Controls Glossary

Building controls has a vocabulary problem: the same system gets described in acronyms by vendors, in brand names by contractors, and in plain frustration by the people who operate it. These are the terms owners and operators actually run into, in plain language, grouped by what they describe.

The system itself

• BAS / BMS (Building Automation System / Building Management System): two names for the same thing, the system of controllers, sensors, and software that runs your HVAC and often lighting and metering. This site says BAS.
• DDC (Direct Digital Control): the modern approach where a programmable controller, not a pneumatic device, makes the control decisions. If your building was built or upgraded after the 1990s, this is probably what you have.
• Controller: the small computer mounted near equipment that reads sensors and drives valves, dampers, and motors. Buildings typically have dozens to hundreds.
• Supervisor / front end: the software layer where operators see graphics, trends, alarms, and schedules for the whole building. It supervises the controllers; it does not replace them.
• Operator workstation: the computer, or these days the browser, where staff interact with the front end.
• Pneumatics: the older control approach that runs on compressed air instead of electronics. Still alive in many buildings, often side by side with DDC.

Points and data

• Point: a single input or output the system knows about, such as one temperature reading or one fan command. Buildings are sized in points; a mid-size building may have thousands.
• AI / AO (analog input / analog output): a measured value coming in (a temperature) or a variable command going out (a valve at 40 percent).
• BI / BO (binary input / binary output): an on-or-off status coming in (fan running or not) or an on-or-off command going out (start the fan).
• Points list: the master schedule of every point, its name, and where it lands. The single most valuable document in a controls project, and the one most often out of date.
• Trend: a recorded history of a point over time. Trends are how problems get diagnosed without standing in the mechanical room.
• Alarm: the system flagging a condition somebody should act on. A healthy system has few alarms and takes all of them seriously.
• Setpoint: the value the system is trying to hold, such as 72 degrees in a zone.
• Override: a manual command that holds a point at a fixed value, bypassing the automatic logic. Useful for a day, corrosive for a year.
• Schedule: the calendar logic that decides when equipment runs. Wrong schedules are one of the most common sources of wasted energy.
• COV (change of value): a device reporting a point only when it changes instead of being asked constantly. Lighter on the network than polling.

Protocols and networks

• Protocol: the language devices use to talk to each other. Shared protocols are what let equipment from different manufacturers work together.
• BACnet: the dominant open protocol in building automation, published as an ASHRAE standard. Comes in network flavors, most commonly BACnet/IP and BACnet MS/TP.
• BACnet/IP: BACnet running over standard Ethernet networks, fast and switch-based.
• BACnet MS/TP: BACnet running over a shared twisted-pair trunk where devices take turns via a passed token. Cheaper to wire, slower, and sensitive to configuration.
• Modbus: a simple, widespread protocol spoken by meters, drives, boilers, and packaged equipment. Common at the equipment edge of a building.
• LonWorks: an interoperability platform from an earlier generation of open systems, still present in many existing buildings.
• Niagara: a software platform, not a protocol, that connects and supervises devices across protocols. A JACE is a hardware controller that runs it. Widely used, and worth naming precisely: a building can speak BACnet without Niagara, and vice versa.
• Gateway: a device that translates between two protocols. Necessary sometimes, but every gateway is a translation layer that can lose detail, so integrations should prefer native protocol support where it exists.
• Integration: making systems from different manufacturers or eras work as one, through shared protocols, gateways, or a supervisory platform.

Equipment the controls touch

• AHU (air handling unit): the machine that conditions and moves air, usually the most instrumented piece of equipment in the building.
• RTU (rooftop unit): a packaged air handler that lives on the roof, common in commercial buildings.
• VAV box (variable air volume): the small terminal unit that meters conditioned air into a zone. Buildings often have one per zone, and their small controllers are a frequent modernization target.
• VFD (variable frequency drive): the device that lets a fan or pump run at variable speed instead of only full blast. Large energy savings live here.
• Economizer: dampers and logic that cool with outside air when conditions allow, instead of running mechanical cooling.
• DCV (demand controlled ventilation): ventilating based on measured occupancy, usually via CO2 sensors, instead of a fixed worst-case rate.

Projects and delivery

• Sequence of operations (SOO): the written description of how the system is supposed to control the equipment. If it does not exist or does not match reality, troubleshooting becomes archaeology.
• Commissioning (Cx): the structured process of verifying the system actually does what the design says, point by point and sequence by sequence.
• Retro-commissioning (RCx): the same discipline applied to an existing building that has drifted from its intended operation.
• Point-to-point checkout: verifying each physical point end to end, sensor to graphic, before trusting the system.
• As-builts: drawings and documents updated to match what was actually installed, as opposed to what was designed. Demand them at handoff.
• Division 25: the section of a construction specification that covers integrated automation, where the integration and documentation requirements live.
• FDD (fault detection and diagnostics): software that watches trends and flags equipment problems automatically. Only as good as the sensor data feeding it.

Missing a term you keep running into? Ask us and we will add it: the glossary grows from real conversations with owners and operators.