Understand the Basic concept of BMS system

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What is a BMS or Building Management System?

In a nutshellBMS-System otherwise called as BAS or building automation is computer-based control system which reduces the workforce, automate the system, and saving the energy consumption in buildings by monitoring and controlling the mechanical and electrical equipment in modern-day buildings or any industrial plants.
Not only that but BMS helps to
  • Increasing productivity.
  • Increase the equipment lifetime and better performance.
  • Identify the systems faults earliest.
  • Manag the hotel tenants in an effective manner.
Nowadays any modern-day buildings built with BMS to support facilities management to accomplish the maintenance and save the energy in the building from one place of computers. 

Essential Features of BMS software

  • monitoring and controlling connected equipment in the building. 
  • The alarm should be a popup in operator workstation for any critical faults in the system. 
  • Any types of equipment on, off status and alarm should be logged or stored in PC to retrieve later.
  • Schedule the equipment to on and off automatically by preset time. 
  • User interface graphics should be available to visualise the field equipment to monitor for BMS operator easily. 

List of Equipment controlled by BMS or BAS in buildings

  • HVAC (Heating, Ventilation, and Air-conditioning or all supply and exhaust fans, ACs etc.). 
  • Lighting control system. 
  • Fire alarm system. 
  • Firefighting system. 
  • Security control system. 
  • CCTV system.
  • Lift control system. 
  • Pumping system. 
  • Water tanks level. 
  • Irrigation system. 
  • Electrical meters.
  • Water Leak detection system.
  • Split units. 
  • VFD-Variable frequency drives. 
  • VRF/VRV-Variable refrigerant flow or volume (both are same but each term copyrighted by a different vendor) 
  • And any other system which has provision for BMS to control and monitor. 

Main components of the BMS System

  1. Hardware
    • DDC-Direct digital controller
    • Sensors
    • Actuators
    • Cables to connect sensors, actuators to DDC.
    • HMI display-Human machine interface.
    • PC Workstation
    • Server to save the extensive database.
  2. Software
    • Programming or configuration tools.
    • Graphics or User interface.
  3. Networking protocols
    • TCP/IPTransfer control protocols/Internet Protocol.
    • BACnetBuilding automation controller network-ASHRAE
    • Modbus
    • LonWorks
    • CANbus
    • and numerous protocols available.
Don’t worry about the various protocols, this all protocol doing the same task to transfer data from one device to another device. anyhow we will discuss these each protocol in an upcoming post

BMS System architecture in the modern-day building

BMS Architecture Schematic
However, BMS System controls and monitor all the electrical and mechanical systems in buildings from BMS workstation or HMI(Human Machine Interfaces),
But not directly because each system has its dedicated functionality and unique purpose like,
  • HVAC System helps to facilitate and provide comfortable and healthy air conditioning to tenants.
  • The lighting control system which has a variety of lightings in buildings that needs to be on and off effectively and save energy while tenants not available.
  • CCTV helps to facility management to secure the building
  • Access control systems may also be used to control access into certain areas located within the interior of buildings.
  • A fire alarm system is the life safety system to warn people by audio and visual to protect their lives from fires, smoke, carbon mono oxide and other toxic elements for the human.
  • In case of fire Firefighting system aims to protect human life and property in the building by a large amount of water and other gas.
  • UPS is to provide to the uninterrupted power supply in the building for electrical equipment.
  • Pumping system used in the building to pump the water to the required area.
  • Still tons of systems evolved in the modern-day building to facilitate the people.
All systems have their dedicated controllers and processing system due to the different functionality of each system.
So BMS controllers or device designed for controlling and monitoring the HVAC system and other small systems and integrate all other systems through dedicated networking protocols like BACnet, Modbus etc.

General BMS System architecture with Levels

BMS System architecture
  • Management Level: This is the front end for the operator and engineer used to visualise the graphics for controlling and monitoring the systems which have computer workstation, server, web browser, printers.
  • Automation Level: BMS Router and other main controllers connected in building network integrate the third-party system and connect BMS devices.
  • Field devices Level: this is Level where BMS controllers connect to field systems sensors, actuators, and other panel circuits to monitor and control.

Real-Time example for BMS System

Any modern-day building client provides huge specifications for BMS System,
Whereas here I am going to take simple requirement to monitor and control the sequence of Air Handling Unit. 
Let us see below the requirement of the client to monitor and control the sequence in BMS System.
Before we go detailed about how to design the BMS System for the requirement, let us see some basics components of the AHU-Air handling unit.
AHU is an HVAC system which consists of the duct, fan, filter, cooling coil, heating element,humidifier, sound attenuators, dampers, valves and many more to regulate the air into the room by heating, ventilation and conditioning to distributes the conditioned air through the building and returns it to the AHU and also called as centralised AC in modern-day building.
Duct – It is the collection of metallic tubes that interconnected and distributes the heated/cooled air to the required rooms.
To monitor the duct air temperature in fresh, return and supply duct. We have to install the duct temperature sensor in the duct.
Fan Motor– Blower is used to circulate the air from fresh and return duct to the supply duct.
This fan motor controlled and monitored by the separate electrical panel by the designed electrical circuit with the help of electrical relay and contactor and providing an option to BMS system to
  • On/Off the fan.
  • Monitor the fan running status.
  • Monitor the Fan motor overload fault status and many more.
Filter– It is one of the main components in AHU to prevent the dust and dirt particles from entering in the AHU.
When the AHU fan motor started, the fresh outside air supplied into the duct where filter components used to filter the dirty particles continuously and to monitor the filter extreme dirty condition,
DPS switch is used to install across the filter and provide signals to BMS,
when the filter gets dirty(technically DPS-Differential pressure switch will send the signal to BMS when the pressure reached more than pre-set across the filter, and this same function can be used to monitor the fan status.
Heating/Cooling element- It is used to cool or heat the water that entered in the coil so that air in the duct can be heated or cooled based on the user requirement.
Either heating or cooling water enters into the coils are controlled and monitored by valves on the pipe with the help of the valve actuator.
Dampers- An HVAC damper is a movable plate, located in the ductwork, that regulates airflow and directs it to areas that need it most.
damper opening and closing position-controlled electrically with the help of damper actuators, and these actuators have terminal for control from BMS and terminal to monitor the feedback of position.

System Description

The variable speeds Air Handling Units are used to serve air conditioning need for all area of


The Air Handling Unit comprises

  • Variable Speed Supply Fan
  • Chilled water coil with the 2-Way modulating control valve
  • Duct mounted supply air pressure sensor
  • Outdoor & re-circulating air modulating damper
  • Carbon dioxide sensor.
  • Supply and Return Air temperature sensors
  • Supply air differential pressure switch
  • Differential pressure switches for 2 set of filters

System Monitoring and Alarm

  1. Software alarms shall be generated at the operator workstation whenever the run status of the supply fan (with differential pressure switch) does not match the current command state.
  2. A failure alarm shall occur when the run status of the load shows no operation, and the load has been commanded to be on.
  3. An advisory alarm shall occur when the run status of the load shows operation and the load has been commanded to be off. All alarms shall be recorded in an alarm log for future review. Provide 15 seconds (adjustable) time delays before generating an alarm.

The sequence of Operation

a. Auto Mode:

When the AHU start is in AUTO mode (i.e. selector switch installed in the MCC must be in Auto Position), the unit is started and stopped from the BMS via a time schedule or BMS override command. When the start for the AHU is initiated, the control program residing in the controller follows the following sequence


The following sequence follows with a preset time interval per interlock equipment start-up:

1) Check Supply fan trip signal – Normal State

2) Supply Air Damper –Open Position

3) Outdoor Air Damper –Open Position

4) Return Air Damper – Open Position

5) Once the above conditions are satisfied, AHU is enabled to start in Auto mode or using a plant enable button on the graphics in manual mode by the operator. Once enabled, BMS will automatically command the supply fan to start.

6) Supply Fan shall start, and it’s associated Interlock equipment in sequence. Through the signal from the Diff. Airflow Switch, if airflow is detected, the System will continuously run, if No airflow is detected by the DP Switch, the Supply Fan will de-activated and send an Alarm to the DDC – for “No Airflow” and shut down the whole system including its associated interlocks. If the Airflow switch signal is proved ‘ON’ then BMS will enable control loops.

b. Shutdown Mode:

When the shutdown command for the AHU is initiated, the control program residing in the

controller follows the following sequence.

1) Send Stop command to stop the supply fan

2) The outdoor air, return and supply air damper move to close

3) Move chilled water valve to close position

b. Manual (Hand) Mode:

When the AHU is the manual mode, the fans are started and stopped from the AHU control panel. Other control except for fan on/off control shall function as per the Auto mode.

c. Fire / Smoke Mode:

Fire condition is determined by the Fire Alarm Control Panel. AHU will automatically shutdowns the whole system with associated interlocks.

4. AHU Control

The control program, on the feedback of air handling unit operation, initiates the control

algorithm. This algorithm consists of three controls. Each temperature, pressure and ventilation control has its own control loop. The pressure control loop is used to modulate the speed of the supply air fan hence supply airflow. The control loops design to function as per the following explanation

a. Temperature Control loop:

The supply air temperature installed in the duct will relay the measured signal(temperature) to the DDC controller, the DDC controller compares this signal with set-point (adjustable by the operator from BMS central) and generates an analog output to the 2-way modulating cooling valve. Based on the difference between the two values, a proportional-integral program will determine the percentage of the cooling coil valves opening to achieve the desired condition. The default set-point value for the supply air temperature is 13ºC (Adjustable).

b. Pressure Control loop:

The supply air pressure sensor shall be installed in the duct  will relay the measured signal (static pressure) to the DDC controller, the DDC controller compares this signal with the set-point (adjustable by the operator from BMS central) and generates an analog output to the variable frequency drive (VFD) of the supply air fan. Based on the difference between the two values, a Proportional-Integral program will determine the percentage of the fan speed to achieve the desired pressure. The set-point value for the supply air pressure for each AHU shall be adjusted.

c. Ventilation Control loop:

Demand control ventilation employs return air carbon dioxide controlling strategy.

A single carbon dioxide sensor sense carbon dioxide concentration in the return air duct and sent to the DDC controller, the DDC controller compares the signals with return air carbon dioxide concentration (Default carbon dioxide level difference value 400 ppm ).

Then DDC controller generates an analogue output to the outside air dampers and returns air damper to modulate, based on the difference between the values, the Proportional integral program will determine the percentage of the modulation of outdoor and return air dampers.

Minimum outdoor air quantity shall be governed either by building pressurisation requirement (Input from Building differential pressure sensor) or 20% of the Maximum outdoor demand of the AHU.

5. Alarms:

The following minimum alarms shall be generated on BMS

1) Filter Dirty Alarm: This is generated when the pressure drop on each filter exceeds the set value to indicate dirt accumulate at filters.

2) Fan Trip Alarm: A normally open “NO” volt free contact at the MCC panel when closed will generate an alarm at the BMS indicating that the fan is tripped

3) Fan Fail: In case the supply air fan fails to start or if the differential pressure switch across

the supply fan is not giving the signal according to the command due to any reason then an alarm shall be generated. In case of a fan fail alarm on the BMS, due to abnormal behaviour, the DDC controller will latch the alarm. The operator has to acknowledge (reset) the alarm on the BMS once the trouble has been checked and removed. The operator shall not be able to start the AHU until the alarm s acknowledged and reset.

4) Temperature High & Low: Temperature HIGH and LOW alarms shall be generated if the supply/return air temperature rises above or falls below the supply /return air temperature alarm limit.


A list of Input and output points are required for the above-discussed sequence of operation for AHU

AHU IO Points
Some basic terms of digital electronics
  • Analog Input: Analog inputs can come from a variety of sensors and transmitters. You can measure a whole bunch of different things. The job of the sensor or transmitter is to transform that into an electrical signal. Here are a few of the things you can measure with analogue sensors:
    • Level
    • Flow
    • Distance
    • Viscosity
    • Temperature
  • Digital Input: It allows a microcontroller to detect logic states either 1 or 0, otherwise called as VFC-Volt free contact.
  • Analog Output: In automation and process control applications, the analogue output module transmits analogue signals (voltage or current) that operate controls such as hydraulic actuators, solenoids, and motor starters.
  • Binary Output: it is nothing but relay output from the controller to trigger on and off any equipment.
Now it’s time to choose the DDC controllers based on the above input and output point list.
Any BMS controllers manufacturer must have the basic controllers types of analogue input-output, binary input, and output controllers either dedicated controllers or mixing of all types in a single controller.
for the above applications, we need to choose controllers that should accommodate 17 AI, 6 BI, 5 AO, and 1 BO(Note that temperature and humidity are two different analogue inputs)
Once controllers are designed, we need to calculate the power load for each controller (available in controller datasheet) and field devices to choose the right transformer rating for our DDC panel.
The next things are to write a program for our controllers to accomplish the above sequence,
First, we need to change English words into the flowchart then we can change it later on to the different programming languages that required for BMS vendors either ladder logic or functional block or plain English and etc.
whatever it is any BMS program functionality that will not go beyond the basic digital logic gates.

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86 thoughts on “Understand the Basic concept of BMS system”

    • Hi Riyz,

      DPS-Differential pressure switch triggers NC to NO when the differential pressure across the positive tube and a negative tube is greater than the pressure range set on the DPS.

      For Fan, DPS Pressure tube connected after the fan and negative tube connected before the fan in order to identify fan running condition, when fan starts to run, the pressure will be high in positive side and lesser in negative where DPS differential pressure will be greater than the negative side and triggers the contact from NC to NO.

      For Filter, we need to install positive tube before filter and negative tube in after filter or the opposite of fan because we need to signal from DPS when the filter gets dirty or technically we need signal across filter when pressure builds up before the filter more than DPS setting range.

      Anyhow, let me post separate post to explain both scenarios in a better way with the graphical presentation as soon as possible.

      Check here
      if you want to know how DPS triggers NC to NO when pressure build-up across the positive and a negative tube.

      Thank you.

  1. Also we will get the filter status dirty(the filter has become dirty) only while fan is running?
    once the fan stops, it will go back to clean status right?

  2. Thanks for showing the sequence operation of Ahu in detail and now show us how the termination is done in DDC panel for Ahu and which cables,controller and protocol use for the Ahu .

  3. Mr.hamza, i dont know , which word i can use, for appreciate u. This is very very useful for the begenners , who looking for learning BMS, may allah bless u, ur presentation is very good, and can understand easly. Waiting for ur next post.

  4. thank you mr hamza now i better clear with bms.i request u to update how sensor working one by one for running the AHU.
    thank you lot.

      • Hi, there is no error in B&D conditions. Let me explain the B & D conditions.
        Condition B : AHU command will be disabled & respective Valve & Damper actuators will be closed (shutoff position)
        Condition D : After AHU command is ON, condition D will check the respective supply fan sir flow status to confirm the fan running status.

  5. Thank you Mr Hamza, you make BMS much easier. Your explanation is simple and to the point instead of telling the stories. I have no words to say thanks. Can you please share data with me on my email. I am beginner and after reading your post i came to know its easy if i followed your steps. Your response will be highly appreciated.

  6. Hey Hamza. This was very informative And connected a lot of dot for me. But I was also looking for tutorials for wiring up an industrial control panel aswell as the HMI. Plz email me if you have something of sort as am really hoping on building these panels from scratch in the near future for my future commercial boiler room installations.

  7. Such a wonderful blog about understand the basic concept of bms system and I appreciate your effort for bringing this in to notice. Great blog indeed, will visit again future to read more!!They offer same information here Comfortgroup.co.nz one must check them also.

    • Hi Hamed,

      Except SP you refer here all hardware points which means physical Input/output points.

      Here SP refers SetPoint which created in program called software points for internal AHU/FCU function/program to give option for customer to change the AHU setpoint of temperature

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  9. This is really so much informative but can you please guide me as how is the future scope for mechanical engineer in BMS field… I am fresher now and looking for jobs in design field. Is it feasible to go for BMS engineer??

    • Hi parag, yes offcourse…you have lot of opportunities since BMS controls most of the HVAC in buildings, only thing you have to do it keep open you mind to learn few electronics, and programming side.thats to easy not complicated as like as software engineer

  10. Thanks dear, its very well explained.
    its my first day on BMS but I understood more than 50% , from your content.
    pls write something for terminations as well.

  11. Thanks, sir, it’s very well explained. Please add some schematic for better understanding and to know the functions of various devices used in BMS.

  12. This information is meaningful and magnificent which you have shared here about the Building management systems. I am impressed by the details that you have shared in this post and It reveals how nicely you understand this subject. I would like to thanks for sharing this article here.

  13. It sure was interesting when you said that to save energy while tenants are not available, a variety of lighting in buildings needs to be on and off effectively. With this in mind, I would assume that it would be helpful to have a building management system implemented in order to efficiently run the operations of their buildings. Maybe, they should consider hiring a building management system company that could expertly ensure that all their systems are working fine.


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