HVAC Building Automation Systems 4 — Questions and Answers
Question 1: What is 'optimal start' in a BAS control strategy?
- Starting all equipment simultaneously at a fixed time each morning
- Calculating the latest possible start time to reach setpoint by occupancy time based on conditions (Correct answer)
- Starting the chiller plant before the AHUs each morning
- A manual override feature activated by the building operator
Correct answer: Calculating the latest possible start time to reach setpoint by occupancy time based on conditions
Optimal start calculates the latest possible equipment start time to achieve setpoint by occupancy, saving energy over a fixed early-start approach.
Question 2: Which of the following BACnet object properties defines how often a controller reads a sensor value?
- COV Increment
- Poll Rate (Correct answer)
- Present Value
- Priority Array
Correct answer: Poll Rate
The Poll Rate property defines the interval at which a BACnet device reads input values from sensors or other objects.
Question 3: A variable air volume (VAV) box controlled by a BAS fails to open beyond 30% even at maximum cooling demand. What should be checked first?
- BACnet network topology
- Actuator wiring, travel limits, and calibration (Correct answer)
- Chiller setpoint in the supervisory controller
- CO2 sensor accuracy
Correct answer: Actuator wiring, travel limits, and calibration
When a VAV actuator doesn't fully travel, the first checks are wiring integrity, actuator calibration, and mechanical travel limit settings.
Question 4: What does 'Change of Value' (COV) subscription mean in BACnet communications?
- A controller updates its program code automatically when values change
- A device sends an unsolicited notification when a monitored value changes by a defined increment (Correct answer)
- The BAS operator is emailed when setpoints are modified
- Controller firmware is updated when a new version is available
Correct answer: A device sends an unsolicited notification when a monitored value changes by a defined increment
COV subscriptions allow a BACnet device to push value updates to subscribers only when the value changes by a defined increment, reducing network traffic.
Question 5: In a BAS, what is the purpose of the 'priority array' in BACnet commandable objects?
- Rank alarm severity from low to high priority
- Manage multiple competing commands to an output by assigning priority levels 1-16 (Correct answer)
- Schedule equipment operation priority during peak demand
- Sort trend data by time priority for archiving
Correct answer: Manage multiple competing commands to an output by assigning priority levels 1-16
The BACnet priority array allows up to 16 commands at different priority levels to compete for control of an output, with the highest active priority winning.
Question 6: What is a 'free cooling' or 'economizer lockout' condition in a BAS?
- Disabling the economizer when outdoor humidity is too high to use outside air for cooling (Correct answer)
- Locking the chiller off during off-peak hours
- Bypassing the cooling coil when duct static pressure is low
- Disabling VAV boxes when outdoor temperature drops below freezing
Correct answer: Disabling the economizer when outdoor humidity is too high to use outside air for cooling
Economizer lockout prevents the use of outdoor air for cooling when outdoor humidity or enthalpy would add more load than it removes.
Question 7: Which BAS function aggregates energy consumption data from multiple meters and sub-meters for reporting?
- Alarm management
- Energy dashboarding and reporting (Correct answer)
- Sequence-of-operations programming
- Network routing
Correct answer: Energy dashboarding and reporting
Energy dashboarding and reporting functions aggregate metered data to provide consumption analysis, trending, and benchmarking reports.
HVAC Building Automation Systems 4
What is 'optimal start' in a BAS control strategy?