1. Home
  2. Modeling Guidelines
  3. 3.0 Building Descriptors Reference
  4. 3.7 HVAC Secondary Systems

3.7.4 Outdoor Air Controls and Economizers

Outside Air Controls

Maximum Outside Air Ratio
Applicability All systems with modulating outside air dampers
Definition The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.
Units Ratio
Input Restrictions As designed
Baseline Rules 1.0
Design Outside Air Flow
Applicability All systems with outside air dampers
Definition The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside air flow for each of the zones served by the system.
Units cfm
Input Restrictions As designed
Baseline Rules Same as proposed design. This input along with occupant density determines if the zones served by the system shall have demand control ventilation. See ventilation control method at the zone level.
Outdoor Air Control Method
Applicability All HVAC systems that deliver outside air to zones
Definition The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system report their outside air requirements on an hourly basis. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information on an hourly basis. Options include:
  • Average Flow. The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air fan.
  • Critical Zone. The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting greater outside air than required.
The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply which modulates the position of the outside air and return dampers to maintain the desired outside air flow. With the average flow, a CO2 sensor in the return air duct is another way to control the position of the outside air and return dampers.
Units List (see above)
Input Restrictions As designed
Baseline Rules Same as proposed design

Air Side Economizers

Economizer Control Type
Applicability All systems with an air-side economizer
Definition An air-side economizer increases outside air ventilation during periods when refrigeration loads can be reduced from increased outside air flow. The control types include:
  • No economizer
  • Fixed dry-bulb. The system shifts to 100% outside air and shuts off the cooling when the temperature of the outside air is equal to or lower than the supply air temperature.
  • Differential dry-bulb. The system shifts to 100% outside air when the temperature of the outside air is lower than the return air temperature but continues to operate the cooling system until the outside air temperature reaches the supply air temperature.
  • Fixed enthalpy. The system shifts to 100% outside air and shuts off the cooling when the enthalpy of the outside air is equal to or lower than the supply air enthalpy.
  • Differential enthalpy. The system shifts to 100% outside air when the enthalpy of the outside air is lower than the return air enthalpy but continues to operate the cooling system until the outside air enthalpy reaches the supply air enthalpy.
Units List (see above)
Input Restrictions As designed
Baseline Rules The control should be no economizer when the baseline building cooling capacity  $\leq$50 tons AND the baseline building system total cooling capacity $\leq$95,000 Btu/h. Otherwise, the control should be based on the prescribed system type as shown in [bookref id="baseline-economizer-control-type"].

[table title="Baseline Economizer Control Type" id="baseline-economizer-control-type"]

Baseline building System Economizer Control
System 1 – PTAC None
System 2 – PTHP None
System 3 – PSZ-AC Differential dry-bulb
None in climate zones 1a-4a and 1b
System 4 – PSZ-HP Differential dry-bulb
None in climate zones 1a-4a and 1b
System 5 – Packaged VAV with Reheat Differential dry-bulb
None in climate zones 1a-4a and 1b
System 6 – Packaged VAV with PFP boxes Differential dry-bulb
None in climate zones 1a-4a and 1b
System 7 – VAV with Reheat Differential dry-bulb
None in climate zones 1a-4a and 1b
System 8 – VAV with PFP boxes Differential dry-bulb
None in climate zones 1a-4a and 1b
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 Rules Not applicable
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 Rules Not used
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 25 Btu/lb.
Baseline Rules No lockout limit

Outside Air Controls

Maximum Outside Air Ratio
Applicability All systems with modulating outside air dampers
Definition The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.
Units Ratio
Input Restrictions As designed
Baseline Rules 1.0
Design Outside Air Flow
Applicability All systems with outside air dampers
Definition The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside air flow for each of the zones served by the system.
Units cfm
Input Restrictions As designed
Baseline Rules Same as proposed design. This input along with occupant density determines if the zones served by the system shall have demand control ventilation. See ventilation control method at the zone level.
Outdoor Air Control Method
Applicability All HVAC systems that deliver outside air to zones
Definition The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system report their outside air requirements on an hourly basis. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information on an hourly basis. Options include:
  • Average Flow. The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air fan.
  • Critical Zone. The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting greater outside air than required.
The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply which modulates the position of the outside air and return dampers to maintain the desired outside air flow. With the average flow, a CO2 sensor in the return air duct is another way to control the position of the outside air and return dampers.
Units List (see above)
Input Restrictions As designed
Baseline Rules Same as proposed design

Air Side Economizers

Economizer Control Type
Applicability All systems with an air-side economizer
Definition An air-side economizer increases outside air ventilation during periods when refrigeration loads can be reduced from increased outside air flow. The control types include:
  • No economizer
  • Fixed dry-bulb. The system shifts to 100% outside air and shuts off the cooling when the temperature of the outside air is equal to or lower than the supply air temperature.
  • Differential dry-bulb. The system shifts to 100% outside air when the temperature of the outside air is lower than the return air temperature but continues to operate the cooling system until the outside air temperature reaches the supply air temperature.
  • Fixed enthalpy. The system shifts to 100% outside air and shuts off the cooling when the enthalpy of the outside air is equal to or lower than the supply air enthalpy.
  • Differential enthalpy. The system shifts to 100% outside air when the enthalpy of the outside air is lower than the return air enthalpy but continues to operate the cooling system until the outside air enthalpy reaches the supply air enthalpy.
Units List (see above)
Input Restrictions As designed
Baseline Rules The control should be no economizer when the baseline building cooling capacity  $\leq$50 tons AND the baseline building system total cooling capacity $\leq$95,000 Btu/h. Otherwise, the control should be based on the prescribed system type as shown in Table 6.7.4-1 .

Table 6.7.4-1: Baseline Economizer Control Type

Baseline building System Economizer Control
System 1 – PTAC None
System 2 – PTHP None
System 3 – PSZ-AC Differential dry-bulb
None in climate zones 1a-4a and 1b
System 4 – PSZ-HP Differential dry-bulb
None in climate zones 1a-4a and 1b
System 5 – Packaged VAV with Reheat Differential dry-bulb
None in climate zones 1a-4a and 1b
System 6 – Packaged VAV with PFP boxes Differential dry-bulb
None in climate zones 1a-4a and 1b
System 7 – VAV with Reheat Differential dry-bulb
None in climate zones 1a-4a and 1b
System 8 – VAV with PFP boxes Differential dry-bulb
None in climate zones 1a-4a and 1b
System 9 – Heating and Ventilation None (no cooling)
System 10 – Heating and Ventilation None (no cooling)
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 Rules Not applicable
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 Rules Not used
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 25 Btu/lb.
Baseline Rules No lockout limit

Outside Air Controls

Maximum Outside Air Ratio

Applicability

All systems with modulating outside air dampers

Definition

The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.

Units

Ratio

Input Restrictions

As designed

Baseline Rules

1.0 for systems 3-8 and 11-13.

 

Design Outside Air Flow

Applicability

All systems with outside air dampers

Definition

The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside air flow for each of the zones served by the system.

Units

cfm

Input Restrictions

As designed

Baseline Rules

Same as proposed design, except for the following cases:

  • When demand control ventilation is modeled in the proposed design for systems less than or equal to 3,000 cfm with an occupant load of 100 people per 1,000 ft² or less.
  • When the proposed design uses the Ventilation Rate Procedure of Standard 62.1, Section 6.2 and has a ventilation effectiveness greater than 1.0.
  • When outside air ventilation is increased above the requirements of Standard 62.1 in order to get credit for recognition programs.
  • When modeling laboratory spaces that are prevented from recirculating return air.

See ventilation control method at the zone level.

 

Outdoor Air Control Method

Applicability

All HVAC systems that deliver outside air to multiple zones

Definition

The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system report their outside air requirements on an hourly basis. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information on an hourly basis. Options include:

  • Average Flow. The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air fan.
  • Critical Zone. The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting greater outside air than required.

The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply which modulates the position of the outside air and return dampers to maintain the desired outside air flow. With the average flow, a CO2 sensor in the return air duct is another way to control the position of the outside air and return dampers.

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

Same as proposed design

Air Side Economizers

Table 3.7.4-1 – Economizer Requirements for Baseline Building Systems 3-8 and 11-13

Thermal Zone

Humid (A)

Dry (B)

Marine (C)

0

Economizer not required

Economizer not required

Fixed drybulb economizer with high-limit shutoff of 75 °F

1

2

 

3

4

5

Fixed drybulb economizer with high-limit shutoff of 70 °F

6

7

 

8

Economizers are not required in the baseline building for systems meeting one or more of the following exceptions:

  1. Systems that include gas-phase air cleaning to meet the requirements of Section 6.1.2 in Standard 62.1. This exception shall be used only if the system in the proposed design does not match the building design.
  2. 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. If the exception is used, an economizer shall not be included in the baseline building design.
  3. System types 3 or 4 that that serve computer rooms. If system type 11 is used to serve a computer room, it shall be equipped with a water economizer.

Economizer Control Type

Applicability

All systems with an air-side economizer

Definition

An air-side economizer increases outside air ventilation during periods when refrigeration loads can be reduced from increased outside air flow. The control types include:

  • No economizer
  • Fixed dry-bulb. The system shifts to 100% outside air and shuts off the cooling when the temperature of the outside air is equal to or lower than the supply air temperature.
  • Differential dry-bulb. The system shifts to 100% outside air when the temperature of the outside air is lower than the return air temperature but continues to operate the cooling system until the outside air temperature reaches the supply air temperature.
  • Fixed enthalpy. The system shifts to 100% outside air and shuts off the cooling when the enthalpy of the outside air is equal to or lower than the supply air enthalpy.
  • Differential enthalpy. The system shifts to 100% outside air when the enthalpy of the outside air is lower than the return air enthalpy but continues to operate the cooling system until the outside air enthalpy reaches the supply air enthalpy.

Differential dry-bulb or enthalpy economizers are also known as integrated economizers.

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

Fixed dry-bulb economizers are required for baseline building systems 3-8 and 11-13 except for climates identified in Table 3.7.4-1

 

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 Rules

75 °F in all climates that require an economizer, except 70 °F in climate zones 6a and 7a. See Table 3.7.4-1.

 

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 Rules

Not used

 

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 25 Btu/lb.

Baseline Rules

Not applicable since no baseline building system has an enthalpy economizer
90.1-2019

This section addresses:

 

Outside Air Controls

Maximum Outside Air Ratio
Applicability All systems with modulating outside air dampers
Definition The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.
Units Ratio
Input Restrictions As designed
Design Outside Air Flow
Applicability All systems with outside air dampers
Definition The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside air flow for each of the zones served by the system.
Units cfm
Input Restrictions As designed
Outdoor Air Control Method
Applicability All HVAC systems that deliver outside air to zones
Definition The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system report their outside air requirements on an hourly basis. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information on an hourly basis. Options include:
  • Average Flow. The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air fan.
  • Critical Zone. The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting greater outside air than required.
The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply which modulates the position of the outside air and return dampers to maintain the desired outside air flow. With the average flow, a CO2 sensor in the return air duct is another way to control the position of the outside air and return dampers.
Units List (see above)
Input Restrictions As designed

Air Side Economizers

Economizer Control Type
Applicability All systems with an air-side economizer
Definition An air-side economizer increases outside air ventilation during periods when refrigeration loads can be reduced from increased outside air flow. The control types include:
  • No economizer
  • Fixed dry-bulb. The system shifts to 100% outside air and shuts off the cooling when the temperature of the outside air is equal to or lower than the supply air temperature.
  • Differential dry-bulb. The system shifts to 100% outside air when the temperature of the outside air is lower than the return air temperature but continues to operate the cooling system until the outside air temperature reaches the supply air temperature.
  • Fixed enthalpy. The system shifts to 100% outside air and shuts off the cooling when the enthalpy of the outside air is equal to or lower than the supply air enthalpy.
  • Differential enthalpy. The system shifts to 100% outside air when the enthalpy of the outside air is lower than the return air enthalpy but continues to operate the cooling system until the outside air enthalpy reaches the supply air enthalpy.
Units List (see above)
Input Restrictions As designed
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
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
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 25 Btu/lb.

Outside Air Controls

Maximum Outside Air Ratio
Applicability All systems with modulating outside air dampers
Definition The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.
Units Ratio
Input Restrictions As designed
Design Outside Air Flow
Applicability All systems with outside air dampers
Definition The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside air flow for each of the zones served by the system.
Units cfm
Input Restrictions As designed
Outdoor Air Control Method
Applicability All HVAC systems that deliver outside air to zones
Definition The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system report their outside air requirements on an hourly basis. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information on an hourly basis. Options include:
  • Average Flow. The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air fan.
  • Critical Zone. The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting greater outside air than required.
The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply which modulates the position of the outside air and return dampers to maintain the desired outside air flow. With the average flow, a CO2 sensor in the return air duct is another way to control the position of the outside air and return dampers.
Units List (see above)
Input Restrictions As designed

Air Side Economizers

Economizer Control Type
Applicability All systems with an air-side economizer
Definition An air-side economizer increases outside air ventilation during periods when refrigeration loads can be reduced from increased outside air flow. The control types include:
  • No economizer
  • Fixed dry-bulb. The system shifts to 100% outside air and shuts off the cooling when the temperature of the outside air is equal to or lower than the supply air temperature.
  • Differential dry-bulb. The system shifts to 100% outside air when the temperature of the outside air is lower than the return air temperature but continues to operate the cooling system until the outside air temperature reaches the supply air temperature.
  • Fixed enthalpy. The system shifts to 100% outside air and shuts off the cooling when the enthalpy of the outside air is equal to or lower than the supply air enthalpy.
  • Differential enthalpy. The system shifts to 100% outside air when the enthalpy of the outside air is lower than the return air enthalpy but continues to operate the cooling system until the outside air enthalpy reaches the supply air enthalpy.
Units List (see above)
Input Restrictions As designed
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
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
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 25 Btu/lb.