Outdoor Air Control
The Outdoor Air Control tab contains property information for conditioning the outside air including flow rates and economizer information. Note, dedicated outside air units have their own category in the system library where their properties are defined.
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Minimum outdoor air flow
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Default: autosize
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Typical Range: N/A
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Min Max: -infinity to infinity
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Units: cfm
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The minimum outdoor air flow rate to the system can be autosized or input as a specific value.
Economizer
Economizer control type
● No Economizer – There is no economizer control in the system.
● Fixed Dry Bulb – The economizer will bring in the minimum outdoor air unless the outdoor air dry bulb is less than the specified economizer dry bulb limit.
● Fixed Enthalpy – The economizer will bring in the minimum outdoor air unless the outdoor air enthalpy is less than the specified economizer enthalpy limit.
● Differential Dry Bulb – The economizer will bring in the minimum outdoor air unless the outdoor air dry bulb is less than the return air dry bulb temperature.
● Differential Enthalpy – The economizer will bring in the minimum outdoor air unless the outdoor air enthalpy is less than the return air enthalpy.
● Fixed Enthalpy – The economizer will bring in the minimum outdoor air unless the outdoor humidity ratio is less than the calculated humidity ratio limit. The humidity ratio limit is calculated based on the dry bulb temperature of the outdoor air and a quadratic/cubic curve.
● Differential Dry Bulb and Enthalpy – The economizer will bring in the minimum outdoor air unless the both the outdoor air dry bulb and outdoor air enthalpy are respectively lower than the return air dry bulb and return air enthalpy.
Economizer Outdoor Air Control
● Modulated Outdoor Airflow – This will control the economizer to increase the outdoor airflow until the mixed air temperature setpoint is met.
● Minimum Flow - Bypass – The economizer will bring in the minimum outdoor air unless high humidity control is used. In this case, the outdoor air flow rate will be equal to the product of the maximum outdoor flow rate and the outdoor air ratio – high humidity. These inputs are found in the Humidity Control section.
Economizer Maximum DB Temperature
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Default: blank
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Typical Range: N/A
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Min Max: -infinity to infinity
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Units: °F, °C
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The economizer won’t run unless the outdoor air dry bulb is below this temperature if the Economizer Control Type is set to Fixed Dry Bulb. If this field is left blank, the economizer can run at any outdoor dry bulb temperature.
Economizer Maximum Enthalpy
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Default: blank
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Typical Range: N/A
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Min Max: -infinity to infinity
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Units: Btu/lb; J/Kg
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The economizer won’t run unless the outdoor air enthalpy is below this value if the Economizer Control Type is set to Fixed Enthalpy. If this field is left blank, the economizer can run at any outdoor air enthalpy.
Economizer Maximum Dew Point Temperature
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Default: blank
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Typical Range: N/A
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Min Max: -infinity to infinity
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Units: °F, °C
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The economizer won’t run unless the outdoor air dew point temperature is below this temperature if the Economizer Control Type is set to Fixed Dew Point And Dry Bulb. If this field is left blank, the economizer can run at any outdoor air dew point temperature.
Economizer Minimum DB Temperature
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Default: blank
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Typical Range: N/A
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Min Max: -infinity to infinity
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Units: °F, °C
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The economizer won’t run if the outdoor air dry bulb temperature is below this temperature. If this field is left blank, there is no low dry bulb temperature limit to economizer operation.
Lockout Type
Lockout is used for packaged units with DX coils.
None: The economizer will not be locked out.
● Economizer Lockout - Heating – The economizer will be locked out if the unit is in heating mode. The economizer will only run in heating if it can meet the entire heating load.
● Economizer Lockout – Cooling and Heating – The economizer will be locked out both when the system is in heating mode and when the system is in cooling mode and the compressor is operating. The economizer dampers will only open if economizing can meet either the entire heating load if the system is in heating or the entire cooling load if the system is in cooling.
Enable Time of Day Economizer
Designates whether or not to use a time of day economizer schedule.
Time of Day Economizer
This is a schedule from the library that controls whether the economizer can operate based on the time of day. If the schedule reads 0, the economizer cannot operate during that time. If the schedule reads something greater than 0, the economizer can operate.
System-Level Ventilation Overrides
Enable Ventilation Overrides
● None – There are no system level ventilation overrides applied
● Maximum Fraction of Outdoor Air Schedule – This is used to define the maximum outdoor air based on a schedule.
● Minimum Outdoor Air fraction of Design Schedule – This is used to define the minimum outside air as a fraction of the total design airflow based on a schedule.
● Minimum Outdoor Air Modifier Schedule – This is used to define the minimum outside air as a fraction of the design minimum outdoor air flow rate based on a schedule.
Maximum Fraction of Outdoor Air Schedule
This is a schedule from the library that is a direct multiplier on the total design air flow rate to define the maximum amount of outdoor air.
Minimum Outdoor Air Fraction of Design Schedule
This is a schedule from the library that is a direct multiplier on the total design air flow rate to define the minimum amount of outdoor air.
Minimum Outdoor Air Modifier Schedule
This is a schedule from the library that is a direct multiplier on the minimum outdoor air flow rate. If this field is left blank, the outdoor air flow will vary according to the Minimum Limit Type.
Ventilation Reset
Ventilation Reset Flag
Setting this field to Yes enables the rest of the fields in this section. This allows different ventilation calculation methods to be used, including ASHRAE 62.1 and Demand Control Ventilation.
DCV
If this field is set to Yes, the system will be able to vary the amount of ventilation to the zones using demand control ventilation. Demand Control Ventilation varies the ventilation to the system based on the people in the system zones. Fewer people require less ventilation, so if the people schedule shows less than the maximum number of people, the maximum outdoor airflow is not required.
If this field is set to No, the system will not be able to use demand control ventilation to vary the ventilation airflow and minimum ventilation requirement will remain constant.
System Outdoor Air Method
● Zone Sum – Sum of Outdoor Air – This method sums the ventilation requirements for all the zones on the system and uses that value for the system level ventilation.
● VRP – ASHRAE 62.1 – This ventilation rate procedure method uses the multizone calculations described in ASHRAE Standard 62.1 to calculate the system level ventilation based on the people and area based ventilation requirements of each zone.
● Proportional Control – Design Occupancy – This method can be used for single zone systems to calculate the ventilation airflow required to vary in proportion to the percent of CO2 using the procedure described in ASHRAE Standard 62.1 based on the design occupancy.
● Proportional Control – Actual Occupancy – This method can be used for single zone systems to calculate the ventilation airflow required to vary in proportion to the percent of CO2 using the procedure described in ASHRAE Standard 62.1 based on the actual occupancy.
● IAQP – This indoor air quality procedure method calculates the ventilation airflow required to keep CO2 levels in the zones below a set point value using the procedure described in ASHRAE Standard 62.1.
Maximum Outdoor Air
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Default: blank
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Typical Range: N/A
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Min Max: 0–1
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Units: dimensionless
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The zone maximum outdoor air fraction represents the maximum fraction of the total supply air that should be ventilation. If the ventilation requirement is higher than the maximum outdoor air fraction, the return airflow will be increased so both the ventilation cfm requirement and the maximum outdoor air fraction are met. This also takes into account unused outdoor air in the return air stream.
Maximum Vent Z Ratio
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Default: blank
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Typical Range: N/A
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Min Max: 0–1
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Units: dimensionless
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When modeling ASHRAE 62.1, the Outdoor Air Fraction in discharge air is calculated for each room, and the “critical zone” in turn sets the percentage of outside air for the entire system. The OA fraction (or Z ratio) for the critical zone or zones can be reduced by increasing the VAV minimum flow for that room. The Z ratio in the program is the same as Zd in the ASHRAE standard.
The “ASHRAE STD62 Max Vent (Z) Ratio Allowed” should only be input after viewing the ASHRAE Standard 62.1 report. Based on the substantial majority of the OA Fractions listed for the rooms on a system, you may then decide to input a maximum OA Fraction, or Max Vent (Z) Ratio, for the entire system. This replaces the need to manually increase the VAV Minimum Flow for each successive critical zone, an iterative process that can be time-consuming.
If a system level Max Vent (Z) Ratio is input to automatically increase the VAV minimum flows of all the rooms that fall below this OA Fraction, care must be taken to avoid raising the VAV minimums above the optimal point in terms of energy consumption. While increasing the VAV minimum flows will decrease the outside air intake flow required, it will also increase both the amount of reheat required and the fan energy consumption.
The “ASHRAE STD62 Max Vent (Z) Ratio Allowed” input is the maximum percentage allowed for the ventilation fraction of the critical zone (Z). During the System Simulation, the VAV minimum flow is increased to prevent the Z ratio from exceeding this limit.
Outdoor Carbon Dioxide Setpoint Control Type
When demand control ventilation is used with proportional control (either based on design occupancy or actual occupancy), the outdoor CO2 setpoint needs to be defined. The setpoint can be controlled either as a variable control or a constant control.
Constant Control Setpoint
When the constant control setpoint is used, the intake airflow is adjusted to maintain the indoor CO2 concentration in the space as defined in this field.
Variable Control Setpoint
The variable control setpoint will vary the outdoor air flow proportionally between the minimum and maximum ventilation airflows by taking a CO2 sample from the space.
Humidity Control
High Humidity Control
The high humidity control can be turned on or off by selecting yes or no respectively from this dropdown. This is used in conjunction with the Minimum Flow with Bypass option in the Economizer Control Action Type control.
Control Zone
This is the zone where the humidistat is located. This can only be defined in the project after the zones have been assigned to the system. Note, in the project this field will be shown even before zones have been assigned but cannot be edited until after zones have been assigned to the system. By default, the first zone assigned to the system will be set as the control zone but any zone assigned to the system can be selected from this dropdown.
Outdoor Air Ratio – High Humidity
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Default: blank
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Typical Range: N/A
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Min Max: 0–1
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Units: dimensionless
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This is the ratio of the current outdoor air flow rate to the maximum outdoor air flow rate. This is used with the high humidity control and the Minimum Flow with Bypass option in the Economizer Control Action Type control.
Control Zone Humidity based on Outdoor Air
When this control is set to No, the high humidity control will dehumidify whenever there is a moisture load in the zone. When this control is set to Yes, the high humidity control will dehumidify when there is a moisture load in the zone and the indoor humidity ratio is higher than the outdoor humidity ratio.
Heat Recovery Integrated with Economizer
Heat Recovery Control
This controller is used to determine whether a heat recovery device will be used or bypassed when the system is in economizer mode.
● None – There is no heat recovery device for the system.
● Heat Recovery Bypass – Economizer – This option forces all of the outside air to bypass the heat recovery device whenever the system is in economizer mode.
● Heat Recovery Bypass – Outdoor Air – This option forces all of the outside air to bypass the heat recovery device whenever the outdoor airflow is above the minimum outdoor air flow. If at the minimum outdoor airflow, the system heating coil turns on, the energy recovery device will turn on as well and the outdoor airflow will no longer be bypassed.