How Does BMS Work?

In the previous article “The Era of Building Automation” we talked about the origin of building automation and how it all started. In this article, we will dig deeper into the architecture of a building management system and how it works.

Building Management System, as the name states, facilitates the management of a building with all of its control and monitoring systems. BMS mainly utilizes BACnet protocol, and can integrate with a wide range of protocols like Modbus,M-Bus,CAN bus,TCP/IP,RS-232,DALI, and others to centralize the control of a facility in one operator.

Building Management System | Binghalib Engineering BMS Division
examples of the systems and protocols that can be integrated to BMS

 just like the human body, BMS is the nervous system of a building. The Control Panels have all the processing power within its controllers. sensors and probes act just like the senses of the human body, to monitor the vital signs of the body and report back to the Control Panel. actuators do all the actions, where they act as the muscles of the building, turning every order from the control panel into physical action.

common applications of BMS control in HVAC are: chillers, pumps, air handling units, fan coil units, boilers,…etc. each one of these systems has a sequence of operation programmed in the control panel. sensors like temperature sensors, pressure sensors, humidity sensors, differential pressure sensors,… and actuators like valves, dampers, dry contact relay,… all of these combined can provide stable operation of the system.

BMS computer interface to control and monitor a chiller plant

Let’s take one simple application, break it down, and see how it works. Assuming that we have a storage facility for a medical company and we have one main chiller on the roof and fan coil units distributed in the storage room to control the temperature and humidity of air in each room.

In such cases, we install a main control panel on the roof to control the chiller and its corresponding pumps, and for each FCU we install a single controller.

a main control panel with the BACnet router in the top right corner

We will dissect the FCU control system as it is the simplest one. The FCU has 2 water coils, heating coil and cooling coil, used to increase or decrease the temperature of the air going through. In order to control that we install a motorized valve on each coil so we can control the flow of water in it from our controller. Temperature sensors are installed on the inputs and the outputs of the coils as a feedback to control the opening of the valves. Temperature and humidity sensor is also installed on the supply duct to compare that to the system setpoint and change the control variables in order to minimize the difference between the setpoint and actual reading.

Module 101: The evolution of fan coils for efficient conditioning ...
Schematic of a Fan Coil Unit

Now for the fan usually we have 3-speed fan controlled by 3 relay outputs, and a Differential Pressure Switch or DPS to monitor its state, for example, if the fan command is ON and the DPS is indicating no difference in pressure before and after the fan, this indicates a problem with the fan. As for the main control of the FCU, this can be either set by the BMS operator or we install a thermostat in the served room so the occupant can set the temperature and occupancy as needed. 

a newly installed FCU with the pipes on the right to be connected to its coils to deliver hot and cold water

All of this equipment mentioned above is connected to one (Application) controller through its Input/Output ports, and with the sequence of operation programmed into the controller, we have a fully functional mini BMS.

an enclosure containing one application controller

Now, we have an independent fully functional controller for each FCU. In order to connect them all together, we BACnet MSTP connection, which stands for BACnet Master-Slave Token Passing protocol. The benefit of such protocol is the wiring of the controllers, it will be a daisy chain network where the cable will go from one controller to another just like wiring a bunch of resistors in series. Physically now, we have a fully connected network of controllers, but there is one missing component to make this whole system work. Just like in your home network you need a router for your network devices to communicate with each other, in BMS we add a BACnet router for each system to route the communication between the controllers. And this is the difference between an application controller and system-level controller, where the application controller, like the one we used for the FCU, doesn’t have routing capabilities, but a system controller can route the communication throughout the whole network. and also this why we called the roof panel, the main control panel, since it contains the router fo the whole system in addition to the application controllers used for the chiller and its pumps.

One more feature the router has is the ability to communicate over TCP/IP, so we can connect our BMS workstation with the BMS software on it to the main panel through via ethernet cable and we will be able to monitor and control all of the systems in our facility, being it the chiller, pumps, or the FCUs. And this connection enables us as well, to pass the demand from the FCU controllers to the main panel so it can automatically control the temperature and pressure of water in the chiller to meet that demand.

a main control panel connected via Ethernet to a PC for testing and commissioning

And with this, we have covered the main working principle of any BMS through a direct HVAC application.

A peek into the future.

With the numerous integrations between BMS and other systems that are being developed nowadays like security systems, home automation systems, power metering systems, and so many other systems, and with the rise of supercomputers and machine learning algorithms, we are already witnessing great applications being developed and many “smart” buildings being built. Check out the video below on the Smartest BMS on earth so far.

So, now that we have smart buildings learning how to behave every day. What do you think comes next?

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