Building Automation Systems Flashcards

What does the acronym 'BAS' stand for in commercial HVAC?

Answer: Building Automation System

BAS stands for Building Automation System. A BAS integrates control of HVAC, lighting, fire, security, and other building systems through a centralized, computer-based control platform.

What communication protocol is most widely used for BAS integration in commercial HVAC?

Answer: BACnet

BACnet (Building Automation and Control Networks) is the ASHRAE/ANSI standard communication protocol (ASHRAE 135) most widely used for BAS integration. It allows devices from different manufacturers to communicate on the same network.

What is a 'DDC' controller in a building automation system?

Answer: Direct Digital Control — uses microprocessors to control HVAC equipment based on sensor inputs

DDC (Direct Digital Control) uses microprocessor-based controllers to receive sensor inputs, execute control logic, and send output commands to HVAC equipment. DDC replaced pneumatic and analog electronic controls in modern systems.

What is the purpose of a Variable Air Volume (VAV) box in a BAS-controlled system?

Answer: To modulate airflow to individual zones to maintain zone temperature setpoints

A VAV box is a terminal unit that modulates the volume of conditioned air delivered to a zone by adjusting a damper. The BAS controls each VAV box to maintain the zone's temperature setpoint, allowing individual zone control.

What is 'demand-controlled ventilation' (DCV) and what sensor drives it?

Answer: Outdoor air ventilation modulated based on actual occupancy, driven by CO2 sensors

DCV uses CO2 sensors to measure occupancy-related CO2 levels and adjusts outdoor air intake to match actual occupancy. When CO2 rises, more outdoor air is introduced; when occupancy drops, outdoor air is reduced, saving energy.

What does 'PID control' stand for and what are its three components in a BAS?

Answer: Proportional-Integral-Derivative — three terms that eliminate steady-state error and improve control response

PID control combines three actions: Proportional (responds to current error), Integral (eliminates steady-state offset by responding to accumulated error over time), and Derivative (responds to the rate of error change to dampen oscillation).

What is an 'occupied/unoccupied' setback schedule in a BAS?

Answer: Temperature setpoint schedules that relax HVAC control setpoints during unoccupied periods to save energy

Setback scheduling programs the BAS to use relaxed temperature setpoints (higher cooling setpoint, lower heating setpoint) during unoccupied periods. This reduces HVAC energy consumption without sacrificing comfort during occupied hours.

What is the primary function of an 'economizer' controlled by a BAS?

Answer: To use outdoor air for free cooling when outdoor conditions are favorable, reducing mechanical cooling

An economizer uses outdoor air for 'free cooling' when outdoor temperature (or enthalpy) is low enough. The BAS opens the outdoor air damper when outdoor conditions are favorable, reducing or eliminating the need for mechanical refrigeration.

What is the difference between a 'hardwired' point and a 'network' point in a BAS?

Answer: Hardwired points are physically wired to the controller; network points communicate via a digital network protocol

Hardwired (or physical) points are sensors/actuators directly wired to controller I/O terminals. Network points are communicated digitally via protocols like BACnet MS/TP, allowing remote devices to share data across the network without individual wiring.

What does 'ASHRAE 90.1' establish that BAS designers must follow?

Answer: Energy efficiency standards including HVAC controls requirements for commercial buildings

ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) establishes minimum energy efficiency requirements for commercial buildings, including HVAC controls requirements like economizers, setback controls, and variable speed drives.

What is the function of a 'supervisory controller' (or 'front-end workstation') in a BAS?

Answer: It provides a central interface for monitoring, overriding, and programming all field-level controllers

A supervisory controller (building controller or front-end workstation) provides centralized monitoring, control, alarming, and trending for all subsystems. It typically does not directly control equipment but communicates with and manages field-level DDC controllers.

What is 'trend logging' in a BAS and why is it important?

Answer: Recording time-stamped sensor and control values over time to analyze performance and diagnose problems

Trend logging records the historical values of points (temperatures, pressures, flows, control outputs) at defined intervals. These logs allow technicians and operators to analyze system performance, diagnose problems, verify setpoint compliance, and demonstrate energy savings.

What is a 'sequence of operations' document in HVAC controls?

Answer: A written description of how an HVAC system and its controls should operate under all conditions

A sequence of operations (SOO) is a narrative description of exactly how the HVAC system should function — startup, normal operation, setback, alarms, shutdown, and emergency modes. It's the primary reference for programming and commissioning DDC controls.

What is 'load shedding' in the context of a BAS?

Answer: Removing non-critical electrical loads to reduce peak demand charges

Load shedding is the programmed reduction of non-critical electrical loads (HVAC, lighting, etc.) during peak demand periods to avoid high demand charges from utilities. The BAS can automatically cycle or reduce equipment operation to manage peak kW demand.

What is an 'alarm management' system in a BAS?

Answer: A system that detects out-of-range conditions, notifies operators, and tracks alarm acknowledgment and resolution

BAS alarm management detects abnormal conditions (equipment faults, sensor out-of-range, setpoint deviations), generates alarms, routes notifications to appropriate personnel, and tracks acknowledgment and resolution to ensure nothing is missed.

What is 'commissioning' of a BAS and why is it required?

Answer: A systematic process to verify that the BAS is programmed and operating according to the design intent

BAS commissioning is the systematic process of verifying that the installed system matches the design intent — checking that sensors are calibrated, control sequences are programmed correctly, setpoints are met, and the system performs as specified.

What is the purpose of 'proportional band' in a DDC control loop?

Answer: The range of measured variable over which the controller output changes from 0% to 100%

Proportional band is the range of the controlled variable (e.g., temperature) over which the controller output varies from minimum to maximum. A narrow proportional band creates aggressive response (potentially unstable); a wide band creates sluggish response.

What is 'chiller sequencing' in a multi-chiller BAS plant?

Answer: Logic that starts and stops chillers in the optimal order to meet load efficiently

Chiller sequencing logic determines when to start additional chillers (as load increases) and when to stop them (as load decreases), optimizing energy efficiency. Proper sequencing avoids short-cycling, prevents simultaneous starts, and maintains optimal chiller loading.

What does 'reset' control mean in a BAS (e.g., supply air temperature reset)?

Answer: Dynamically adjusting a setpoint based on changing conditions (e.g., lowering chilled water temperature on hot days)

Reset control dynamically adjusts setpoints based on operating conditions. For example, supply air temperature reset raises the supply air temperature setpoint on mild days (reducing cooling energy) and lowers it on hot days when more cooling is needed.

What is a 'network topology' in BAS communication, and which type is most common for field-level HVAC controllers?

Answer: Daisy-chain (bus) topology — controllers connected in series on a common communication bus

BACnet MS/TP and many other field-level protocols use a daisy-chain (bus) topology where controllers are connected in series on a two-wire communication bus. This is cost-effective and practical for connecting many devices in a building.