CET supply air VAV box, external flow control (CetVavSu11)

Overview

The application function "CET room pressurization supply air VAV box 11, external air volume flow controller, air damper" (CetVavSu11) connects to an external airflow controller / actuator. It outputs a setpoint signal (VavSuSpAirFl) to the controller and receives a measured flow signal from the controller. The setpoint signal is calculated based on demand signals received for heating, cooling and ventilation. Flow control performance depends on the external airflow controller / actuator.

Main features:

VAV pressurization, temperature and ventilation
Airflow setpoint calculation
Interface logic for external airflow controller / actuator
Interlocks (box coils)
Supply chain interface

This AF does not include functions to support air balancing.

Function

The figure below shows BACnet objects associated with this application function. Primary signal flow is summarized as follows:

Basic function: Accept request signal(s) (VavSuAirFlReq and VavSuAflScale) from associated room controller and map it to the appropriate airflow setpoint; Pass the signal through a device mode logic switch prior to sending it (as VavSuSpAirFl) to the external airflow controller / actuator.

	Command or request (or related)
	Notification of condition or status, or availability
	Device mode
	Interlock (internal signal, not a BACnet object; see Interlocks section for additional information)

Device mode: The input signal for device mode is a multistate value.

VavSuDevMod supports the following states:

Off
Control mode
Max.air vol.flow
Min.air vol.flow
Smoke ctrl.air flow setp.

Available status: When the VAV supply damper is available for heating or cooling or ventilation, the respective binary output signal(s) that indicate availability (VavSuAvlH, VavSuAvlC, VavSuAvlVnt) will be "Yes" (available).

For available status to be "Yes", device mode must equal "Control mode" (modulation) and VavSuChovrCnd (Supply air VAV changeover condition) cannot equal "Neither".

Airflow control loop (cascade control)

The airflow control loop and airflow sensing are implemented outside this AF, in a separate device. Other aspects of the cascade control are implemented in this AF.

Airflow setpoint for external airflow controller / actuator: The airflow setpoint AO (VavSuSpAirFl) derives from the selection of input values; one request signal for lab and a signal to determine how the request should be scaled for heating, cooling and ventilation (VavSuRqdH, VavSuRqdC, and VavSuAirFlReq respectively). The demand values are calculated by room control functions and normalized 0 to 100%.
Input signals from external airflow controller / actuator: When an external controller / actuator is used, the AF requires a volumetric airflow input signal from a differential pressure sensor. This AI signal (VavSuAirFl) is in units of airflow. It is used to determine airflow status. It is also used by the supply chain logic. The AI for supply damper position (VavSuPos) is used only for display purposes.

The damper AO object, VavSuPos, drives the damper (0-10vdc or PlLink).

ABT 5.x and later:

Airflow setpoint selection: The AF calculates the airflow setpoint for the terminal to satisfy the demand of one of the supported airflow drivers:

Heating
Cooling
Ventilation
Make-up

The AF maps the demand level in percent to the flow range (min/max values) configured for the active driver. The result is the airflow setpoint for the terminal in physical flow units (ft³/min, m³/h, l/s). Mapping of the demand level to the flow range is as shown in the diagram.

The objects indicating demand level, (VavSuAirFlReq) and the active driver (VavSuAflScale) are inputs to the AF. They are written by other functions. The airflow limits for the terminal are configured here.


Cooling	Heating

Ventilation	Make-up

That setpoint mapping is normal operation. It applies when the device mode (VavSuDevMod) is Control Mode. Special values of device mode alter the setpoint as follows:

Off – Setpoint is 0 and damper is closed
Control Mode – Setpoint follows % demand
Maximum airflow setpoint – Setpoint is maximum for the currently active airflow scale
Minimum airflow setpoint – Setpoint is minimum for the currently active airflow scale
Manual airflow setpoint – Setpoint is value configured for smoke control

Supply Air Flow Limits for Air Terminals - Configuration

The Supply Air Flow serves multiple functions: heating, cooling, ventilation, and pressurization. The following section explains how to configure each supply terminal for different use cases.

	Cooling	Heating	Ventilation
Single CV (Constant Volume) Supply Terminal	Min = Max = 0 (or minimum flow value needed for the coil to cool the room)	Min = Max = 0 (or minimum flow value needed for the coil to heat the room)	Min = 0 Max ≥ The largest value configured in Ventilation AF
Single VAV (Variable Air Volume) Supply Terminal (no fume hood)	Min = 0 (or minimum flow value needed for the coil to cool the room) Max = Value from box schedule	Min = 0 (or minimum flow value needed for the coil to heat the room) Max = Min (if sequence does not vary flow to heat) If sequence does vary flow to heat: Max = Value from box schedule	Min = 0 Max Ventilation ≥ Largest value configured in Ventilation AF
Single VAV Supply Terminal (fume hood)	Min = 0 (or minimum flow value needed for the coil to cool the room) Max = Value from box schedule	Min = 0 (or minimum flow value needed for the coil to heat the room) Max = Min (if sequence does not vary flow to heat) If sequence does vary flow to heat: Max = Value from box schedule	Min Ventilation = 0 Max Ventilation ≥ Largest value configured in Ventilation AF or value needed to balance maximum fume hood flow

Check the project specification to see if the various terminals are to be sequenced differently from each other: if they respond to different needs. If no difference is specified, follow the next table's instruction. If the flow limits are set the same for each terminal, then they run at equal flows.

	Cooling	Heating	Ventilation
Multiple Supply Terminals (same size, same function)	Min = 0 (or minimum flow value needed for the coil to cool the room) Max = Value from box schedule (specified per terminal)	Min = 0 (or minimum flow value needed for the coil to heat the room) Max = Min (if sequence does not vary flow to heat) If sequence does vary flow to heat: Max = Value from box schedule	Min = 0 Max ≥ The largest of configured ventilation levels for the room and the supply flow needed to balance fume hoods, divided by the number of supply terminals

Check the project specification to see if the various terminals are to be sequenced differently from each other: if they respond to different needs. If the supply terminals are different sizes but have the same function, follow the next table's instruction.

	Cooling	Heating	Ventilation
Multiple Supply Terminals (different sizes, same function)	Min = 0 (or minimum flow value needed for the coil to cool the room) Max = Value from box schedule (specified per terminal)	Min = 0 (or minimum flow value needed for the coil to heat the room) Max = Min (if sequence does not vary flow to heat) If sequence does vary flow to heat: Max = Value from box schedule	Min = 0 Max ≥ 1. The largest value of the terminal being configured or 2. The value needed to balance maximum fume hood flow of the terminal being configured

In the case of multiple supply terminals with different functions, the following tables provide an example of how to configure one CV (Constant Volume) Terminal serving a chilled beam, and one VAV (Variable Air Volume) Terminal without cooling.

Flow through the CV Terminal does not vary to heat or cool.
Flow is always set by ventilation or support for coil (minimum heat or minimum cooling) unless reducing it is the only way to maintain room pressurization.

Flow through the VAV Terminal may vary for heating, cooling, and ventilation, or to balance fume hood flow.

	Cooling	Heating	Ventilation
Multiple Supply Terminals (different functions) CV Terminal	Min = Max = 0 (or minimum flow value needed for the coil to cool the room)	Min = Max = 0 (or minimum flow value needed for the coil to heat the room)	Min = Max = 0 (or value needed to support a heating or cooling coil)
Multiple Supply Terminals (different function) VAV Terminal	Min = 0 (or minimum flow value needed for the coil to cool the room) Max = Value from box schedule (specified per terminal)	Min = 0 (or minimum flow value needed for the coil to heat the room) Max = Min (if sequence does not vary flow to heat) If sequence does vary flow to heat: Max = Value from box schedule	Min = 0 Max ≥ 1. The largest value of the terminal being configured or 2. The value needed to balance maximum fume hood flow of the terminal being configured

ABT 4.x and earlier

Airflow setpoint selection: The AF calculates the airflow setpoint for the terminal to satisfy the demand of one of the supported airflow drivers:

Heating
Cooling
Ventilation
Make-up


Cooling	Heating

Ventilation	Make-up

That setpoint mapping is normal operation. It applies when the device mode (VavSuDevMod) is Control Mode. Special values of device mode alter the setpoint as follows:

Off – Setpoint is 0 and damper is closed
Control Mode – Setpoint follows % demand
Maximum airflow setpoint – Setpoint is maximum for the currently active airflow scale
Minimum airflow setpoint – Setpoint is minimum for the currently active airflow scale
Manual airflow setpoint – Setpoint is value configured for smoke control

Feedback loop: The airflow control loop is implemented on a separate device. The relative values are used to communicate with the external flow controller. The configuration property, NominalFlow is the scaling value; it is not altered by the maximum flow values. The value of nominal flow must be set to match the scaling of the flow controller. The calculated value of relative setpoint is limited between 0 and 100%.

Flow sensor failure: Response of the flow control loop to a sensor failure depends on the separate flow controller.

Sensor calibration: No sensor calibration feature is implemented in this AF.

Airflow sensing: The airflow control loop and airflow sensing are implemented outside this AF, in a separate device.

Interlocks

Room automation interlocks are internal signals that coordinate interaction between HVAC devices. They are not visible in ABT Site or web interface, but parameters associated with them are. See comment column for hints on parameters that affect interlock functionality.

Signal	Type	Dir.	Description	Comment
AirFlCReq	Boolean	In	Airflow cooling request ▶ Initiates airflow (damper opens) to satisfy airflow support request from cooling coil.
AirFlHReq	Boolean	In	Airflow heating request ▶ Initiates airflow (damper opens) to satisfy airflow support request from heating coil.
AirFlHldH	Boolean	In	Airflow hold for heating ▶ Interlock signal from electric heating coil indicating the coil needs equipment protection airflow support. The damper is prevented from closing for a period of time.
AirFlHldC	Boolean	In	Airflow hold for cooling ▶ Interlock signal from cooling coil indicating the coil needs equipment protection airflow support. The damper is prevented from closing for a period of time.
AirFlSta	Boolean	Out	Airflow status ▶ Interlock signal that keeps the coil locked at equipment protection priority (PrPrio = 5) unless the supply damper provides air flow.	See Configuration section for parameters named "...AirFlSta"

Supply chain interface: There are four demand output signals for air.

VavSuCDmd - Supply air VAV cooling demand
When device mode is in control, VavSuCDmd is the percentage value of the cooling request from the room (otherwise it is 0).
VavSuHDmd - Supply air VAV heating demand
When device mode is in control, VavSuHDmd is the percentage value of the heating request from the room (otherwise it is 0).
VavSuVntDmd - Supply air VAV ventilation demand
When device mode is in control, VavSuVntDmd is the percentage value of the ventilation request from the room (otherwise it is 0).
VavSuAirDmd - Supply air VAV air demand for plant mode
VavSuAirDmd is a multistate value that equals PltOpMod (VavSuSpAirFl must be greater than the switch-on point for airflow demand, SwiOnAirFlDmd). If PltOpMod is "Emergency Off" or "Smoke Negative Pres.", VavSuAirDmd will equal PltOpMod regardless of VavSuSpAirFl value.

Airflow deviation signal: The supply airflow deviation (VavSuAirFlDvn) signal (in percent) is used for fan speed (static pressure) reset strategies at the air handling unit. It is obtained by measuring the airflow from the supply duct and comparing it to the airflow setpoint.

VavSuAirFlDvn = VavSuSpAflRel minus VavSuAirFlRel

VavSuAirFlDvn will equal 0 in case of invalid condition(s).

Saturation signal: The saturation signal VavSuAflStrtn is a binary object that is True ("Starved") when the airflow control loop cannot get enough air to reach setpoint for a time exceeding a built-in time delay.

Note
VavSuAflStrtn is always off if parameter EnStrtnCal = 0 (No). This allows the user to exclude a particular terminal from the saturation pressure reset system.

In order for Saturation Signal to be True:
1. The Enable Saturation Calibration parameter must be set to Yes (EnStrtnCal=Yes)
2. The output of the VAV controller must be greater than the saturation level (VavEhAirFlCtr>StrtnLvl)
3. The air flow error, which is the setpoint minus airflow value, must be greater than the air flow error limit (AirFlEr>AirFlErLm)

Saturation Signal can only be True when all three parts are satisfied for the duration of DlyOnStrtn.

AHU changeover condition signal: The multistate VAV supply changeover condition input signal (VavSuChovrCnd) comes from the room coordinating function (RCoo). It indicates the current heating / cooling available status for central AHU. If the supply air system does not include a changeover function, do not connect this value to a source of data. Configure the value to the normal AHU operation state. The default is Cooling.

Four states are supported:

Neither
Heating
Cooling
Neutral

"Neutral" means that heating and cooling PID controllers are both enabled and that the system is providing air that is not hot or cold.

Note
In some cases, damper position data is not available for collection by the central AHU application or for calculating the saturation signal.

Configuration

Objects

Description	Object	Type	Default value
Supply air VAV smoke control air volume flow setpoint ▶ Airflow setpoint if Supply air VAV device mode=5 unless overwritten.	VavSuSpAflSmk	ACnfVal	50 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV maximum air volume flow for cooling ▶ Maximum airflow setpoint when in cooling mode. Typically this is related to the size of the terminal.	VavSuAirFlMaxC	ACnfVal	100 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Supply air VAV minimum air volume flow for cooling ▶ Minimum airflow setpoint when in cooling mode. Minimum cooling flow should be set to zero (0) in most installations without a cooling coil in the terminal. If the terminal has a cooling coil, minimum cooling flow should be set high enough for effective heat transfer.	VavSuAirFlMinC	ACnfVal	50 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV maximum air volume flow for heating ▶ Maximum airflow setpoint when in heating mode. Often set at a level needed to transfer heat from the reheat coil to the room at maximum heating demand.	VavSuAirFlMaxH	ACnfVal	100 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Supply air VAV minimum air volume flow for heating ▶ Minimum airflow setpoint when in heating mode. Often set at a level needed to transfer heat from the reheat coil to the room at low load and can be set lower if primary air is warm.	VavSuAirFlMinH	ACnfVal	50 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV maximum air volume flow for ventilation ▶ Maximum ventilation airflow setpoint for indoor air quality (IAQ) controller and for Rapid Ventilation. Sometimes this can be equal to cooling maximum, sometimes less. This allows ventilation and cooling capacities to be specified separately.	VavSuAflMaxVnt	ACnfVal	100 [m³/h] 58.9 [ft³/min] 27.78 [l/s]

Parameters

Description	Parameter	Default value
Switch delay for tracking method to air volume flow ▶ The airflow tracking signal that is sent to the room will automatically switch from setpoint to airflow when: Absolute Value (Relative Setpoint – Relative Air Flow) > SwiTolTckMthd, for a time longer than SwiTolTckMthd. SETTING TOLERANCE LEVEL (SwiTolTckMthd) TO 100% WILL NOT ALWAYS PREVENT TRACKING FROM SWITCHING FROM SETPOINT TO AIRFLOW.	SwiDlyTckMthd	60 [s]
Switch tolerance for tracking method to air volume flow ▶ The airflow tracking signal that is sent to the room will automatically switch from setpoint to airflow when: Absolute Value (Relative Setpoint – Relative Air Flow) > SwiTolTckMthd, for a time longer than SwiTolTckMthd. SETTING TOLERANCE LEVEL (SwiTolTckMthd) TO 100% WILL NOT ALWAYS PREVENT TRACKING FROM SWITCHING FROM SETPOINT TO AIRFLOW.	SwiTolTckMthd	5.0 [%]
Enable deviation calculation ▶ Enables the calculation of the deviation value (airflow setpoint minus airflow value) to be available for AHU fan static setpoint reset. See also section Airflow deviation signal. 0:No 1:Yes	EnDvnCal	1:Yes
Enable saturation calculation ▶ Enables the saturation signal calculation logic for AHU fan static pressure reset. See also section Saturation signal. Set to "Yes" to include this VAV in the AHU static fan pressure reset logic. Set to "No" to exclude undesired VAV from the AHU static fan pressure reset logic. An example of an undesired VAV would be a VAV that is part of a rogue zone. 0:No 1:Yes	EnStrtnCal	1:Yes
Saturation level ▶ The minimum VAV damper position before the saturation logic would send a starved signal (if other conditions are true; see Saturation signal section).The saturation level is used for AHU static fan pressure reset to maximize energy efficiency by reducing airflow resistance in the duct system due to the damper. A higher value prioritizes energy efficiency while a lower value prevents insufficient airflow.	StrtnLvl	90 [%]
Air volume flow error limit ▶ The maximum allowable tolerance below airflow setpoint before the saturation logic would send a starved signal (if other conditions are true; see Saturation signal section). The airflow error limit is used to allow deviation below the airflow setpoint when the saturation level is exceeded. When set to 0, ignores calculation for AirFlErLm.	AirFlErLm	0 [%]
Switch-on delay saturation ▶ The time after the saturation conditions are met that the saturation signal to the AHU changes from False to True.	DlyOnStrtn	60 [s]
Switch-on point for air flow demand ▶ Airflow value (in percent of the largest configured max airflow) that defines the airflow demand switch-on point. The airflow setpoint VavSuSpAirFl must equal or exceed this value before the terminal box begins to open.	SwiOnAirFlDmd	4 [%]
Hysteresis for air flow demand ▶ Hysteresis offset used to determine the airflow demand cutoff value. SwiOnAirFlDmd minus HysAirFlDmd = the airflow value that VavSuSpAirFl needs to fall below before the damper closes.	HysAirFlDmd	2 [%]
Switch-on point for air volume flow state ▶ The airflow above which the VavSuAirFlTck signal switches from the supply airflow setpoint to the actual supply airflow (assuming SpSelVavEx is set to True).	SwiOnAirFlSta	10 [%]
Hysteresis for air volume flow state ▶ The airflow below SwiOnAirFlSta, below which the VavSuAirFlTck signal switches from the supply airflow to the supply airflow setpoint (assuming SpSelVavEx is set to True).	HysAirFlSta	5 [%]

Interface

Interface		Description	Type	Ref.	Owned by
VavSuSpAirFl		Supply air VAV setpoint for air volume flow	AO	◉	Room segment / Field device
TrndVavSuSpAfl		Trend for supply air VAV setpoint for air volume flow	FtrSel	●	-
VavSuSpAflRel		Supply air VAV setpoint for relative air volume flow	ACalcVal	●	-
VavSuAirFl		Supply air VAV air volume flow	AI	◉	Room segment / Field device
TrndVavSuAirFl		Trend for supply air VAV air volume flow	FtrSel	●	-
VavSuAirFlRel		Supply air VAV relative air volume flow	ACalcVal	●	-
VavSuAirFlDvn		Supply air VAV air volume flow deviation	ACalcVal	●	-
VavSuAflStrtn		Supply air VAV air volume flow saturation 0:Satisfied 1:Starved	BCalcVal	●	-
VavSuPos		Supply air VAV position	AI	◉	Room segment / Field device
VavSuDevMod		Supply air VAV device mode 1:Off 2:Control mode 3:Maximum air volume flow 4:Minimum air volume flow 5:Smoke control air volume flow setpoint	MPrcVal	●	-
VavSuAirFlReq		Supply air VAV air volume flow request	ACalcVal	●	-
VavSuAflScale		Supply air VAV air volume flow scale 1:Heating 2:Cooling 3:Ventilation 4:Make-up air	MCalcVal	●	-
VavSuRqdC		Supply air VAV required for cooling 0:Off 1:On	BCalcVal	●	-
VavSuRqdH		Supply air VAV required for heating 0:Off 1:On	BCalcVal	●	-
VavSuAvlC		Supply air VAV available for cooling 0:No 1:Yes	BCalcVal	●	-
VavSuAvlH		Supply air VAV available for heating 0:No 1:Yes	BCalcVal	●	-
VavSuAvlVnt		Supply air VAV available for ventilation 0:No 1:Yes	BCalcVal	●	-
VavSuChovrCnd		Supply air VAV changeover condition 1:Neither 2:Heating 3:Cooling 4:Neutral	MPrcVal	●	-
VavSuAirDmd		Supply air VAV air demand for plant mode 1:Off 2:Protection 3:Economy 4:Pre-Comfort 5:Comfort 6:Warm-up 7:Cool down 8:Room low temperature protection 9:Not used 10:Free cooling 11:Night cooling 12:Ventilation 13:Not used 14:Air volume flow off 15:Smoke extraction positive pressure 16:Smoke extraction negative pressure 17:Purge	MCalcVal	●	-
VavSuCDmd		Supply air VAV cooling demand	ACalcVal	●	-
VavSuHDmd		Supply air VAV heating demand	ACalcVal	●	-
VavSuVntDmd		Supply air VAV ventilation demand	ACalcVal	●	-
VavSuAirFlTck		Supply air VAV air volume flow tracking	ACalcVal	●	-
VavTckMthd		VAV tracking method 1:Setpoint 2:Air volume flow	MCalcVal	●	-
CdnMsgCol		Collection of condensation message	ColView	●	-
	CdnMsgRs	Result of condensation message 0:Normal 1:Alert	BCalcVal	●	-
	CdnMsg	Condensation message Enumeration see CdnMsgRs	BCalcVal	◉	CcgChw11 / HCcg2Pipe11 / HCcg4Pipe11 / HCcg4Pipe13
	CclCdnMsg	Cooling coil condensation message Enumeration see CdnMsgRs	BCalcVal	◉	CclChw13
VavSuSplyAir		Supply air VAV supply chain for air	GrpMbr	●	-
VavSuSpAflSmk		Supply air VAV smoke control air volume flow setpoint	ACnfVal	●	-
VavSuAirFlMaxC		Supply air VAV maximum air volume flow for cooling	ACnfVal	●	-
VavSuAirFlMinC		Supply air VAV minimum air volume flow for cooling	ACnfVal	●	-
VavSuAirFlMaxH		Supply air VAV maximum air volume flow for heating	ACnfVal	●	-
VavSuAirFlMinH		Supply air VAV minimum air volume flow for heating	ACnfVal	●	-
VavSuAflMaxVnt		Supply air VAV maximum air volume flow for ventilation	ACnfVal	●	-

Engineering and commissioning

Check for correct damper actuator installation. Actuator mis-wiring or improper installation is a major cause of common problems.

The relative airflow (VavSuAirFlRel) is normalized as a percentage (0 - 100%) of VavSuAirFl based on the nominal (rated) value for the box airflow (AirFlNom).

The value of VavSuSpAirFl must never exceed the value of its Present-Value-2 property. Present-Value-2 is used as the nominal air flow. Thus, VavSuSpAirFl cannot exceed the nominal airflow value.