Unit Ventilators
Unit ventilators are zone level equipment units which are assembled from other components. They contain a built-in outdoor air mixer, a fan, a heating coil, and a cooling coil. These components are defined in other areas of the equipment library and are referenced by the unit ventilator library members.
In general, the unit ventilator input is very similar to the fan coil unit in that it is connected to a hot water loop through its hot water coil and to a chilled water loop through its cooling coil. The main difference between the fan coil and unit ventilator is that the unit ventilator has a built-in outdoor air mixer with its own specialized controls.
The unit ventilator is controlled to meet the zone heating or cooling demand. If there is a heating demand, the cooling coil is off and the hot water flow through the heating coil is throttled to meet the demand. If there is a cooling demand from the zone, the hot water coil is off and the chilled water flow through the cooling coil is throttled to meet the load.
Note: Standard TRACE library members cannot be modified or deleted but they can be copied and edited.
Operating Mode
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Default: Cooling and Heating
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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Three operating modes are able to be selected for the Unit Ventilator library members: Cooling and Heating, Cooling Only, and Heating Only. The number of additional input fields used to define the Unit Ventilator are dependent on the mode selected.
Nominal Cooling Capacity
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Default: Auto Size box checked
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Typical Range: 1 to 200 Mbh
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: % Clg Capacity; W; kW; W/m2; W/ft2; tons; Btuh; MBh; tons/ft2; Btuh/ft2
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Cooling Only. If available, this checkbox indicates whether or not the capacity used for the selected cooling coil will be auto sized by the program or referenced from the user defined input.
Nominal Heating Capacity
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Default: Auto Size box checked
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Typical Range: 41.3 to 705 Mbh
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: % Htg Capacity; W; kW; W/m2; W/ft2; tons; Btuh; MBh; tons/ft2; Btuh/ft2
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Heating Only. If available, this checkbox indicates whether or not the capacity used for the selected heating coil will be auto sized by the program or referenced from the user defined input.
Supply Fan
Supply Fan Placement
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Default: Blow Through
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This input is used to determine if the supply fan is placed up stream or downstream of the coils. Two options are available: Blow Through or Draw Through.
Cycling Mode
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Default: Cycling Fan
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This is the input used to describe the operational control of the supply fan. Two options are available: Cycling Fan or Continuous Fan.
If the Cycling Fan option is selected, then the supply fan for the unit ventilator will cycle on and off in operation in step with the coil operation of the equipment. If the Continuous Fan option is selected, the supply fan for the unit ventilator will continue to operate whether or not there is a cooling or heating load that needs to be met.
Supply Fan Type
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Default: Constant Volume
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This is the type of supply fan that is associated with the library member. Two options are available: Constant or Variable Volume.
Supply Air Fan
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Default: Direct Drive Plenum Fan
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This input selection refers to the name of the fan library member that is used in this object.
Fan Full Load Energy Rate
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Default: 0.0002 kW/(cfm) ; Auto Size box is unchecked
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Typical Range: 186 to 200 W
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: W; kW; W/(m3/sec); W/(m3/sec)-Pa; W/(cfm); W/(L/sec); W/(L/sec)-Pa; kW/(cfm); kW/(L/sec); kW/(cfm)-in H2O; kW/(L/sec)-Pa; W/(cfm)-in H2O; bhp
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This checkbox indicates whether or not the full load energy rate used for the selected fan will be auto sized by the program or referenced from the user defined input.
Pressure Rise
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Default: 4 inches H2O (From fan library member selected)
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Typical Range: 0.25 to 3 in H2O
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: in H2O; lb/ft2; psig; Pa; kPa; mm Hg; mm H2O; bar; in Hg; atm; mm CE; m CE; m H2O; mbar
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The value will be populated by default based on the fan library member selected. You can override this value and the units of measure.
Dead Band Airflow
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Default: Auto size box selected
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Typical Range: 0 to 1500 CFM
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: m3/sec; cfm; cfm/ft2; (m3/s)/m2; % Clg Airflow; % Htg Airflow; L/s; m3/hr; (m3/hr)/m2
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This numeric field defines the supply air flow rate leaving the Unit Ventilator when no cooling or heating is required (i.e. neither coil is operating).
Unitary Cooling
Unitary Cooling Type
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Default: Water (Detailed)
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Cooling Only. This field specifies the type of cooling coil used for this Unit Ventilator. Two options are available to choose from: Water (Detailed) or Water (Simple).
Cooling Coil
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Default: 10 Row - Copper Fins - Prima Flo
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Cooling Only. This field allows you to select the cooling coil library member that will be used for this Unit Ventilator equipment.
Cooling Design Airflow
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Default: Auto size box checked
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Typical Range: 750 to 1500 CFM
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: cm3/s; m3/s; cfm; cfm/ft2; (m3/s)/m2; % Htg Airflow; cfm/ton; (m3/s)/(kW); (m3/hr)/(kW); (m3/s)/(W); m3/hr ; m3/hr/m2
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Cooling Only. This input refers to the design cooling airflow value associated with the Unit Ventilator library member.
Unitary Heating
Unitary Heating Type
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Default: Electric
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Heating Only. This field specifies the type of heating coil to model for this Unit Ventilator. Four options are available to choose from: Electric, Gas, Steam, Water.
Unitary Heating
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Default: Electric Heating Coil
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Heating Only. This field allows you to select the heating coil library member that will be used for this Unit Ventilator equipment.
Unitary Heating Design Airflow
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Default: Auto Size box checked
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Typical Range: 750 to 1500 CFM
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: cm3/s; m3/s; cfm; cfm/ft2; (m3/s)/m2; % Htg Airflow; cfm/ton; (m3/s)/(kW); (m3/hr)/(kW); (m3/s)/(W); m3/hr ; m3/hr/m2
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This option will not be available for the Unit Ventilator unless the Operation Mode is set to Cooling and Heating or Heating Only. This input refers to the design heating airflow value associated with the Unit Ventilator library member.
Outdoor Air Control
Outdoor Air Control
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Default: Fixed Minimum Airflow
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Typical Range: N/A
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Min Max: N/A
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Units: N/A
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This input refers to the outdoor air control used by the Unit Ventilator library member. Three options exist: Fixed Minimum Airflow, Variable Minimum Airflow, and Variable Min. Airflow – Fixed Temp.
The variable minimum airflow – fixed temp control option varies the amount of outdoor air between the minimum outdoor air fraction and 100% outdoor air to obtain a mixed air temperature as close as possible to the space set point temperature for the given time frame. The variable minimum airflow control option will also vary the amount of outdoor air between the minimum and maximum values defined by the user to meet the load without the use of a coil if possible. For the fixed minimum airflow control, the outdoor air flow rate is fixed to the specified value by the user.
These control types are based on the 2004 ASHRAE Systems and Equipment Handbook, pages 31.1 to 31.3, description of unit ventilator systems.
Minimum Outdoor Airflow
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Default: Auto Size box checked
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Typical Range: 0 to 1500 cfm
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Min Max: 0 to 1,000,000,000,000,000 (this field cannot be left blank)
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Units: L/s; m3/s; cfm; m3/hr
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This input sets the minimum airflow value associated with the Unit Ventilator library member.