ASHRAE ASHRAE Controls Automation and Building Systems Integration 1 — Questions and Answers
Question 1: What communication protocol is widely recognized by ASHRAE as the standard for building automation and control network (BACnet) interoperability?
- BACnet (ASHRAE Standard 135) (Correct answer)
- Modbus RTU only
- LonWorks only
- DALI (Digital Addressable Lighting Interface)
Correct answer: BACnet (ASHRAE Standard 135)
BACnet, defined in ASHRAE Standard 135, is the open communication protocol developed specifically for building automation and control systems to enable interoperability between devices from different manufacturers.
Question 2: In ASHRAE building control terminology, what is a 'setpoint' in a control loop?
- The desired target value that the controller attempts to maintain for a controlled variable (Correct answer)
- The maximum allowable pressure in a refrigerant circuit
- The minimum outdoor air fraction required for ventilation
- The rated capacity of the HVAC equipment at design conditions
Correct answer: The desired target value that the controller attempts to maintain for a controlled variable
A setpoint is the desired value for the controlled variable (such as room temperature or supply air pressure) that the control system continuously works to maintain.
Question 3: What type of control action does a proportional-integral-derivative (PID) controller use to eliminate steady-state offset in HVAC systems?
- Integral action (Correct answer)
- Proportional action
- Derivative action
- On-off (bang-bang) action
Correct answer: Integral action
Integral action in a PID controller accumulates the error over time and adjusts the output until the steady-state error (offset) is eliminated, which proportional-only control cannot achieve.
Question 4: According to ASHRAE guidelines, what is 'economizer control' in an air-handling unit?
- A control strategy that uses outdoor air for free cooling when outdoor conditions are favorable, reducing mechanical cooling energy (Correct answer)
- A method to reduce fan speed during low-load periods to save fan energy
- A sequence that pre-heats outdoor air in cold climates to prevent freezing of coils
- A control mode that cycles the compressor to maintain supply air temperature within a dead band
Correct answer: A control strategy that uses outdoor air for free cooling when outdoor conditions are favorable, reducing mechanical cooling energy
Economizer control modulates the outdoor air damper to increase free cooling from outdoor air whenever outdoor conditions (temperature and/or enthalpy) allow, reducing or eliminating mechanical refrigeration.
Question 5: In ASHRAE Variable Air Volume (VAV) system controls, what is the primary variable that the VAV terminal unit modulates?
- Airflow rate to the zone (Correct answer)
- Supply air temperature at the AHU
- Chilled water flow through the cooling coil
- Static pressure in the main supply duct
Correct answer: Airflow rate to the zone
A VAV terminal unit modulates the volume of conditioned supply air delivered to a zone to match the zone's varying cooling or heating load while maintaining a relatively constant supply air temperature.
Question 6: What does ASHRAE define as a 'direct digital control (DDC)' system in building automation?
- A control system where a digital computer or microprocessor directly performs the control algorithm and sends output signals to controlled devices (Correct answer)
- An analog pneumatic control system with digital display readouts
- A system where building operators manually enter setpoints via a digital touchscreen
- A control network that transmits analog 4-20 mA signals between field devices
Correct answer: A control system where a digital computer or microprocessor directly performs the control algorithm and sends output signals to controlled devices
DDC systems use microprocessors to execute control algorithms digitally, replacing pneumatic or analog electronic controls with programmable digital controllers that offer greater flexibility and accuracy.
ASHRAE ASHRAE Controls Automation and Building Systems Integration 1
What communication protocol is widely recognized by ASHRAE as the standard for building automation and control network (BACnet) interoperability?