Extract air VAV box, pressure dependent control (VavEx15)

This application function operates the modulation of a damper. The resulting change of the air volume flow is pressure dependent.

The air volume flow setpoint is calculated based on request for extract flow, relative to the volume of air flowing through the supply terminal following the principle of «extract tracks supply».

In this AF, duct area is for user reference only; it is not used for damper position calculation. There is no coefficient object or coefficient calculation logic.

Air balancing functions are provided by manually changing min and max damper position setpoints.

Required elements and how to configure them:

Element	I/O	Signal type
VavExPos "Extract air VAV position"	On-board output, Extract air VAV position	0…10V or 3-position
VavExPos "Extract air VAV position"	or KNX PL-Link device, Extract air VAV	Position control

Function

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

VAV extract ventilation request (VavExVntReq) is received and processed into the output signal (in percent) for VAV extract damper position (VavExPos).

Basic function: Accept request signal from associated room controller and map it to damper position setpoint; Pass result through a device mode logic switch prior to outputting as a command to the object that controls the extract damper position.

	Command or request (or related)
	Notification of condition or status, or availability
	Device mode

Saturation signal: The saturation signal (VavExPosStrtn) is a binary object that is True ("Starved") when the VAV extract air terminal cannot get enough air for a time exceeding a built-in time delay.

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

Available status: When the VAV extract damper is available for extract air ventilation, the binary output signal that indicates availability (VavExVntReq) will be "Yes" (available).

For available status to be "Yes", device mode must equal "Control mode" (modulation).

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

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

When these conditions are true, the boolean extract fan coasting signal (FanExCstVal) 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 extract duct over-pressure.

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

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

Configuration

Objects

Description	Object	Type	Default value
Air flow settings
Extract air VAV smoke control setpoint position ▶ Air flow setpoint if VavExDevMod = 5. (manually entered damper setpoint value for smoke control)	VavExSpPosSmk	ACnfVal	50 [%]
Extract air VAV maximum position for ventilation ▶ Maximum ventilation damper position setpoint for indoor air quality (IAQ) controller and for Rapid ventilation.	VavExPosMaxVnt	ACnfVal	100 [%]
Extract air VAV mininimum position for ventilation ▶ Minimum damper position setpoint for ventilation, whether fixed or controlled by indoor air quality (IAQ) controller; see also Minimum ventilation flow section.	VavExPosMinVnt	ACnfVal	0 [%]
Duct
Extract air VAV duct area ▶ Duct cross-sectional area, user-entered or calculated by automation station using the duct shape, dimension A and dimension B values. For user-entered method, VavExDuctShape must be set to 4:Direct entry. ▶ In a pressure dependent AF, VavExDuctArea is for user reference only; not used for damper position calculation.	VavExDuctArea	ACnfVal	0.05 [m²] 0.54 [ft²]
Extract air VAV duct shape ▶ Duct cross-sectional shape; Direct entry uses user-entered area, not controller calculation. 1:Rectangular 2:Round 3:Flat oval 4:Direct entry	VavExDuctShape	MCnfVal	2:Round
Extract air VAV dimension A ▶ For rectangular duct: Width; for round duct: Diameter; for flat oval duct: Total width (major dimension); for Direct entry: Not used.	VavExDmsnA	ACnfVal	20 [cm] 7.9 [in]
Extract air VAV dimension B ▶ For rectangular duct: Height; for round duct: Not used; for flat oval duct: Height (minor dimension); for Direct entry: Not used.	VavExDmsnB	ACnfVal	20 [cm] 7.9 [in]

Parameters

Description	Parameter	Default value
Saturation calculation
Enable saturation calculation ▶ When EnStrtnCal = 1:Yes, the calculation logic for the saturation signal (VavExPosStrtn) is enabled and active. Saturation signal calculation logic: VavExPosStrtn will equal 1 ("Starved") if VavSuPos > StrtnLvl for longer than DlyOnStrtn. This feature is used for AHU fan static pressure reset. See also Saturation signal section. 0:No 1:Yes	EnStrtnCal	1:Yes
Saturation level ▶ Damper position level for use in determining whether the pressure dependent VAV extract box is handling enough air flow / getting enough air flow support to satisfy the functional needs of the application. If VavExPos > StrtnLvl for longer than DlyOnStrtn, the binary calculated value object VavExPosStrtn (Extract air VAV position saturation) switches from Satisfied to "Starved".	StrtnLvl	90 [%]
Switch-on delay saturation ▶ The time after the saturation conditions are met that the saturation signal to the central fan system goes from False to True.	DlyOnStrtn	60 [s]
Position demand
Switch-on point for position demand ▶ Damper position value above which the fan coast timer is not triggered.	SwiOnPtPosDmd	4 [%]
Hysteresis for position demand ▶ Damper position value below SwiOnPtPosDmd, below which the fan coast timer is triggered. See also Fan coast section.	HysPosDmd	2 [%]
Rate limitation for position ▶ A rate change limiter function that is provided to minimize rapid damper repositioning if the damper is changed by a large amount (for example, manually by a user). The rate limiter delay is only applied when VavExSpPos is in normal runtime priority – not protection or emergency priority.	RateLmPos	60 [s]

Interface

Interface	Description	Type	Ref.	Owned by
VavExPos	Extract air VAV position	AO	◉	Room segment / Device
VavExSpPos	Extract air VAV setpoint position	APrcVal	●	-
VavExPosStrtn	Extract air VAV position saturation 0:Satisfied 1:Starved	BCalcVal	●	-
VavExDevMod	Extract 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	●	-
VavExVntReq	Extract air VAV ventilation request	ACalcVal	●	-
VavExAvlVnt	Extract air VAV available for ventilation 0:No 1:Yes	BCalcVal	●	-
FanExCstVal	Coasting value of extract air fan 0:Off 1:On	BCalcVal	●	-
VavExSplyAir	Extract air VAV supply chain for air	GrpMbr	●	-
VavExBalSta	Extract air VAV balancing state 1:Initial 2:Balancing 3:Balanced	MCnfVal	●	-
VavExDuctArea	Extract air VAV duct area	ACnfVal	●	-
VavExDuctShape	Extract air VAV duct shape 1:Rectangular 2:Round 3:Flat oval 4:Direct entry	MCnfVal	●	-
VavExDmsnA	Extract air VAV dimension A	ACnfVal	●	-
VavExDmsnB	Extract air VAV dimension B	ACnfVal	●	-
VavExSpPosSmk	Extract air VAV smoke control setpoint position	ACnfVal	●	-
VavExPosMaxVnt	Extract air VAV maximum position for ventilation	ACnfVal	●	-
VavExPosMinVnt	Extract air VAV mininimum position 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.