Building Automation System (BAS)
The networked controls that monitor and run a building's equipment.
Quick Answer
A building automation system (BAS) is a network of sensors, controllers, and software that monitors and controls building equipment, mainly HVAC but often also lighting, metering, and alarms. Operators use a central interface to set schedules and setpoints, view trends, and respond to alarms. It is also called a building management system (BMS).
The Full Picture
A BAS has layers. Field devices such as temperature sensors, actuators, valves, and dampers sit at the equipment. Controllers run the control logic for each air handler, chiller, or zone. A network connects the controllers, and a supervisory front end gives operators graphics, scheduling, alarms, and trend logs.
Communication between devices relies on protocols. BACnet, an ASHRAE standard, and Modbus are widely used in building controls, and LonWorks appears in some older installations. Open protocols make it easier to integrate equipment from different manufacturers, though integration still depends on careful specification and testing.
The BAS carries out the sequences of operation that the mechanical engineer writes: how a fan speed responds to duct pressure, when economizers open, how setpoints reset, and what happens at unoccupied times. Good sequences and good programming are what turn equipment into an energy-efficient system, and commissioning verifies them.
For contractors, controls scope is a notorious boundary issue. Specifications may place the BAS under the mechanical contractor, a separate controls subcontractor, or a mix, and electrical power, low-voltage wiring, and IT network connections each have their own interfaces. Confirming who provides each device, wire, and software license early avoids bid gaps.
Real Examples
Common Misconceptions
People assume: A BAS and a thermostat are the same thing.
Actually: A thermostat controls one zone locally. A BAS networks many controllers, applies programmed sequences across equipment, logs data, and gives operators central visibility and alarms.
People assume: An open protocol means any device will work together automatically.
Actually: Protocols like BACnet make integration possible, but points, naming, and testing still have to be specified and verified for the pieces to work together.
Frequently Asked Questions
What is the difference between BAS and BMS?
The terms are used interchangeably. Some firms use BMS for broader management that includes security or energy dashboards, and BAS for HVAC-focused controls.
What does a BAS control?
Mainly HVAC equipment such as air handlers, chillers, boilers, and terminal units. Many also integrate lighting, metering, and alarm monitoring.
What is BACnet?
BACnet is a data communication protocol for building automation and control networks, developed under ASHRAE, that lets controllers from different manufacturers exchange information.
Who installs the building automation system?
Usually a controls subcontractor, which may be a manufacturer's branch or a controls integrator, sometimes under the mechanical contractor, depending on the project's specifications.