CET supply air VAV box, pressure sensor, flow conversion (CetVavSu13)

Overview

The application function "CET room pressurization supply air VAV box 13, differential pressure sensor, flow conversion, internal air flow controller, air damper" (CetVavSu13) operates a damper, using closed loop control to drive the measured flow to an airflow setpoint (VavSuSpAirFl) that it calculates based on demand signals received for heating, cooling and ventilation. It can control airflow with a settling time of 1 to 2 seconds when applied with the right peripheral devices. It can also be set up for slower operation.

The main output is a modulating output for damper position (VavSuPos) that is generated by a PID airflow controller in this AF.

Note
To calculate supply air volume flow, this AF uses an OEM box coefficient and input from a differential pressure sensor. This means CetVavSu13 has no air balancing functionality or duct area calculation.

Main features:

Function

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

The input signals VavSuAflScale and VavSuAirFlReq are received and processed into the output signal for VAV supply damper position (VavSuPos).

 

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:

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.

Airflow control loop (cascade control)

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

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:

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:

  1. Off – Setpoint is 0 and damper is closed
  2. Control Mode – Setpoint follows % demand
  3. Maximum airflow setpoint – Setpoint is maximum for the currently active airflow scale
  4. Minimum airflow setpoint – Setpoint is minimum for the currently active airflow scale
  5. 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:

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.

Cooling

Heating

Ventilation

Make-up

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.

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:

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

Feedback loop: A feedback controller drives the AO object for the damper. The feedback loop uses relative values for flow and setpoint. Both physical values are converted to a percentage of a scaling value for use in the PID loop. These relative values for flow and setpoint are written to calculated value objects. (This is different from the scaling used to express the demand for heating, cooling or ventilation.) The scaling value used is the highest flow setpoint expected in the programmed control sequence. It is the largest of the various maximum flow limits configured for the terminal:

Flow sensor failure: If the airflow sensor object is invalid (not including over range) then, the flow control damper position will be set based on the setting of the configuration extension AirFlFailMod.

While the sensor is failed, PID operation is suspended. When status of the sensor is valid again, the PID resumes operation.

When the AI is unreliable, airflow calculations continue, however, airflow value might not change because the value of the AI stops updating. The AF also sets a binary value object indicating normal or faulty state of the airflow data.

Network communication loss: If network communication loss occurs, the flow setpoint will be set based on the setting of the configuration extension AirFlFailMod.

When the network communication is lost, the flow control loop continues to operate with the setpoint selected from above.

Sensor calibration: While the airflow sensor is in active calibration, the closed loop flow control is suspended; the output does not change. Calibration only occurs during periods of steady state control, not when the flow setpoint is changing.

Airflow sensing: The AF calculates volumetric airflow from a measured differential pressure value and works with the OAVS pressure sensor, that includes an Auto-zero function. The AF does not include calculations or data to zero a sensor.

Low sensor value: Configuration settings specify a switch-on point and hysteresis value in units of pressure.

Duct area calculation: None. This AF has no air balancing functionality.

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.

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:

  1. Neither
  2. Heating
  3. Cooling
  4. 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.

Configuration

Objects

Description

Object

Type

Default value

Supply air VAV smoke control air volume flow setpoint
▶ Air flow setpoint if VavSuDevMod=5 unless overwritten.

VavSuSpAflSmk

ACnfVal

50 [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 [m3/hSqrtPa]
4022.01 [ft3/min/inWC]
41.67 [l/s/SqrtPa]
88.29 [ft3/min/SqrtPa]

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 air flow 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 air flow 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 air flow 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

Failure mode for air volume flow sensor

▶ Defines how the terminal responds if the air volume flow sensor fails.
1:Hold supply air
2:Open supply air
3:Close supply air

▶Also defines how the terminal responds if loss of network communication occurs.
1. Hold: Setpoint does not change
2. Close: Setpoint is set to the minimum flow of the mode the terminal was in during failure
3. Open: Setpoint is set to the maximum flow of the mode the terminal was in during failure

AirFlFailMod

1:Hold supply air

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]

Time constant for air volume flow
▶ The time constant for the attenuation filter on the air volume flow sensor.
Increase the value to help stabilize fluctuating air volume flow sensor values, leaving at 0 disables the filter.

TiConAirFl

0 [s]

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 [%]

Nominal air volume flow
▶ Optional. Maximum airflow capacity of the VAV boxSet no larger than 1.2 times the greater of the values of maximum airflow for ventilation or heating or cooling.

AirFlNom

0 [m3/h]
0.0 [ft3/min]
0.00 [l/s]

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 Air flow 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 air flow 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 air flow to the supply air flow setpoint (assuming SpSelVavEx is set to True.)

HysAirFlSta

5 [%]

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 / Field device

VavSuDiffP

Supply air VAV differential pressure

AI

Room segment / Field device

VavSuSpAirFl

Supply air VAV setpoint for air volume flow

APrcVal

-

TrndVavSuAirFl

Trend for supply air VAV air volume flow

FtrSel

-

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

-

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

-

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

-

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

-

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

-

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

-

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).