systems without a fan or that use induced airflow, where the total capacity of these systems is less than 1,000,000 Btu/h in Climate Zones 0, 1B, and 2 through 4; less than 1,400,000 Btu/h in Climate Zones 5 through 8; or any size in Climate Zone 1A. Systems that include nonparticulate air treatment as required by Standard 62.1, Section 6.2.1. In hospitals and ambulatory surgery centers, where more than 75% of the air designed to be supplied by the system is to spaces that are required to be humidified above 35°F dew-point temperature to comply with applicable codes or accreditation standards; in all other buildings, where more than 25% of the air designed to be supplied by the system is to spaces that are designed to be humidified above 35°F dew-point temperature to satisfy process application needs. This exception does not apply to computer rooms. Systems that include a condenser heat recovery system with a minimum capacity as defined in 90.1 Section 6.5.6.2.2. Systems that serve residential spaces where the system capacity is less than five times the requirement listed in 90.1 Table 6.5.1-1. Systems that serve spaces whose sensible cooling load at design conditions, excluding transmission less than or equal to transmission losses at an outdoor temperature of 60°F. Systems expected to operate fewer than 20 hours per week. Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems. For comfort cooling, where the cooling efficiency meets or exceeds the efficiency improvement requirements in 90.1 Table 6.5.1-2. Systems primarily serving computer rooms where a. the total design cooling load of all computer rooms in the building is less than 3,000,000 Btu/h and the building in which they are located is not served by a centralized chilled-water plant; b. the room total design cooling load is less than 600,000 Btu/h and the building in which they are located is served by a centralized chilled-water plant; c. the local water authority does not allow cooling towers; or d. less than 600,000 Btu/h of computer-room cooling equipment capacity is being added to an existing building. Dedicated systems for computer rooms, where a minimum of 75% of the design load serves. a. those spaces classified as an essential facility, b. those spaces having a design of Tier IV as defined by ANSI/TIA-942, c. those spaces classified under NFPA 70 Article 708—Critical Operations Power Systems (COPS) d. those spaces where core clearing and settlement services are performed such that their failure to settle pending financial transactions could present systemic risk as described in “The Interagency Paper on Sound Practices to Strengthen the Resilience of the U.S. Financial System” (April 7, 2003). Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1 requirements are inapplicable.

Economizer Integration
Applicability	Airside economizers
Definition	This input specifies whether 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	G3.2 New Construction/Major Alterations Integrated G3.3 Minor Alterations Integrated according to 90.1-2022 Section 6.5.1.3. Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1.3 requirements are inapplicable.

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	G3.2 New Construction/Major Alterations 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 G3.3 Minor Alterations According to 90.1 2022 Table 6.5.1.1.3 based on climate zone. The requirements are summarized directly below. 0B, 1B, 2B, 3B, 3C, 4B, 4C,5B, 5C, 6B, 7, 8: 75°F 5A, 6A: 70°F 0A, 1A, 2A, 3A, 4A: 65°F Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1.1.3 requirements are inapplicable.

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	G3.2 New Construction/Major Alterations None G3.3 Minor Alterations Same as proposed if applicable, otherwise 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	G3.2 New Construction/Major Alterations Not applicable since no baseline building system has an enthalpy economizer. G3.3 Minor Alterations 28 Btu/lb according to Standard 90.1-2022 Table 6.5.1.1.3. Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1.1.3 requirements are inapplicable.