### Airflow Control The information below provides a FULL description of this microblock and all of its properties. What information you see and what you can do with it depends on your license and the application you are in, and in some cases, a controller's driver version.
Microblock family | I/O Point microblocks
Icon and symbol |
What it does | Maintains VAV zone airflow at setpoint. Its inputs, outputs, and properties interface with a controller's built-in airflow control algorithm. The algorithm ensures that zone airflow stays above the specified minimum for zone indoor air quality standards. This applies to ZN, ME, and SE line controllers. Enables VAV testing and balancing through your system interface or through the stand-alone Airflow Test and Balance Utility. This microblock allows the Airflow Test and Balance Utility to control the VAV damper and other key zone operations, such as the fan (FAN) and auxiliary heat (AUX HEAT), during commissioning and flow sensor calibration. For more information on testing and balancing, see your system's Help or the Airflow Test and Balance Utility help.
Click for details below How it works Limitations Inputs and outputs Properties Flow input properties Alarms Simulation Programming example Tips and tricks BACnet object property addresses
#### How it works A patented algorithm provides fast response while minimizing overshoot and damper movements, leading to longer actuator life. The algorithm measures the damper curve slope (change in airflow / damper movement) with each damper movement and uses that information to predict the movement required for the next flow adjustment or setpoint change. The algorithm dynamically calculates the deadband around the flow control setpoint based on the current slope. If the damper requires less than a 1-second movement to bring the measured flow to its setpoint, the damper does not move. This dynamic deadband provides accuracy at low flow settings while maintaining stability throughout the damper range. The algorithm provides additional stability by averaging flow sensor readings over a 10-second period and reacting to average, rather than instantaneous readings, and by requiring at least a 5% change in flow setpoint to initiate a damper movement. If measured flow falls below the Occupied Min Airflow while the zone is occupied, the algorithm sends a 1-second open signal to the dampers to ensure that zone airflow stays above the specified minimum for zone indoor air quality standards. The algorithm calculates the flow setpoint based on the microblock's current operational mode. Cooling Mode
If HEAT MODE is... | and the zone is... | the flow setpoint is...
--- | --- | ---
Off | occupied | Occupied Min Airflow + Cooling % x (Cooling Max Airflow - Occupied Min Airflow)
not occupied | Unoccupied Min Airflow + Cooling % x (Cooling Max Airflow - Unoccupied Min Airflow)
NOTE If the Cooling % input is 100%, the flow setpoint is the Cooling Max Airflow. The damper will be at the position required to maintain the flow at this setpoint, which may not be 100% open. Heating Mode For a VAV air handling unit that provides heat, check Use supply air for heating when Heat Mode is ON and connect a reverse-acting controller to the Heating % input.
If HEAT MODE is... | and the zone is... | the flow setpoint is...
--- | --- | ---
On | occupied | Occupied Min Airflow + Heating % x (Heating Max Airflow - Occupied Min Airflow)
not occupied | Unoccupied Min Airflow + Heating % x (Heating Max Airflow - Unoccupied Min Airflow)
For VAV boxes with reheat coils that require a certain amount of airflow from the air handling unit to operate safely and effectively, use the Aux Heat Min Airflow to specify the minimum airflow across the coils.
If AUX HEAT is... | and the zone is... | the flow setpoint is...
--- | --- | ---
On | occupied | The largest of Heating Max Airflow x Heating % (if HEAT MODE is on) or Auxiliary Heat Min Airflow or Occupied Min Airflow
not occupied | The largest of Heating Max Airflow x Heating % (if HEAT MODE is on) or Auxiliary Heat Min Airflow or Unoccupied Min Airflow
#### Limitations This microblock is designed for comfort VAV flow control applications. Used in other applications, some properties may not apply and features intended to extend actuator life, such as the 5% threshold on setpoint adjustments, may not be compatible. If the Automated Logic® controller has an integrated flow sensor, it is a velocity sensor, not a velocity pressure sensor. This can be confusing to technicians who have only worked with velocity pressure sensors. Velocity sensors do not have a K-factor. Flow varies linearly with the sensor reading, not with the square root of the sensor reading, resulting in more precise readings than velocity pressure sensors at low flows. You calibrate the combined flow sensor and VAV box pitot tube array by entering measured flow values in a table. This microblock provides exceptional control of VAV boxes, but it cannot compensate for mechanical problems such as duct restrictions, damper actuator slippage, an oversized VAV box, or a damper motor that is too fast to provide accurate control. Although this microblock applies to various controllers, some sensor and damper configurations apply only to controllers with an integrated flow sensor and damper actuator.
#### Inputs and outputs Inputs
Occupied | True (on) when the zone is occupied. Not true (off) when the zone is unoccupied. Connect to a time clock microblock or to other logic that indicates the zone's occupancy status.
Heat Mode | True (on) when the AHU is in a heating mode. Not true (off) if the AHU provides cooling only or is in a cooling mode.
Cooling % | Cooling called for (%). Connect to a direct-acting controller such as the Zone Controller microblock's CLG% output.
Heating % | Heating called for (%). Connect to a reverse-acting controller such as the Zone Controller microblock's HTG% output.
Fan | Fan start/stop signal. Usually passed directly to the Fan output, unless controlled by the Airflow Test and Balance Utility for testing, balancing, and flow sensor calibration.
Aux Heat Auxiliary Heat | Signal to control the VAV box's auxiliary heat. Usually passed directly to the Aux Heat output, unless controlled by the Airflow Test and Balance Utility for testing, balancing, and flow sensor calibration.
Outputs
Fan | Fan start/stop signal. Usually passed directly from the Fan input, unless controlled by the Airflow Test and Balance Utility for testing, balancing, and flow sensor calibration.
Aux Heat Auxiliary Heat | Signal to control the VAV box's auxiliary heat. Usually passed directly from the Aux Heat input, unless controlled by the Airflow Test and Balance Utility for testing, balancing, and flow sensor calibration.
Actual Flow Flow | Measured airflow (units defined by Flow Measurement Units).
Flow Setpt Flow Setpoint | Airflow setpoint (units defined by Flow Measurement Units) calculated by the airflow control algorithm.
Damper Pos Damper Position | (0–100%). For External damper types, connect to the analog output or floating motor output that controls the damper actuator. For ZNv+ Built In and ZASF damper types, displays the damper position.
#### Properties TIPS Alt+click any value in the WebCTRL® or Field Assistant interface to view property details, including its editing privilege and expression (location path) for use on graphics. You can right-click some properties in the EIKON® Property Editor and select Make editable or Make read-only to determine that property's functionality in your system.
Display Name | The microblock label used in the EIKON® application and the WebCTRL® interface. You can use any characters except the " character.
Reference Name RefName | Use the default reference name unless you want a more descriptive name for graphics or network links. Limitations: lower case only limited to 40 characters cannot begin with a number must be unique within a control program
Description | (optional) A BACnet-visible microblock description.
Lock... ...fan ...auxiliary heat ...Flow Setpoint ...Damper Position | Check to output the locked value from the microblock instead of the microblock's calculated value.
Editing Privilege | Preset - Each microblock property has an appropriate privilege or role assigned to it. You can use Global Modify in the WebCTRL® interface to find out what the actual privilege is. CAUTION If you change the Editing Privilege from Preset, the privilege you select will be used for all properties of this microblock, which is not always desirable.
Flow Measurement Units | The unit of measurement of the microblock's present value. Select from the BACnet engineering units in this droplist. For some microblocks, you can customize the droplist by selecting Options > Preferences > Droplist Options. For display and documentation purposes only. Set to the airflow unit of measurement used during system calibration.
Hardware Configuration
Primary Use | For a typical single-duct VAV system, select Cooling. For a dual-duct system, use 1 Airflow Control microblock for each duct. Select Cooling in the microblock that controls the primary cooling duct. Select Heating in the microblock that controls the heating or ventilating duct.
Sensor | Select the airflow sensor type used by your equipment.
Damper | Select the damper type used by your equipment.
Damper Motor Travel Time | The time (seconds) the damper motor takes to travel from its fully open to its fully closed position.
Direction: CW = | Close - Turn the damper motor clockwise to close the damper. Open - Turn the damper motor clockwise to open the damper. NOTE Applies to integrated actuators only.
Design Properties
Cooling Max Airflow | The maximum zone airflow specified for the cooling mode (HEAT MODE input is off).
Heating Max Airflow | The maximum zone airflow specified for a heating or warm-up mode (HEAT MODE input is on). Typically used if the air handling unit supplies warm air to heat the zone.
Occupied Min Airflow | The minimum airflow specified for ventilation when the zone is occupied. Applies in heating and cooling modes. Usually based on health and safety criteria such as ASHRAE Standard 62-1.
Unoccupied Min Airflow | The minimum airflow specified for ventilation when the zone is unoccupied (usually 0).
Auxiliary Heat Min Airflow | The minimum airflow specified to ensure adequate airflow over a VAV box's auxiliary heating coil. Applies when the AUX HEAT input is greater than zero. Type`0` if the VAV box does not have an auxiliary heating coil or if the box contains a fan that ensures sufficient flow across the coil.
Use supply air for heating when Heat Mode is ON | Check to control the VAV damper with the Heating % input when the air handling unit supplies warm air to the VAV box. Uncheck to provide the appropriate occupied or unoccupied minimum zone airflow during the warm-up period.
Property Page Text
Show Property Page Text | Check to show this microblock's value on the equipment's Properties page.
Property Page Text | You can edit the microblock description that appears on the Properties page. See "Editing Properties page text using special characters" and "Formatting a microblock property" in EIKON® Help.
BACnet configuration
Object Instance | Auto-assign - A BACnet Object ID is assigned by the system. Use specific value - (0–3999999) Assign a number that is unique within the controller.
#### Flow input properties These properties apply to the flow input BACnet object embedded in the Airflow Control microblock. You can think of this object as a microblock within a microblock.
Display Name | The microblock label used in the EIKON® application and the WebCTRL® interface. You can use any characters except the " character.
Description | (optional) A BACnet-visible microblock description.
Input Resolution | The increment by which the microblock updates the value on its output wire in the WebCTRL® or Field Assistant system. The Resolution format is used to truncate the microblock’s actual value. For example, if you enter a value from: 0.1 to 0.9, the wire displays 1 digit to the right of the decimal 0.01 to 0.99, the wire displays 2 digits to the right of the decimal 1 or greater, the wire displays a whole number The Resolution value determines the increment by which the present value is updated. For example, if you enter: .2, the wire displays 8.4, 8.6, 8.8, ... .03, the wire displays 5.09, 5.12, 5.15, ... 10, the wire displays 30, 40, 50, ...
Lock | Check to output the locked value from the microblock instead of the microblock's calculated value.
BACnet Configuration
Network Visible | Check to allow other BACnet equipment to read or change the microblock's present value. Must be checked for this microblock to generate alarms.
Object Instance | Auto-assign - A BACnet Object ID is assigned by the system. Use specific value - (0–3999999) Assign a number that is unique within the controller.
COV Increment | An Analog Network Input (ANI) that references this microblock in its Address field tries to subscribe to this microblock's COV (Change of Value) service. If subscription succeeds, the ANI receives a value from this microblock only when this microblock's present value changes by at least the COV Increment. If subscription fails, the ANI reads this microblock's value at intervals specified in the ANI's Refresh Time field.
Trend
Enable Trend Log | Check to have the controller collect trend data for the microblock's present value.
Sample every ___ (hh:mm:ss) | Records the microblock's present value at this interval. EXAMPLE Type `00:10:00` to record the microblock's present value every 10 minutes.
Sample on COV (change of value) | Records the microblock’s present value only when the value changes by at least the COV Increment.
Max samples | The number of data samples the controller allocates memory for. Memory consumption is 10 bytes for each sample plus 48 bytes. For example, for 100 samples: (100 x 10 bytes) + 48 = 1048 bytes of memory The allocated memory is constant regardless of how many samples are actually recorded. If you do not enable trending, no memory is consumed. NOTE Click Reset on the Properties page in the WebCTRL® or Field Assistant system to delete all samples currently stored in the controller.
Enable Trend Historian | Check this field to archive the controller's collected trend data to the system database after every 129 data samples. NOTES You must check Enable Trend Log if you want to Enable Trend Historian. You can change Enable Trend Historian archival settings and other trend properties on the Properties page in the WebCTRL® or Field Assistant system.
Keep historical trends for __ days | This is based on the date that the sample was read. Set this field to 0 to use the system default value.
Write to historian: Every ___ trend samples Use default (45% of Max samples) | Writes all trend data in the controller to the system database each time the controller collects the specified number of samples. You can select Every __ trend samples and enter a number greater than zero and less than the number in the Max samples field, or you can select Use default. The number of trends specified must be accumulated at least once before the historical trends can be viewed.
In the WebCTRL® or Field Assistant system only: |
Stop When Full | Check this field to stop trend sampling when the maximum number of samples is reached.
Enable trend log at specific times only | Collects trend data for the specific period of time you define in the time and date fields.
Store Trends Now | Writes all trend data in the controller to the system database without having to enable trend historian.
Trend samples accumulated since last notification | Shows the number of samples stored in the controller since data was last written to the database.
Last Record Written to Historian | Shows the number of trend samples that were last written to the database.
Delete | Deletes all trend samples stored in the database for the microblock.
BACnet Configuration | The Object Name is a unique alphanumeric string that defines the BACnet object. Although the Object Name field can be edited, it is not recommended. The Notification Class is set to 1 to receive alarms generated by Automated Logic® controllers.
#### Alarms
Potential alarm source | Check to make this microblock available in the system's Alarm Sources list.
Critical | Determines the color of the system-wide alarm button when the alarm comes in if the alarm's Template field is set to Universal. = Critical = Non-critical
Category | The category you want to use to filter this microblock's alarms on the system's Alarms page > View tab.
Template | Universal - Allows your system to use the Alarm text and Return text defined in the microblock, and the Critical checkbox to determine the color of the system-wide alarm button when the alarm comes in.
Alarm
Low Limit Enable | Check to send an alarm when the microblock's present value remains below the Low Limit value for the defined Delay Seconds.
Low Limit | The value the microblock's present value must drop below to send an alarm.
High Limit Enable | Check to send an alarm when the microblock's present value remains above the High Limit for the defined Delay Seconds.
High Limit | The value the microblock's present value must rise above to send an alarm.
Dead Band | The amount inside the normal range by which an alarm condition must return before a return-to-normal notification is generated. EXAMPLE
Delay Seconds | The time the microblock's present value must remain in an alarm condition before the microblock sends an alarm.
Alarm text | The message displayed on the WebCTRL® Alarms page > View tab when an alarm condition occurs. A relative path is useful for alarm messages that you will reuse in multiple WebCTRL® locations because the path is relative to the item that contains the path.
Alarm requires acknowledge | Check to require that an operator acknowledge alarm notifications on the system's Alarms page > View tab.
Return to Normal
Return Enabled | Check to send a message when an alarm condition has returned to normal.
Return text | The message displayed on the WebCTRL® Alarms page > View tab when an alarm condition returns to normal. A relative path is useful for alarm messages that you will reuse in multiple WebCTRL® locations because the path is relative to the item that contains the path.
Return requires acknowledge | Check to require that an operator acknowledge return-to-normal notifications on the system's Alarms page > View tab.
Fault
Fault Enabled | Check to send a message when a fault condition occurs, such as a misconfigured or non-existent sensor.
#### Simulation Define the value(s) the microblock will use when you simulate the control program.
#### Programming example This simple VAV application with no fan and no auxiliary heat compares the zone temperature to the supply air temperature from the air handling unit (AHU) to determine whether the AHU is in a heating mode. This airflow microblock is configured for a controller with an integrated actuator, so the microblock controls the actuator directly and no additional output points are needed for control.
#### Tips and tricks Cooling-only VAV boxes Uncheck Use supply air for heating when Heat Mode is ON. The microblock will maintain the applicable minimum flow. Dual-duct VAV boxes Use 2 Airflow Control microblocks. In the microblock that controls the cooling damper, lock the HEAT MODE off and set Primary Use to Cooling. In the microblock that controls the heating damper, lock the HEAT MODE on and set Primary Use to Heating. Examples of dual duct applications can be found in EquipmentBuilder. Industrial process ventilation or off-hours skeleton crews You can use the Unoccupied Min Airflow to provide a different minimum flow during unoccupied periods for industrial process ventilation, for skeleton staffs, or for other reasons. Deadhead protection To prevent deadheading the fan (running the fan with the outlet blocked), some VAV systems require a minimum number of open dampers before the fan starts. Typically, when a building is unoccupied and the AHU fan is off, VAV dampers are closed. To provide deadhead protection, set Unoccupied Min Airflow to a non-zero value in the VAV boxes you want to leave open. When the system is unoccupied and the fan is turned off, these VAV boxes will open their dampers fully in an attempt to maintain the Unoccupied Min Airflow setpoint. Smoke control - unbounded inputs Cooling % and Heating % are not limited to 0–100%. A PID microblock or Zone Controller microblock output stays within the 0–100% range, but for special circumstances like smoke control you can switch these inputs to other signals. For example, to force the dampers fully open, switch HEAT MODE to an off signal and switch Cooling % to a value such as 500%. This forces the damper to open fully trying to reach an unrealistically high flow setpoint. Pressure-dependent control To control a pressure-dependent VAV box (a box with no flow pickup or flow measurement, where the damper moves from 0% to 100% open proportionally with the zone temperature), check Lock Damper Position to on the microblock's Properties page and use a BACnet Analog Network Output microblock to write the desired damper position to the Airflow Control microblock's Damper Lock property. See BACnet object property addresses below. You can connect a Zone Controller microblock's CLG% output to the Network Output microblock's input, and use a Constant Low Limit microblock to impose a minimum damper position. BACnet properties The Airflow microblock is a proprietary BACnet object. If you make this object Network Visible, you can address a BACnet Analog Network Input or BACnet Analog Network Output microblock to access many of its configuration and control properties in other control programs. See BACnet object property addresses below.
#### BACnet object property addresses The Airflow Control microblock is a proprietary BACnet object (object type 768). The format for a BACnet address is bacnet://device/object/property@priority. See To format a BACnet address for BACnet address syntax options and information. EXAMPLE To set up a microblock to read the Cooling Output (%) from the first Airflow Control microblock in the same controller, use the following address: `bacnet://this/768:1/4114` In the above address, 768:1 indicates the first instance of an Airflow Control microblock in the controller. If writing to a dual-duct application with two Airflow Control microblocks, the address in the second microblock would have 768:2. NOTE These properties are only available with a v2-03-009 or newer driver.
BACnet property identifier # | BACnet property identifier | Description | Read/ Write
--- | --- | --- | ---
4096 | Max Occ Cooling Flow | Real - units determined by flow AI. | r/w
4097 | Max Occ Heating Flow | Real - units determined by flow AI. | r/w
4098 | Min Occupied Flow | Real - units determined by flow AI. | r/w
4099 | Min Unoccupied Flow | Real - units determined by flow AI. | r/w
4100 | Min Aux Heat Flow | Real - units determined by flow AI. Minimum airflow to maintain while aux heat is active. | r/w
4101 | Flow at 1 inch WC | Real - units determined by flow AI. VAV manufacturer-provided data used for baseline control. | r/w
4102 | Flow Calibration | Real - units determined by flow AI. Array of 4 measured flow calibration properties. | r/w
4103 | Sensor Calibration | Real - units determined by flow AI. Array of 4 raw sensor calibration properties. | r/w
4104 | Aux Heat Lock | Real - lock value (%) for AUX HEAT output wire. | r/w
4105 | Damper Lock | Real - lock value (% open) for VAV damper. | r/w
4106 | Flow Setpoint Lock | Real - lock value, units determined by flow AI. | r/w
4107 | Auto-Zero | Boolean - indicates completion of auto-zero. | r/w
4108 | Test and Balance Mode | Enumeration of states of damper control while test and balance is performed. | r/w
4109 | Parent Program Device ID | Device ID of device containing air source object. | r
4110 | Parent Program ID | Program AI of program containing air source object. | r
4111 | Air Source Object ID | Air source BACnet object ID. | r/w
4112 | Occupied Mode | Boolean - true if occupied. | r
4113 | Heat Mode | Boolean - true if AHU in heating mode. | r
4114 | Cooling Output | Real - percentage cooling demand. | r
4115 | Heating Output | Real - percentage heating demand. | r
4116 | Auxiliary Heat Output | Real - percentage of aux heat demand. | r
4117 | Damper Output | Real - damper position (% open). | r
4118 | Actual Flow | Real - units determined by flow AI. | r
4119 | Flow Setpoint | Real - units determined by flow AI. | r
4120 | Air Flow Object ID | BACnet Object ID of embedded flow AI object. | r
4121 | Obsolete. Use 4141. |  |
4122 | Loop config | Cooling or Heating (Primary Use). | r/w
4123 | Sensor Config | Sensor configuration—Internal, ZASF, or External. | r
4124 | Damper Config | Damper configuration—Internal, ZASF, or External. | r
4125 | Raw sensor reading | Real - raw sensor value, % of scale of 0-1.0" WC. | r
4126 | Raw sensor setpoint | Real - raw sensor Setpt, % of scale of 0-1.0"WC. | r
4127 | Moves Today | Number of damper moves, current day. | r
4128 | Moves Yesterday | Number of damper moves yesterday. | r
4129 | Fan Lock | Fan lock value—on or off. | r/w
4130 | Fan Output | Status of fan pass-through wire. | r
4131 | Status Flags | BACnet status flags. | r
4132 | Test and Balance Tech Name | Name of test and balance technician. | r/w
4133 | Test and Balance Org Name | Name of test and balance company. | r/w
4134 | Last Test and Balance Date | Date of last test and balance activity through stand-alone Airflow Test and Balance Utility. | r/w
4135 | Display Name | Copy of Display Name provided by microblock. | r/w
4136 | Owning Program ID | Object ID for program containing this instance. | r
4137 | Override Flags | Unsigned - Status flags of property overrides in effect. | r
4138 | Max Occ Cooling Flow Override | Real - volatile override for Max Occ Cooling Flow (4096). | r/w
4139 | Min Occ Flow Override | Real - volatile override for Min Occ Flow (4098). | r/w
4140 | Pars Stamp | BACnet DateTime - volatile, used to detect writes by external test and balance software. | r/w
4141 | Lock Flags | Binary lock bits whether to apply locks. Include one of the following index numbers in the address field: 4141(4) Holds the damper in its current position. 4141(5) Locks the fan to the Fan Lock State property value. 4141(6) Locks the damper to the Damper Position Lock Value property. 4141(7) Locks the aux heat to the Aux Heat Percentage Lock Value property. 4141(8) Locks the flow setpoint to the Flow Setpoint Lock Value property. NOTE 4141(0) creates a "Write access denied" error message. For example, `bacnet://this/768:1/4141(4)` | r/w