6.7.4 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 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

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