Supply air VAV box, pressure sensor, flow conversion (VavSu13)

This application function operates a damper, using internal air volume flow controller to drive the measured air volume flow to an air volume flow setpoint.

To calculate air volume flow, this AF uses an OEM box flow coefficient (K-factor) and input from a differential pressure sensor.

The air volume flow setpoint is calculated based on request signals (from the room controllers) received for heating, cooling and ventilation.

There is an optional collection input for condensation detection at chilled beam(s) or chilled ceiling(s).

Required elements and how to configure them:

Element	I/O	Signal type
VavSuPos "Setpoint for supply air VAV position"	On-board output, Setpoint for supply air VAV position	0…10V or 3-position
VavSuPos "Setpoint for supply air VAV position"	or KNX PL-Link device, Supply air VAV	Position control
VavSuDiffP "Supply air VAV differential pressure"	On-board input, Supply air VAV differential pressure	0…10V or P1

Function

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

VAV supply heating, cooling, or ventilation request (VavSuHReq / VavSuCReq / VavSuVntReq) is received and processed into the output signal for VAV supply damper position (VavSuPos).

Basic function: Accept request signal from associated room controller and map it to appropriate air volume flow setpoint; Compare setpoint to air volume flow in terminal box; Pass result through a device mode logic switch prior to outputting as a command to the object that controls the device.

	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)

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	Direction	Description	Comment
AirFlCReq	Boolean	In	Air flow cooling request ▶ Initiates air flow (damper opens) to satisfy air flow support request from cooling coil.
AirFlHReq	Boolean	In	Air flow heating request ▶ Initiates air flow (damper opens) to satisfy air flow support request from heating coil.
AirFlHldH	Boolean	In	Air flow hold for heating ▶ Interlock signal from electric heating coil indicating the coil needs equipment protection air flow support. The damper is prevented from closing for a period of time.
AirFlSta	Boolean	Out	Air flow 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"
AirFlVavReq	%	Out	Air flow vav request ▶ In a fan-powered box (FPB) application, the VAV damper sends a proportional analog request signal (AirFlVavReq) requesting air flow, so that the box can meet an air flow support or ventilation demand.	Fan-powered box application.
FanSta	Boolean	In	Fan state ▶ Interlock signal used in series fan-powered box applications: FanSta must be True (fan is running) before damper is allowed to open. This ensures that air flow from the damper does not rotate the series fan while at rest.	In series fan-powered box applications, parameter EnMonFanSta must = Yes. See Configuration section for additional information.

Air flow controller

The PID air flow controller (VavSuAirFlCtr) compares the air volume flow of the terminal box to the current VAV supply air flow setpoint, and modulates VavSuPos as necessary to keep the box flow at setpoint.

Two-position operation, if needed, is achieved by providing a 2-position setpoint signal.

Air flow setpoint selection: In control mode (VavSuDevMod = 2) the air flow setpoint (VavSuSpAirFl) is the largest of the values needed to satisfy the requests for:

Cooling
Heating
Ventilation
Air flow support for cooling coil
Air flow support for heating coil

Although request values are in percent, the comparison to select the max is made in physical air flow units. This is because the air flow limits (minimum flow and maximum flow) are different for each request type.

The mapping from percent to air flow units for heating, cooling, and ventilation includes a step-up at the low end, with a switch on point of 4% and a hysteresis of 2%. Thus, the request percent must rise above 4% before its corresponding air flow value rises from 0 to the request’s minimum air flow.

Potential uses of the step-up include:

Keeping the flow setpoint high enough to support heat transfer from a coil
Keeping the flow setpoint high enough for effective sensing

Low air flow: If the box cannot satisfy air flow demands due to not having enough available air flow (or the air flow sensor object is invalid), then open loop operation begins. In open loop operation, damper position is controlled without using the measured air volume flow value from the sensor; instead, damper position = air flow setpoint in % of nominal air flow.

When the air volume flow setpoint falls below 5% of nominal or the measured air volume flow value rises above 7% of nominal, closed loop operation resumes.

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

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.

Air flow deviation signal: The supply air flow 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 air flow from the supply duct and comparing it to the air flow setpoint.

VavSuAirFlDvn = VavSuSpAflRel minus VavSuAirFlRel

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

Relief input signal: A VAV box’s binary air flow relief object (VavSuAirFlRlf) can be set to True by the air handling unit. This is done to relieve flow resistance in the duct system in order to maintain proper static pressure. The VAV controller responds by increasing its air flow setpoint (VavSuSpAirFl).

When device mode = control mode and both EnRlf (Enable relief) and VavSuAirFlRlf equal On, VavSuSpAirFl will be set to the maximum of either its current value or the parameter AirFlRlf.

Note
Relief has no effect on the supply chain VAV air demand signal VavSuAirDmd.

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".

The output signal VavSuAflPvdVnt is the amount of air flow available for ventilation. When VavSuAvlVnt is True, a maximum value (VavSuAflMaxVnt) is transferred to VavSuAflPvdVnt.

VavSuAflPvdVnt (VAV supply air flow provided for ventilation) is the sum of maximal flow (in engineering units) from the supply VAV boxes. These values are set in each supply VAV for the maximum flow in the segment.

Minimum ventilation flow: In an installation without DCV (demand control ventilation), the minimum primary air flow is applied whenever ventilation is required. This effectively replaces minimum cooling and minimum heating values. In an installation with DCV, the primary air flow value is applied when the calculated ventilation demand is at minimum and ventilation is required.

See the room level ventilation control AF (VavVntCtl.xx) for detailed information on configuring minimum ventilation levels.

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

VavSuDevMod supports the following states:

Off
Control mode (modulation)
Max.air vol.flow
Min.air vol.flow
Smoke ctrl.air flow setp (manually entered damper setpoint value for smoke control)

Fan coasting feature: The fan coasting feature prevents premature closing of the room supply damper while the AHU fan is coasting down. This might occur during a switch between operating modes (for example, switching from Comfort to Economy).

The fan coasting feature is active when:

The room supply air damper is the only damper that is open – all other VAV supply air dampers are closed.
The AHU supply fan has been turned off and is coasting down.
The air flow setpoint signal (VavSuSpAirFl), while on its way to zero, falls below the air flow demand hysteresis setting (HysAirFlDmd).

When these conditions are true, the boolean supply fan coasting signal (FanSuCstVal) will equal 1 (True) and the damper is reset to its last (previous) operating position / value. This keeps the damper open while the fan coasts down to prevent supply duct over-pressure.

In addition to the fan coasting signal freezing the damper in place, an internal timer function was activated when VavSuSpAirFl initially dropped below HysAFlSta. The fan coasting signal and the timer (60 seconds) must both finish (return to False) before the room supply damper can close. See the figure below.

Note
The internal timer operates independently and regardless of FanSuCstVal. Once the timer is activated, the room supply air damper is frozen in place until the counter reaches zero, after which the damper may close.

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

VavSuCDmd - Supply air VAV cooling demand
When cooling is available, VavSuCDmd is the percentage value of the cooling request from the room (otherwise it is 0).
VavSuHDmd - Supply air VAV heating demand
When heating is available, VavSuHDmd is the percentage value of the heating request from the room (otherwise it is 0).
VavSuVntDmd - Supply air VAV ventilation demand
When ventilation is available, 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 air flow demand, SwiOnAirFlDmd). If PltOpMod is "Depressurize" or "Emergency off", VavSuAirDmd will equal PltOpMod regardless of VavSuSpAirFl value.

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. Four states are supported:

Neither
Heating
Cooling
Neutral

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

Configuration

Objects

Description	Object	Type	Default value
Air flow settings
Supply air VAV smoke control air volume flow setpoint ▶ Air flow setpoint if VavSuDevMod = 5 (unless overwritten).	VavSuSpAflSmk	ACnfVal	50.0 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV box coefficient ▶ User-entered number that includes an engineering units conversion factor, a constant for calibration of the sensor span, and the duct area.	VavSuBoxCoef	ACnfVal	150.0 [m³/h/SqrtPa] 4022.01 [ft³/min/inWC] 41.67 [l/s/SqrtPa]
Supply air VAV maximum air volume flow for cooling ▶ Maximum air flow setpoint when in cooling mode.	VavSuAirFlMaxC	ACnfVal	100.0 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Supply air VAV minimum air volume flow for cooling ▶ Minimum air flow setpoint when in cooling mode.	VavSuAirFlMinC	ACnfVal	50.0 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV maximum air volume flow for heating ▶ Maximum air flow setpoint when in heating mode.	VavSuAirFlMaxH	ACnfVal	100.0 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Supply air VAV minimum air volume flow for heating ▶ Minimum air flow setpoint when in heating mode.	VavSuAirFlMinH	ACnfVal	50.0 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Supply air VAV maximum air volume flow for ventilation ▶ Maximum ventilation air flow setpoint for indoor air quality (IAQ) controller and for Rapid ventilation.	VavSuAflMaxVnt	ACnfVal	100.0 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Supply air VAV minimum air volume flow for ventilation ▶ Minimum air flow setpoint for ventilation, whether fixed or controlled by air quality (IAQ) controller; see also section Minimum ventilation flow.	VavSuAflMinVnt	ACnfVal	0.0 [m³/h] 0.0 [ft³/min] 0.00 [l/s]

Parameters

Description	Parameter	Default value
Nominal air volume flow ▶ Maximum air flow capacity of the VAV box. Must be no larger than 1.2 times the greater of the values of maximum air flow for ventilation or heating or cooling.	AirFlNom	100.0 [m³/h] 58.9 [ft³/min] 27.78 [l/s]
Setpoint selector for extract air VAV box ▶ Selects whether the supply air flow setpoint or the supply air volume flow is used to determine the extract air flow setpoint. 0:False (Supply air flow setpoint used) 1:True (Supply air volume flow used)	SpSelVavEx	1:Supply air flow
Differential pressure and air flow monitoring
Switch-on point for differential pressure ▶ The differential pressure sensor signal value must be above this value or else zero is used.	SwiOnPtDiffP	0.2 [Pa] 0.001 [inWC]
Hysteresis for differential pressure ▶ Amount below SwiOnPtDiffP below which the pressure value used by the controller will be zero.	HysDiffP	0.1 [Pa] 0.000 [inWC]
Enable monitoring for missing air volume flow ▶ Set to No to remain with pressure independent control if very low / missing air flow is detected. Set to Yes to switch to pressure dependent control if very low / missing air flow is detected. With Yes or No, an invalid air flow signal causes pressure dependent control. 0:No 1:Yes	EnMonNoAirFl	0:No
Interlocks
Switch-on point for air volume flow state ▶ The air volume flow above which the VavSuAirFlTck signal switches from the supply air flow setpoint to the actual supply air volume flow (assuming SpSelVavEx is set to True).	SwiOnAirFlSta	10 [%]
Hysteresis for air volume flow state ▶ The air volume flow below SwiOnAirFlSta, below which the VavSuAirFlTck signal switches from the supply air flow to the supply air flow setpoint (assuming SpSelVavEx is set to True.)	HysAirFlSta	5 [%]
Enable monitoring for fan state ▶ Enables box fan state to affect VavSuSpAirFl. Must = Yes for series fan powered box (VavSuSpAirFl is set to 0 @ priority 5 if fan = off). Must = No for parallel fan powered box (VavSuSpAirFl is unaffected by fan state). 0:No 1:Yes	EnMonFanSta	0:No
Deviation calculation
Enable deviation calculation ▶ When set to 1:Yes, enables the calculation of the deviation value (Airflow setpoint minus airflow value) to be available for AHU fan static setpoint reset. See also section Air flow deviation signal. 0:No 1:Yes	EnDvnCal	1:Yes
Saturation calculation
Enable saturation calculation ▶ When EnStrtnCal is set to 1:Yes, the calculation logic for the saturation signal (VavSuAflStrtn) is enabled and active. Saturation signal calculation logic: VavSuAflStrtn will equal 1 ("Starved") if (VavSuPos > StrtnLvl) and (VavSuAirFlDvn > AirFlErLm). This feature is used for AHU fan static pressure reset. See also Saturation signal section. 0:No 1:Yes	EnStrtnCal	1:Yes
Saturation level ▶ If VAV damper position VavSuPos > StrtnLvl and the air flow error (setpoint minus air flow value) > AirFlErLm and EnStrtnCal = Yes, then the Saturation signal will be True.	StrtnLvl	90 [%]
Air volume flow error limit ▶ If the air flow error (setpoint minus air flow value) > AirFlErLm and VavSuPos > StrtnLvl and EnStrtnCal = Yes, then the Saturation signal will be True.	AirFlErLm	0 [%]
Switch-on delay saturation ▶ The time after the saturation conditions are met that the saturation signal to the AHU goes from False to True.	DlyOnStrtn	60 [s]
Air flow demand
Switch-on point for air flow demand ▶ The air volume flow value, in percent of nominal air flow, above which the plant mode is written to VavSuAirDmd.	SwiOnAirFlDmd	4 [%]
Hysteresis for air flow demand ▶ The air volume flow value below SwiOnAirFlDmd, in percent of nominal air flow, below which the plant mode is no longer written to VavSuAirDmd and at which the fan coast timer is triggered. See also section Fan coasting feature.	HysAirFlDmd	2 [%]
Relief functionality
Enable relief ▶ If set to Yes, allows the value of AirFlRlf to be written to VavSuSpAirFl if the AHU writes a True to VavSuAirFlRlf and AirFlRlf > the previous air flow setpoint value. See also section Relief input signal. 0:No 1:Yes	EnRlf	0:No
Air volume flow relief ▶ The air flow value written to VavSuSpAirFl if EnRlf = True and if the AHU writes a True to VavSuAirFlRlf and if AirFlRlf > the previous air flow setpoint value.	AirFlRlf	50.0 [m³/h] 29.4 [ft³/min] 13.89 [l/s]
Internal settings – do not change
Pressure unit ▶ Defined by selected engineering unit – do not change.	PUnit	[Pa] [inWC]
Air volume flow unit ▶ Defined by selected engineering unit – do not change.	AirFlUnit	[m³/h] [ft³/min] [l/s]

Interface

Interface		Description	Type	Ref.	Owned by
VavSuPos		Supply air VAV position	AO	◉	Room segment / Device
VavSuDiffP		Supply air VAV differential pressure	AI	◉	Room segment / Device
VavSuSpAirFl		Supply air VAV setpoint for air volume flow	APrcVal	●	-
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	ACalcVal	●	-
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	●	-
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	●	-
VavSuAirFlCtr		Supply air VAV air flow controller	Controller	●	-
VavSuCReq		Supply air VAV cooling request	ACalcVal	●	-
VavSuHReq		Supply air VAV heating request	ACalcVal	●	-
VavSuVntReq		Supply air VAV ventilation request	ACalcVal	●	-
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	●	-
FanSuCstVal		Coasting value of supply air fan 0:Off 1:On	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:Condensation overflow protection 10:Free cooling 11:Night cooling 12:Ventilation 13:Equipment temperature protection 14:Air volume flow off 15:Smoke extraction positive pressure 16:Smoke extraction negative pressure 17:Purge	MCalcVal	●	-
VavSuAirFlRlf		Supply air VAV air volume flow relief 0:Off 1:On	BPrcVal	●	-
VavSuCDmd		Supply air VAV cooling demand	ACalcVal	●	-
VavSuHDmd		Supply air VAV heating demand	ACalcVal	●	-
VavSuVntDmd		Supply air VAV ventilation demand	ACalcVal	●	-
VavSuAflPvdVnt		Supply air VAV provided air volume flow for ventilation	ACalcVal	●	-
VavSuAirFlTck		Supply air VAV air volume flow tracking	ACalcVal	●	-
CdnMsgCol		Collection of condensation message	ColView	●	-
	CdnMsgRs	Result of condensation message 0:Normal 1:Alert	BCalcVal	●	-
	CdnMsg	Condensation message 0:Normal 1:Alert	BCalcVal	◉	Room segment / Field device
	CclCdnMsg	Cooling coil condensation message 0:Normal 1:Alert	BCalcVal	◉	Room segment / Field device
VavSuSplyAir		Supply air VAV supply chain for air	GrpMbr	●	Room segment
VavSuSpAflSmk		Supply air VAV smoke control air volume flow setpoint	ACnfVal	●	-
VavSuBoxCoef		Supply air VAV box coefficient	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	●	-
VavSuAflMinVnt		Supply air VAV minimum 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 air flow (VavSuAirFlRel) is normalized as a percentage (0 - 100%) of VavSuAirFl based on the nominal (rated) value for the box air flow (AirFlNom).

- VavSuSplyAir Group Member – provides information to the central AHU for duct static pressure reset:
RHvacCoo_xx contains a GroupMember (SplyAir) that monitors/receives data from the central function SplyAir group manager. The central function should be located in the room automation station that contains ALL central coordination functions that come from rooms (e.g., child) serviced by the central system (e.g., parent). Verify the assignment of the group numbers in the group manager objects and in the assigned group member objects. Make sure that the group numbers of the group managers in the same group category are unique on the network.

See the room level ventilation control AFs (VavVntCtl, FcuVntCtl) for detailed information on configuring minimum ventilation levels.