1. Air Side System Evaluation
VAV System Theory
Air Side System Evaluation
General temperature control
Zone temperature selection
Flexibility to adapt to change
Energy efficiency
Ease of maintenance
Fire / smoke control
Indoor air quality
Vibration / noise

2. General Temperature Control
VAV System Theory
General Temperature Control

3. General Temperature Control
VAV System Theory
General Temperature Control
Issue #1 : Control of Cooling
Modulating control of the valve provides good general temperature control.
Multiple stages of cooling provide fair general temperature control.
On-off control of a valve provides poor general temperature control.

4. General Temperature Control
VAV System Theory
General Temperature Control
Issue #2 : Sensor Placement
The system is controlled to achieve the setpoint only at the sensor location.
Sensor in the room ensures comfort in that specific room.
Sensor in return air duct reacts to average load; concentrated loads cause problems.
DDC controllers can do averaging, high select or low select of multiple sensors.

5. General Temperature Control
VAV System Theory
General Temperature Control
Ratings of Systems
Split Unit Fair
WCPU Poor
CAV Good
VAV Excellent

6. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection

7. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
Different zones have different loads requiring different setpoints and different amounts of cooling.

8. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
The biggest contributors to air conditioning loads are lighting load and solar load. People and equipment loads are smaller.
Incoming solar radiation - 100%
Solar heat excluded - 16%
Solar heat admitted - 84%
Clear Plate Glass

9. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
The cooling load in a zone varies through the day. Because of thermal inertia, the greatest building load occurs about 15:00. S E
Interior Zone W

10. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
Lighting : Relatively Fixed
Solar : Varies through day
Conduction : Varies through day
Ventilation : Varies with other loads
People : Varies with occupancy
Equipment : Varies with use of space

11. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
Any air conditioning system must deal with varying loads.
Distributed systems such as fan coil units and package units allow localized control but are energy inefficient.
Not all zones will call for maximum cooling at the same time. A centralized system can be designed with a diversity factor (20%).

12. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
A centralized Constant Air Volume (CAV) system is energy efficient but only responds to the average load, and does not allow individual zones to be controlled. For constant loads, a CAV system is best.
A centralized Variable Air Volume (VAV) system is the most economical way to provide zone control, both in terms of first cost and operating cost.

13. Zone Temperature Selection
VAV System Theory
Zone Temperature Selection
Ratings of Systems
Split Unit Fair
WCPU Fair
CAV Poor
VAV Excellent

14. Flexibility to Adapt to Change
VAV System Theory
Flexibility to Adapt to Change

15. Flexibility to Adapt to Change
VAV System Theory
Flexibility to Adapt to Change
One of the key criteria of an “Intelligent Building” is the flexibility to adapt to change. This is because as they way in which we do business changes, so too the way in which we use the office changes.
For example, increasing computerization changes the heat load, power, lighting, communication and LAN wiring requirements in the office.

16. Flexibility to Adapt to Change
VAV System Theory
Flexibility to Adapt to Change
As partitioning layout changes, the air conditioning system must be capable of cost effectively change the airflow patterns and temperature control requirements in each area.

17. Flexibility to Adapt to Change
VAV System Theory
Flexibility to Adapt to Change
Ratings of Systems
Split Unit Fair
WCPU Poor
CAV Fair
VAV Excellent

18. VAV System Theory
Energy Efficiency

19. VAV System Theory Energy Efficiency
Centralized systems are far more energy efficient than distributed systems.
Variable Air Volume systems are more energy efficient than Constant Volume Systems as they deliver the minimum amount of cooling required and do not overcool.

20. VAV System Theory Energy Efficiency Ratings of Systems Split Unit Poor
WCPU Poor
CAV Fair
VAV Excellent

21. VAV System Theory
Ease of Maintenance

22. VAV System Theory Ease of Maintenance
The ease of maintenance of a system relates to the availability of skilled labour to service the equipment.
The increased complexity associated with the centralized systems is partially offset by the diagnostic information available from a BAS.

23. VAV System Theory Ease of Maintenance Ratings of Systems
Split Unit Excellent
WCPU Good
CAV Fair
VAV Good

24. VAV System Theory
Fire / Smoke Control

25. VAV System Theory Fire / Smoke Control
Only centralized systems can help control the egress of smoke during a fire.
If properly designed, the air conditioning system can work together with the pressurization system to provide a smoke spill system or supplementary pressurization.

26. VAV System Theory Fire / Smoke Control Ratings of Systems
Split Unit Poor
WCPU Poor
CAV Excellent
VAV Excellent

27. VAV System Theory
Indoor Air Quality

28. VAV System Theory Indoor Air Quality
The issue of Indoor Air Quality (IAQ) and Sick Building Syndrome started to become an issue in the west in the early 1980s.
The energy crisis of the 1970s resulted in energy efficient office buildings with sealed envelopes to minimize fresh air intake.

29. VAV System Theory Indoor Air Quality
At the same time, new types of building materials (carpets, partitions, etc.) became popular. The new building materials used formaldehyde-based adhesives which gave off a irritating odour.
Chlorinated Hydrocarbons and Volatile Organic Compounds (VOCs) were also introduced into the space from cleaning solutions and other sources.

30. VAV System Theory Indoor Air Quality
Legionnaire's disease increased awareness of bacteria, fungus and mould which lives in air conditioning systems.
Peoples’ jobs became more sedentary as personal computers became more common in the workplace.

31. VAV System Theory Indoor Air Quality Reduced fresh air ventilation +
Increased number of pollutants in the office
Less activity in the job =
Complaints (and litigation!)
Complaints are of headaches, irritated eyes, nose and throat and flu-like symptoms.

32. ASHRAE increased ventilation requirements in an office environment.
VAV System Theory
Indoor Air Quality
ASHRAE increased ventilation requirements in an office environment.
ASHRAE recommends measuring CO2 as an indicator of ventilation effectiveness. People generate CO2 and the ventilation system disperses it. Office CO2 should be <1000 ppm (ambient is 400 ppm).

33. VAV System Theory Indoor Air Quality
VAV systems have been linked to indoor air quality problems. With a conventional VAV system, at part load the amount of fresh air introduced is reduced.
This can be countered by modulating the fresh air dampers or increasing the off-coil temperature setpoint but these approaches add to first cost or to operating cost.

34. VAV System Theory Indoor Air Quality
Indoor air quality has not become an issue with local owners or tenants. It is unlikely to become a serious issue locally.
Local “air quality” problems are usually related to improper placement of fresh air intakes (i.e. next to a durian stand or beside a toilet exhaust).

35. VAV System Theory Indoor Air Quality Ratings of Systems
Split Unit Poor
WCPU Poor
CAV Excellent
VAV Good

36. VAV System Theory
Vibration / Noise

37. VAV System Theory Vibration / Noise
Localized units are inherently more noisy than centralized system.
Conversely, the ductwork can distribute noise from the fan or noise resulting from airflow.
The box in a VAV system provides additional acoustic buffering.

38. VAV System Theory Vibration / Noise Ratings of Systems Split Unit Poor
WCPU Poor
CAV Fair
VAV Excellent

39. VAV System Theory
VAV System Components

40. Mechanical Components
VAV System Theory
Return Air
Cooling Coil
Fan
Outdoor
Air
VAV
Box
Chilled Water
Valve
Mechanical Components
Ceiling Diffusers

41. VAV System Theory VAV System Components Return Air
Typically from ceiling plenum
Represents average of all zones
Outdoor Air
Used to dilute contaminants in Return Air
Cooling Coil
Cools air stream by exposing it to coil with chilled water passing through

42. VAV System Theory VAV System Components Chilled Water Valve
Controls the water passing through coil and thus controls off-coil air temperature
Fan
Controls the amount of air in the system
VAV Box
Controls the amount of air going to a zone

43. Room Temperature Sensor
VAV System Theory
Room Temperature Sensor T
Actuator
Off-coil Air
Temperature
Sensor
P Sensor
Static
Pressure
Sensor
Valve Actuator
Variable
Speed
Drive
Control
Control Components

44. VAV System Theory VAV System Components Valve Actuator
Controls Chilled Water Valve
Off - Coil Temperature
Used to control valve actuator
Variable Speed Drive Control
Controls fan speed
Static Pressure Sensor
Used to control fan speed

45. VAV System Theory VAV System Components VAV Box Actuator
Controls air flow through box
P Sensor
Used to measure air flow through box
Temperature Sensor
Used to determine amount of air required

46. VAV System Theory
VAV Control Theory

47. VAV System Theory VAV Control Theory
Room temperature sensor determines amount of cold air required
P sensor measures actual amount of cold air
Actuator controls cold air to match required amount

48. VAV System Theory VAV Control Theory
When VAV box actuators open or close, the static pressure sensor detects the change
The variable speed drive control changes the fan speed to maintain a static pressure setpoint

49. The off-coil sensor measures the air temperature
VAV System Theory
VAV Control Theory
The off-coil sensor measures the air temperature
The chilled water valve is controlled to achieve a desired off-coil temperature

50. VAV System Theory
VAV / CAV Comparison

51. As compared to a CAV System:
VAV System Theory
As compared to a CAV System:
VAV System allows precise control of each zone
VAV System automatically compensates for shifting solar load during day
VAV System allows each occupant to establish their own setpoint
VAV System saves energy as fan only supplies as much air as is required
VAV System has improved acoustical performance as box baffles duct noise

52. As compared to a CAV System:
VAV System Theory
As compared to a CAV System:
VAV System saves energy when connected to BAS as individual zones can be set to occupied mode
VAV System allows improved diagnostics when connected to BAS as individual zone information is available
VAV System has slightly higher first cost and slightly higher maintenance cost due to boxes and controls