1. VAV Box Control Loop
The purpose of the VAV Box control loop is to adjust airflow through the box to maintain a space temperature.
The difference between the space temperature and the setpoint will establish a desired airflow. The actual flow is monitored and controlled by adjusting the damper to achieve the desired airflow.

2. VAV Box Control Loop Space Temperature Analysis Desired Flow Setpoint
DDC Control Calculations
Signal to VAV Box Damper Actuator
Air flow through box into space

3. Differential Pressure
VAV Box Control Loop
Crossflow Sensor
Damper
Damper Actuator
Differential Pressure
Transducer
Metal Enclosure
DDC
Controller T
24 VAC Transformer
Temperature Sensor
BAS Communication Trunk

4. VAV Box Control Loop
VAV Box Control Logic

5. VAV Box Control Loop VAV Box Control Logic Desired Air Flow Rate
Between Flowmin and Flowmax
Zone Temp.
Temperature Control Loop
Span Routine
Flow Control Loop
Output Control
Offset
Amount of Reset Required
0% to 100%
Calculated Damper Position
0% to 100%
Differential Pressure

6. Temperature Control Loop Offset is typically ±2°C.
VAV Box Control Loop
Temperature Control Loop
The temperature control loop setpoint is calculated by taking either the occupied or unoccupied setpoint and adding the offset from the slider on the thermostat.
Offset is typically ±2°C.

7. Temperature Control Loop 0% : min. cooling 100% : max cooling
VAV Box Control Loop
Temperature Control Loop
The temperature control loop calculates an 0% to 100% output using a proportional plus integral algorithm. The output is based on the difference between the zone temperature and setpoint, the proportional band and the integral time. If the temperature is within the deadband of the setpoint, the output is 0%.
0% : min. cooling 100% : max cooling

8. Temperature Control Loop
VAV Box Control Loop
Temperature Control Loop
Unoccupied Setpoint
Occupied Setpoint
Proportional Band
Select Mode
Integral Time
Deadband (0.17°C)
Remote
Setpoint
Offset
Setpoint
Output
Command
Temperature
Control
Loop +
0% - 100%
Zone Temperature

9. Temperature Control Loop Network Visible Points
VAV Box Control Loop
Temperature Control Loop
Network Visible Points
Zone Temp Offset
Occ SP Unocc SP
Setpoint Temp. PB
Temp. IT Temp. Cmd

10. VAV Box Control Loop SPAN Function
The SPAN Function calculates the desired flow through the VAV box. The desired flow is between the minimum flow value and the maximum flow value based on temperature control loop output. Minimum flow value and maximum flow value will depend on mode of box. Output of SPAN Function (Flow SP) can be overridden for testing.

11. VAV Box Control Loop SPAN Function Select Mode Select Mode SPAN
Unoccupied Minimum
Unoccupied Minimum
Occupied Minimum
Occupied Minimum
Select Mode
Select Mode
Minimum
Maximum
From Temperature
Control Loop
Desired Flow To
Flow Control Loop
SPAN
0% = Minimum
100% = Maximum
0% to 100%
Minimum to Maximum

12. Network Visible Points
VAV Box Control Loop
SPAN Function
Network Visible Points
Occ Min. Unoc Min.
Occ Max Unoc Max
Temp. Cmd Flow SP

13. Actual Flow is calculated from velocity pressure sensor input.
VAV Box Control Loop
Flow Control Loop
Actual Flow is calculated from velocity pressure sensor input.
Actual flow is compared with desired flow as calculated by SPAN function using a Proportional + Integral (PI) control loop. If the difference between the desired air flow and the measured air flow is less than the deadband, the output is 0%.

14. VAV Box Control Loop Flow Control Loop Proportional Band
Box “K” Factor
Integral Time
Deadband
Box Area
Desired Flow from
SPAN Function
Flow
Control
Loop
Output
Signal from
P Sensor
0% - 100%
Calculated Flow

15. Network Visible Points
VAV Box Control Loop
Flow Control Loop
Network Visible Points
DiffPress Airflow
Box Area Box Mult
Flow SP Flow PB
Flow IT Flow DB
Dpr Posn

16. VAV Box Control Loop Output Control
The Output Control Module receives the signal to position the damper from the Flow Control Loop and either opens or closes the damper to achieve the desired position.
The signal is not passed to the output if the shutdown open or shutdown close is active or if the damper is less than the minimum position.

17. VAV Box Control Loop Output Control Output Control Damper Stroke Time
Minimum Damper Position
Shutdown
Open
Close
From Flow
Control Loop
Damper Open
Output
Control
0% to 100%
Damper Close

18. Network Visible Points
VAV Box Control Loop
Output Control
Network Visible Points
Dpr Posn Dmpr Cmd
Dpr Time Dpr Min.
Dmpr Opn Dmpr Cls

19. VAV Box Control Loop
Diagnostics

20. VAV Box Control Loop Diagnostics
A VAV box is a small mechanical system mounted above the ceiling in an office area.
Diagnostics are required to recognize and report common system problems.

21. VAV Box Control Loop Diagnostics Starved Box
If the damper is 100% open and the desired airflow cannot be achieved, the starved box flag is set.
Common causes of starved boxes are: - Duct static pressure too low - Actuator motor burned out - Damper blade is jammed

22. This is a good first place to look to identify VAV box problems
VAV Box Control Loop
Diagnostics
Space Temperature
The difference between the actual space temperature and the space temperature setpoint is averaged over time.
This is a good first place to look to identify VAV box problems

23. Large average deviations (>1°C) caused by:
VAV Box Control Loop
Diagnostics
Space Temperature
Large average deviations (>1°C) caused by:
unreasonable setpoint entered
box unable to provide sufficient cooling due to low duct static pressure or maximum airflow parameter too low
temperature sensor incorrectly located

24. Large average deviations (>5% flowmax) caused by:
VAV Box Control Loop
Diagnostics
Box Flow
The difference between the actual air flow through the box and the calculated air flow setpoint is averaged over time.
Large average deviations (>5% flowmax) caused by:
Calculated setpoint is not achievable
Wrong flow control loop parameters

25. The duty cycle for the actuator is calculated:
VAV Box Control Loop
Diagnostics
Actuator
The duty cycle for the actuator is calculated:
Time actuator is receiving positioning signal Time controller is running
A duty cycle in excess of 5% indicates that box is cycling and the flow control loop parameters are wrong.

26. VAV Box Control Loop
Temperature Sensor

27. Mounted at 1.5m on inside wall, away from sunlight.
VAV Box Control Loop
Temperature Sensor
Mounted at 1.5m on inside wall, away from sunlight.
Local setpoint adjustment allows occupant to modify the setpoint by ±2°C (adjustable). This has a psychological effect on occupants.

28. Sensor has connection point for hand held device used for balancing.
VAV Box Control Loop
Temperature Sensor
Button allows occupant to request overtime or put system in boost mode.
Sensor has connection point for hand held device used for balancing.

29. VAV Box Control Loop
Air Flow Measurement

30. VAV Box Control Loop Air Flow Measurement H L
This tube has holes facing away from air flow
This tube has holes facing toward air flow
Direction of
Air Flow
H = Total Pressure
L = Static Pressure
H - L = Velocity Pressure
CFM = 4005 x Velocity Pressure x Box Area H L
K factor

31. VAV Box Control Loop Air Flow Measurement
DDC Controller measures differential pressure (velocity pressure) from cross flow sensor and converts into air flow.
Auto zero algorithm automatically calibrates reading on request or at a scheduled time.

32. VAV Box Control Loop
Damper Actuator

33. VAV Box Control Loop Damper Actuator Output can be positioned
between 0° and 90° (±1°)
by sending timed pulses
to either open or close.
DDC Controller keeps track of damper position.
With the damper position, the DDC Controller ensures that the actuator is not overdriven.

34. VAV Box Control Loop Damper Actuator Damper has a torque of 4 N-m.
Common/Open Voltage
24VAC
Damper has a torque of 4 N-m. t
Common/Close Voltage
24VAC
Open
Damper
Actuator
Common t
Close
Open for
4.5 sec
(3.4 °)
Close for
3 sec
(2.3 °)
Stroke time (0° to 90°) = 120 seconds

35. Pressure Dependent VAV Box
VAV Box Control Loop
Pressure Dependent VAV Box

36. Pressure Dependent VAV Box
VAV Box Control Loop
Pressure Dependent VAV Box
A Pressure Dependent VAV Box includes:
Temperature Sensor
Damper and actuator
DDC Controller
A Pressure Dependent VAV Box does not include:
Crossflow sensor and differential pressure sensor

37. Pressure Dependent VAV Box
VAV Box Control Loop
Pressure Dependent VAV Box
Damper
Damper Actuator
Metal Enclosure
DDC
Controller T
24 VAC Transformer
Temperature Sensor
BAS Communication
Trunk

38. Pressure Dependent VAV Box
VAV Box Control Loop
Pressure Dependent VAV Box
Damper Stroke Time
Minimum Damper Position
Proportional Band
Integral Time
Shutdown
Deadband (0.17°C)
Open
Close
Damper Open
Zone Temp.
Output Control
Temperature Control Loop
Damper Close
Offset
Calculated Damper Position
0% to 100%

39. Pressure Dependent VAV Box
VAV Box Control Loop
Pressure Dependent VAV Box
A pressure dependent box is slightly less expensive than a pressure independent box but has the following disadvantages:
Less responsive to changes in load
More difficult to balance and troubleshoot
Use more energy as static pressure reset is not possible

40. VAV Box Control Loop Conclusion
Integrating VAV Box DDC Controllers into the BAS has the following advantages when compared to stand-alone DDC controllers:
Save energy by using static pressure reset
With Diagnostics, the operator can know about problems before a tenant calls
Operator can display and keep record of space temperatures