ASHRAE ASHRAE Controls Automation and Building Systems Integration 2 — Questions and Answers
Question 1: According to ASHRAE, what is the purpose of 'demand-controlled ventilation (DCV)' in building HVAC control?
- To modulate outdoor air ventilation rates based on actual occupancy measured by CO₂ sensors, saving energy when spaces are less than fully occupied (Correct answer)
- To control the building's peak electrical demand by shedding HVAC loads during utility demand periods
- To automatically increase ventilation when indoor contaminant levels exceed ASHRAE 62.1 limits
- To cycle supply fans on and off based on outdoor air quality index readings
Correct answer: To modulate outdoor air ventilation rates based on actual occupancy measured by CO₂ sensors, saving energy when spaces are less than fully occupied
DCV uses CO₂ concentration as a surrogate for occupancy to dynamically adjust outdoor air ventilation rates, reducing over-ventilation of under-occupied spaces and saving fan and conditioning energy.
Question 2: In ASHRAE chilled water plant controls, what is 'chiller sequencing'?
- The staged start/stop and load distribution strategy for operating multiple chillers to maximize efficiency at varying building loads (Correct answer)
- The programmed order in which cooling coils are activated across multiple air handling units
- The sequence of refrigerant circuit steps during chiller startup to prevent compressor damage
- The daily schedule for chiller pre-cooling operation before building occupancy
Correct answer: The staged start/stop and load distribution strategy for operating multiple chillers to maximize efficiency at varying building loads
Chiller sequencing determines when to start or stop additional chillers and how to distribute load among operating chillers to keep each unit operating near its most efficient point.
Question 3: What is 'optimal start' control in building automation systems as referenced in ASHRAE guidelines?
- An algorithm that calculates the latest possible startup time for HVAC equipment to achieve comfort conditions by occupancy, using current building thermal mass and outdoor conditions (Correct answer)
- A control feature that starts chillers at minimum load to prevent refrigerant slugging
- A sequence that energizes all HVAC systems simultaneously at a fixed pre-scheduled time before occupancy
- A strategy that starts the HVAC system only after occupants trigger a motion sensor
Correct answer: An algorithm that calculates the latest possible startup time for HVAC equipment to achieve comfort conditions by occupancy, using current building thermal mass and outdoor conditions
Optimal start calculates the latest time to begin heating or cooling a building before occupancy based on current indoor/outdoor temperatures and the building's learned thermal response, saving energy versus fixed early-start schedules.
Question 4: In ASHRAE building controls, what does 'reset control' (e.g., supply air temperature reset) accomplish?
- It adjusts a control setpoint upward or downward in response to measured conditions to improve efficiency while maintaining comfort (Correct answer)
- It resets the controller to factory defaults when a fault is detected
- It restores the previous day's operating schedule after a power failure
- It re-tunes PID gain values automatically when process dynamics change
Correct answer: It adjusts a control setpoint upward or downward in response to measured conditions to improve efficiency while maintaining comfort
Reset control varies a setpoint (such as chilled water or supply air temperature) based on outdoor air temperature or zone demand, allowing the plant to operate at less extreme conditions and save energy during mild weather.
Question 5: According to ASHRAE, what is the role of a 'building automation system (BAS)' in modern HVAC operation?
- To monitor and control all building mechanical, electrical, and life-safety systems from a centralized platform to optimize performance, comfort, and energy use (Correct answer)
- To replace all manual thermostats with programmable setback controls only
- To directly operate refrigeration compressors and eliminate local equipment controllers
- To interface only with fire alarm and security systems without controlling HVAC equipment
Correct answer: To monitor and control all building mechanical, electrical, and life-safety systems from a centralized platform to optimize performance, comfort, and energy use
A BAS integrates and automates control of HVAC, lighting, and other building systems from a central supervisory platform, enabling energy optimization, fault detection, and remote monitoring.
Question 6: In ASHRAE VAV system design, what is 'duct static pressure reset' and why is it recommended?
- Reducing the duct static pressure setpoint as VAV boxes open toward maximum to save fan energy (Correct answer)
- Increasing static pressure when a zone calls for more airflow to ensure adequate delivery
- Setting the static pressure to a fixed maximum value to guarantee airflow to all zones
- Resetting the duct static sensor calibration during off-hours to prevent sensor drift
Correct answer: Reducing the duct static pressure setpoint as VAV boxes open toward maximum to save fan energy
Static pressure reset lowers the duct pressure setpoint when zone VAV dampers indicate they can be satisfied at lower pressure, allowing the supply fan to slow down and save significant energy.
ASHRAE ASHRAE Controls Automation and Building Systems Integration 2
According to ASHRAE, what is the purpose of 'demand-controlled ventilation (DCV)' in building HVAC control?