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Building Automation System – Components and How it Works


Unless you are a controls engineer or are around the system often, a Building Automation System (BAS) may seem like magic. The following article will break down the different parts, levels, and implementations of a BAS.

Note: Building automation system, building management system, BAS, BMS are all interchangeable and represent the same thing.


BMS Levels

There are typically 4 levels to a BAS system:

Level 1: Supervisor

Level 2: Network Controller

Level 3: Field Controller

Level 4: Sensor/Edge


Level 1, the Supervisor, is the software and user interface portion of the BAS where the end user can view the different devices being controlled and monitored. The physical component at Level 1 is a computer, data rack, or server. Interfaces typically consist of a software program that comes with the specific system and can control any point within the entire BAS. Figure 1 displays a sample BAS interface that is monitoring an air-handling unit.




Figure 1: Air-handling unit BAS interface display



Level 2 contains the network controller, often referred to in the industry as the JACE. Even though JACE is a product of a network controller manufacturer, it is the controls industry’s equivalent of a ‘Kleenex’; JACE and Kleenex are both single names used to represent a wide umbrella of different manufacturers. If Level 1 is the user interface of the BAS, the JACE is the brains of the operation. All information being collected and transmitted by Levels 3 and 4 push to the JACE which then translates and transmits to Level 1. The word ‘translate’ is used because many times the subsystems within the BAS will utilize different coding languages. BACNET, IPT, LON, use different connections (ethernet or twisted pair) and work with different languages. A JACE can accept both ethernet and twisted pair connections and, as mentioned, will compile these signals and translate them to a language that can be interpreted by the Level 1 supervisor.


Any data that is being monitored by a system’s BAS is considered a ‘point’. Let us take the office (or spare bedroom since most of us are working from home) you are sitting in right now and hypothetically say temperature and humidity are being monitored by the BAS. Your office’s temperature would be a single point and the humidity would be considered another single, separate point. Points can be further broken down into 4 types, but if I start to get into that you may not have the desire to read any further.

Most JACE’s are licensed for up to 200 devices with 50 points per device for a total of 10,000 points per JACE. A device example may be a VAV. This may seem like more than enough, but if you are designing a 300,000 square foot high school with many rooms containing half a dozen points or more (temperature, humidity, lighting, VAV damper positions, etc.) it can add up quickly. Not to fear, though, multiple JACEs can be incorporated into a single system.


Level 3 consists of the BAS’s field controllers and input/output devices. The field controllers are your local equipment controllers such as a lighting control panel, an AHU controller, etc. Your field controllers are connected to your JACE via ethernet or twisted pair. The most common controller languages are BACnet, MODBUS, and LON, or LonMark.


The final level, Level 4, are your individual sensors. These are your occupancy sensors, thermostats, CO2 sensors, etc. The sensors are connected back to the field controllers and send local data back to make decisions on whether to adjust settings or maintain. A good example of this would be a room occupancy sensor for lighting. If the sensor detects occupants within its space, it will send an output to a lighting control panel, which will then send a signal to a JACE that the room is occupied, and last, that will show up on the interface as the lights being on with the space occupied.



Figure 2: BAS backbone and levels diagram



Pros and Cons

As with any system, there are going to be pros and cons to designing a smart building. It is up to the owner to decide whether the advantages outweigh the disadvantages.


Reduced daily expenditure might be the biggest advantage to having a BAS. With the thousands of points in a system, a BAS can automatically cutdown output. Example, based on daylight conditions, a BAS can lower or raise shades then decrease the lighting output, reducing the energy used for lighting. Also, by reducing the amount of time a device is used, a BAS can extend the lifetime of systems.

Enhanced productivity through more efficient and effective cooling/heating will allow the occupants of a space to be more comfortable. Studies have shown that this leads to better productivity, meaning potentially more money for the building/business owner.


Cost of installation can run high with a BAS. Just to control lighting or shades requires extra conduit, cabling, switch ports, etc., all leading to more upfront cost. Not to be forgotten in this cost is commissioning of a building. I can personally say that I have had multiple projects go sideways when the building is 98% complete. This is because all the components incorporated into the BAS need to integrate smoothly to work the way it was designed. This can lead to headaches for all parties involved, especially when the building appears to be complete, but a room sits at 80% humidity for no reason.

Technology is ever changing, and this is a major drawback for a BAS system. A smart building completed in 2020 will most likely be out of date in the year 2035 or sooner. This means that as the life cycle of each component nears the end it may be a challenge to replace those parts and still have the BAS work as designed 10-15 years ago.



Like a complex math equation, once the backbone of a building automation system is broken down it is much easier to understand. Each level has a job to fulfill. And as with all things in this world, nothing is perfect and can have disadvantages. But, with research and references, these pros and cons can be weighed out by the owner to decide. So, the next time you are thinking about getting a controls degree when the acronym ‘BAS’ is brought up in your design meeting, take a deep breath and reference this article for help; you will be happy you did!

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