BMS glossary for FM managers: JACE, AHU, BACnet and the rest

If you manage a building, the BMS is probably the system you rely on most and understand least. Contractors talk in acronyms, reports are full of point names, and it’s hard to know whether a quote is reasonable.

This glossary explains the terms you’re most likely to hear, grouped by what they relate to. It’s written for facilities managers, building owners and anyone new to the industry.

The system itself

BMS (Building Management System). The network of controllers, sensors and software that monitors and controls building services: heating, cooling, ventilation, and often lighting, metering and other systems. Also called a BEMS (Building Energy Management System) or BAS (Building Automation System). In practice the terms are used interchangeably.

Controller / outstation. A box, usually in a plant room or control panel, that reads sensors and drives equipment. It runs the control strategy locally, so plant keeps running even if the network or front end goes down. “Outstation” is the older UK term.

Front end / head end / supervisor. The software where people view graphics, change setpoints, see alarms and look at trends. It might run on a server on site, a virtual machine, or be accessed through a web browser.

Graphics. The screens on the front end showing plant diagrams with live values. Good graphics make problems obvious; poor ones hide them.

Niagara (Niagara Framework). A widely used software platform, made by Tridium, that many BMS brands build on. It’s popular for integrating equipment from different manufacturers onto one front end.

JACE. A Niagara-based controller that typically sits between the field controllers and the supervisor, connecting to equipment over BACnet, Modbus and other protocols and serving graphics and alarms. Engineers often refer to it as “the JACE” on a site.

Station. In Niagara, the running application on a JACE or supervisor that holds the points, logic, graphics and alarms.

Points, signals and logic

Point. Any single value the BMS knows about: a temperature, a valve position, a fan’s run status, an alarm. A site’s points list or points schedule lists every one.

Inputs and outputs (I/O). Signals going into and out of a controller. You’ll hear AI (analogue input, e.g. a temperature), AO (analogue output, e.g. a valve position), DI (digital input, e.g. fan running) and DO (digital output, e.g. start a pump).

Setpoint. The target value, such as 21 °C for a room. Changing setpoints is the most common BMS adjustment, and the most common cause of complaints when done without understanding the wider strategy.

Control strategy / sequence of operation. The written description of how plant should behave: when it starts, what it’s controlling to, how it responds to faults. It should be in the O&M manual. If it isn’t, ask for it.

PID loop. The standard control method that adjusts an output (a valve, a damper, a fan speed) to keep a measured value at its setpoint. Badly tuned loops cause hunting, where plant swings back and forth around the target.

Time schedule / time zone. When plant is switched on and off. A surprising amount of wasted energy comes from schedules that no longer match how the building is used.

Optimiser. Logic that works out when to start heating or cooling so the building reaches temperature just in time for occupancy, rather than at a fixed early time.

Override / hand mode. Forcing equipment on, off or to a fixed position, bypassing automatic control. Overrides left in place after a fault are a common hidden cause of high energy use.

Interlock. A rule that stops something happening unless something else is true, for example a heater battery that can’t run unless its fan is proven running.

Frost protection. Logic that runs pumps or heating to stop pipes and coils freezing when it’s cold outside, even when the building is unoccupied.

The plant being controlled

AHU (Air Handling Unit). A large unit that conditions and moves fresh air around the building, often with fans, filters, heating and cooling coils and heat recovery.

FCU (Fan Coil Unit). A smaller unit, usually in a ceiling void, that heats or cools a local area.

VAV (Variable Air Volume). A box in the ductwork that varies the amount of air supplied to a zone to control its temperature.

LTHW (Low Temperature Hot Water). The hot water circuit used for heating, typically from boilers or heat pumps.

CHW (Chilled Water). The cold water circuit used for cooling, from chillers.

DHW / HWS (Domestic Hot Water / Hot Water Services). Hot water for taps and showers. Legionella control requirements often involve the BMS monitoring temperatures here.

VSD / inverter. A drive that controls a motor’s speed, usually for fans and pumps. Running plant at lower speeds saves a lot of energy.

Actuator. The motor that moves a valve or damper in response to a signal from the controller.

DP sensor (differential pressure). Measures the pressure difference across something, often a filter (to show when it’s dirty) or a fan (to prove it’s moving air).

Networks and protocols

BACnet. The most common open communication protocol for building automation. It lets equipment from different manufacturers talk to each other and to the front end.

BACnet/IP and BACnet MS/TP. Two ways BACnet travels: over an IP network (like your office network), or over a two-wire RS-485 cable that daisy-chains controllers together. Many sites use both.

Modbus. A simple protocol commonly used for meters, chillers, boilers and drives. Getting values out of it relies on the manufacturer’s register map.

LON (LonWorks). An older protocol still found on many existing sites.

KNX and DALI. Protocols mostly used for lighting and room controls.

Integration. Connecting third-party equipment, such as chillers, meters or lighting systems, into the BMS so it can be monitored and controlled from one place.

BBMD. A device that lets BACnet messages cross between IP subnets. If your IT team re-arranges the network, the BMS contractor may need to reconfigure these.

Contracts, documents and maintenance

O&M manual (Operation and Maintenance). The handover documentation for the building. For the BMS it should include the control strategies, points schedule, wiring diagrams, network drawings and backups.

As-built drawings. Drawings updated to show what was actually installed, not what was designed. Out-of-date as-builts make every fault slower and more expensive to fix.

Commissioning. The process of checking and setting up the system so it works as designed: every point tested, every loop tuned and every sequence proven.

Witness testing. When the client or their representative watches the commissioning engineer demonstrate that the system works.

PPM (Planned Preventative Maintenance). Scheduled maintenance visits. For a BMS this typically covers checking sensors and actuators, reviewing alarms and overrides, checking backups and confirming the front end is healthy.

SFG20. The industry standard maintenance specification for building services in the UK, often used to define what a PPM visit should include and how often.

Reactive maintenance / call-out. Work done in response to a fault, as opposed to planned visits.

Backup. A saved copy of the controller programs and front end configuration. Ask your contractor where the latest backups are kept and how often they’re taken. Losing them can turn a hardware failure into a full rebuild.

Questions worth asking your BMS contractor

• Where are the current backups, and when was the last one taken?
• Do we have up-to-date control strategies and a points schedule?
• Which plant is currently in override, and why?
• Which alarms are we ignoring, and should they be fixed or re-graded?
• Are any controllers, JACEs or software versions approaching end of support?

Knowing the vocabulary won’t make you a controls engineer, but it will help you get straight answers, spot vague quotes, and keep your BMS working for the building rather than against it.