HVAC Controls and Thermostats 2 — Questions and Answers
Question 1: What communication protocol, standardized by ASHRAE, was specifically designed for building automation systems?
- Modbus
- BACnet (Correct answer)
- LonWorks
- Profibus
Correct answer: BACnet
BACnet (Building Automation and Control Networks) is the ASHRAE standard 135 communication protocol designed specifically for HVAC and building automation interoperability.
Question 2: What does PID stand for in HVAC control systems?
- Pneumatic Integrated Damper
- Proportional-Integral-Derivative (Correct answer)
- Pressure-Induced Differential
- Programmable Interface Device
Correct answer: Proportional-Integral-Derivative
PID control combines Proportional, Integral, and Derivative actions to minimize error, eliminate steady-state offset, and dampen rapid oscillations.
Question 3: What is the primary function of an economizer in an HVAC control system?
- To reduce the refrigerant charge in a system
- To use outdoor air for free cooling when outdoor conditions are favorable (Correct answer)
- To limit maximum heating output during peak demand
- To control indoor humidity by bypassing the cooling coil
Correct answer: To use outdoor air for free cooling when outdoor conditions are favorable
An economizer uses outdoor air for cooling when outdoor temperature and/or enthalpy is suitable, reducing or eliminating mechanical cooling energy consumption.
Question 4: In a zone control system, what device modulates airflow to individual zones based on each zone's thermostat demand?
- Balancing damper
- Zone damper or VAV box (Correct answer)
- Fire damper
- Pressure relief valve
Correct answer: Zone damper or VAV box
Zone dampers (in simpler systems) or VAV (Variable Air Volume) boxes modulate airflow to each zone individually based on that zone's thermostat signal.
Question 5: What is 'hunting' in an HVAC control system?
- Searching for the optimal setpoint value
- Continuous oscillation of the controlled variable above and below setpoint due to overcorrection (Correct answer)
- Increasing system capacity to find the maximum output
- Locating a refrigerant leak using electronic detectors
Correct answer: Continuous oscillation of the controlled variable above and below setpoint due to overcorrection
Hunting occurs when a control system repeatedly overcorrects, causing the controlled variable to swing continuously above and below the setpoint without stabilizing.
Question 6: What is the primary energy-saving feature of a programmable thermostat?
- Higher heating setpoints during occupied periods
- Automatic temperature setback during unoccupied periods (Correct answer)
- Continuous fan operation to improve air circulation
- Integrated humidistat control for dehumidification
Correct answer: Automatic temperature setback during unoccupied periods
Programmable thermostats save energy by automatically reducing heating or cooling demand during unoccupied hours (setback), lowering energy consumption when conditioning is not needed.
Question 7: What does an enthalpy controller measure to determine if economizer free-cooling is beneficial?
- Only outdoor air dry-bulb temperature
- Both outdoor air temperature and humidity to calculate total heat content (Correct answer)
- Indoor CO2 levels and occupancy
- Refrigerant pressure and temperature
Correct answer: Both outdoor air temperature and humidity to calculate total heat content
An enthalpy controller measures both outdoor dry-bulb temperature and wet-bulb temperature (or relative humidity) to calculate total heat content (enthalpy) and enable economizer mode only when outdoor air is truly beneficial.
HVAC Controls and Thermostats 2
What communication protocol, standardized by ASHRAE, was specifically designed for building automation systems?