HVAC Control Systems and Building Automation System

In the previous topics, which are as follows:

I explained the HVAC systems Main Equipments that need electrical power and Today I will explain the HVAC Control systems as follows. 

HVAC Control systems 

fig (1): HVAC control system elements

HVAC control system, from the simplest room thermostat to the most complicated computerized control, has four basic elements (see fig.1):
  1. Sensors.
  2. Controllers.
  3. Controlled devices.
  4. Source of energy.

1- Sensors 

Sensor measures actual value of controlled variable such as temperature, humidity or flow and provides information to the controller.

Type of Sensors
Different types of sensors produce different types of signals as follows:

  • Analog sensors are used to monitor continuously changing conditions. The analog sensor provides the controller with a varying signal such as 0 to 10V. 
  • Digital sensors are used to provide two position open or closed signal such as a pump that is on or off. The digital sensor provides the controller with a discrete signal such as open or closed contacts. 

Classification of Sensors
Typical sensors used in electronic control systems are: 

  • Resistance sensors are ‘Resistance Temperature Devices (RTD’s)’and are used in measuring temperature. Examples are BALCO elements, Copper, Platinum, 10K Thermistors, and 30K Thermistors. 
  • Voltage sensors could be used for temperature, humidity and pressure. Typical voltage input ranges are 0 to 5 Vdc (Volts direct current), 1 to 11 Vdc, and 0 to 10 Vdc. 
  • Current sensors could be used for temperature, humidity, and pressure. The typical current range is 4 to 20 mA (milliamps). 
  • Temperature Sensors which include:
  1. Bi-Metallic Strip.
  2. Sealed Bellows.
  3. Bulb & Capillary Sensors.
  • Electronic Sensors which include:
  1. Resistance Temperature Devices (RTD).
  2. Thermistors.
  3. Thermocouples.
  • Relative Humidity Sensors which include:
  1. Resistance Relative Humidity Sensor.
  2. Capacitance Relative Humidity Sensor.
  3. Temperature Condensation.
  4. Condensation & Wetting.
  5. Quartz Crystal Relative Humidity Sensor.
  • Pressure Sensors which include:
  1. Variable Resistance 
  2. Capacitance 
  • Flow Sensors which include:
  1. Orifice.
  2. Venturi Tube.
  3. Flow Nozzles.
  4. Vortex Shedding Sensors.
  5. Positive Displacement Flow Sensors.
  6. Turbine Based Flow Sensors.
  7. Magnetic Flow Sensors.
  8. Ultrasonic Flow Sensors.
  • Air Flow Measurements which include:
  1. Hot Wire Anemometers.
  2. Pitot – Static Tube.
  • Liquid Level Measurements which include:
  1. Hydrostatic Sensors.
  2. Ultrasonic Sensors.
  3. Capacitance Sensors.

2- Controllers 

Controller receives input from sensor, processes the input and then produces intelligent output signal for controlled device.

Controller Types:
  1. Temperature Controllers.
  2. Relative Humidity Controllers.
  3. Enthalpy Controllers.
  4. Universal Controllers.

3- Controlled devices

Controlled device acts to modify controlled variable as directed by controller.

Controlled Devices Types
  1. Control Valves.
  2. Heating and Cooling Coils.
  3. Dampers.
  4. Actuators.
  5. Pumps & Fans.

4- Source of energy 

Source of energy is needed to power the control system. Control systems use either a pneumatic or electric power supply.

Aims of HVAC Control Systems:

HVAC Control Systems are used for the following benfits:
  1. Lower energy cost.
  2. Lower operations cost.
  3. Increase flexibility.
  4. Ensure quality building environment.

Type of Control Systems

there are (5) different type of HVAC Control Systems as follows:

1- Direct Acting Systems 

The simplest form of controller is direct-acting, comprising a sensing element which transmits power to a valve through a capillary, bellows and diaphragm. The measuring system derives its energy from the process under control without amplification by any auxiliary source of power which makes it simple and easy to use. The most common example is the thermostatic radiator valve which adjusts the valve by liquid expansion or vapor pressure.

2- Electric / Electronic Systems 

Electric controlled devices provide ON / OFF or two-position control. In residential and small commercial applications, low voltage electrical controls are most common. A transformer is used to reduce the 115 volt alternating current (AC) to a nominal 24 volts. This voltage signal is controlled by thermostats, and can open gas solenoid valves, energize oil burners or solenoid valves on the DX cooling, control electric heat, operate two position valves and damper or turn on-off fans and pumps.

A relay or contactor is used to switch line voltage equipment with the low voltage control signal. An electronic control system can be enhanced with visual displays that show system status and operation.

3- Pneumatic Systems


The most popular control system for large buildings historically has been pneumatics which can provide both On-Off and modulating control. Pneumatic actuators are described in terms of their spring range.

Compressed air with an input pressure can be regulated by thermostats and humidistat. By varying the discharge air pressure from these devices, the signal can be used directly to open valves, close dampers, and energize other equipment. The copper or plastic tubing carry the control signals around the building, which is relatively inexpensive. The pneumatic system is very durable, is safe in hazardous areas where electrical sparks must be avoided, and most importantly, is capable of modulation, or operation at part load condition. While the 24-volt electrical control system could only energize a damper fully open or fully closed, a pneumatic control system can hold that damper at 25%, 40% or 80% open. This allows more accurate matching of the supply with the load.

Pneumatic controls use clean, dry & oil free compressed air, both as the control signal medium and to drive the valve stem with the use of diaphragms

4- Microprocessor Systems


Direct Digital Control (DDC) is the most common deployed control system today. The sensors and output devices (e.g., actuators, relays) used for electronic control systems are usually the same ones used on microprocessor-based systems. The distinction between electronic control systems and microprocessor-based systems is in the handling of the input signals. In an electronic control system, the analog sensor signal is amplified, and then compared to a set point or override signal through voltage or current comparison and control circuits. In a microprocessor-based system, the sensor input is converted to a digital form, where discrete instructions (algorithms) perform the process of comparison and control. Most subsystems, from VAV boxes to boilers and chillers, now have an onboard DDC system to optimize the performance of that unit. A communication protocol known as BACNet is a standard protocol that allows control units from different manufacturers to pass data to each other.

5- Mixed Systems 

Combinations of controlled devices are possible. For example, electronic controllers can modulate a pneumatic actuator. Also, proportional electronic signals can be sent to a device called transducer, which converts these signals into proportional air pressure signals used by the pneumatic actuators. A sensor-transducer assembly is called a transmitter.

The definitions for Transducer and Transmitter will be as follows:

Transducer changes the sensor signal to an electrical signal (e.g. a pressure into a voltage)

Transmitter is the electronic circuitry to enable a suitable strength voltage proportional to the sensed parameter to be sent to a controller.

Supervisory control (building automation) system

The role of supervisory control is to control the scheduling and interaction of all the subsystems inside a building to meet building needs with appropriate operator input.

Supervisory control systems have many names; each used for a particular emphasis. Among the names and their acronyms are the following:

1. BMS: Building management system

2. EMCS: Energy monitoring and control system

3. FMS: Facility management system

4. EMS: Energy management system

5. BAS: Building automation system (The most generic of these terms)

BAS is where mechanical and electrical systems and equipment are joined with microprocessors that communicate with each other and possibly to a computer. This computer and controllers in the building automation system can be networked to the internet or serve as a standalone system for the local peer to peer controller network only. Additionally, the BAS controllers themselves do not need a computer to process the control functions as the controllers have their own internal processors.. These microprocessors, communication network in addition to central controller constitute what is called a direct digital control DDC system.

What is DDC?

It is a control system that uses a computer or many computers linked together via a network that controls the infrastructure of a building. DDC control consists of microprocessor-based controllers with the control logic performed by software. Most systems distribute the software to remote controllers to eliminate the need for continuous communication capability. The computer is primarily used to monitor the status of the system, store back-up copies of the programs and record alarming and trending functions. Complex strategies and energy management functions are readily available at the lowest level in the system architecture. The central diagnostic capabilities are also a significant asset.

The DDC is actually a distributed control system with several distributed control components which are as follows:

1- Controllers 

Microprocessor based controllers in DDC system may be divided to two types; Central (system) Controller and Dedicated Unit (Zone) Controllers as follows:

A- The central controller 


Modern building management systems consist of a central computer, which normally has a screen that an experienced operator can use to interpret and alter performance. This is linked to controllers by a network, which operates as a two-way channel for information and commands throughout the system. Local controllers and sensors are connected to this data-bus and do the work of controlling individual systems.

In some systems, there may not be any screen, but just box with a small LCD readout.

Such systems normally have the capability of providing a full screen readout. However, without such a screen they are very difficult for the inexperienced operator to work with and represent a barrier between the building manager, the energy manager, or any external consultant and the control system.

B- Dedicated Unit Controllers 

 Dedicated Unit Controllers

Also called a terminal controller, it usually has limited capacity to execute factory-loaded computer programs and to provide functional control for a terminal or a piece of HVAC&R equipment. Unit controllers are often connected in a separate network and supported by a system controller.

2- I/O Points

The information presented upon a BMS is derived from monitoring the status of inputs and outputs. Input points provide information to the controls system, e.g. temperatures, pressures etc. Output points are the control signals to equipment. Each input or output

(Termed an I/O point) provides an electronic interface between the BMS computer equipment and the wires in the building.

Inputs and outputs can be either:
  1. Digital, i.e. on or off, such as the signal to turn a system on and off at the beginning and end of a day; or 
  2. Analogue, i.e. continuously variable over a range, such as a temperature signal. 

Special modules are required to provide the I/O points and each system has a limitation on the total number of points that can be connected. Naturally the more points a system is capable of monitoring or controlling, the higher the system costs. Analogue points are also usually more expensive than digital points.

The I/O is arranged in blocks to suit a computer addressing method so they are usually purchased as standard I/O modules containing 4, 8, 16 or 32 points. Separate modules are usually required for the dedicated type of I/O i.e. input, output, analogue, digital, range or voltage level etc. When new points are required a complete module may have to be installed. 

3- Network 

The network is a communications system that connects the central controller to the local controllers. Some systems use 'open' communication protocols, which allow controllers of many manufacturers to talk to each other. These systems are generally preferable to systems that are manufacturer-specific.

Introduction to DDC system design 

There are six steps of DDC system design as follows:

1- Draw a System schematic

A System schematic Diagram is a schematic drawing of the arrangement of the entire system to be controlled including all interacting components. The location of all control system inputs and outputs associated with the HVAC system should be included on this diagram.

2- Write Control point designations.

3- Write Point list

Each physical point on the project as well as key virtual points should be identified on a points list. The list should include important parameters such as sensor accuracy requirements, alarm limits, and trending requirements.

4- Draw DDC system architecture.

5- Write Sequence of operation

the sequence of operation describes the required HVAC control process in detail including all operational and interlock requirements.

6- Write Specifications

The performance and installation requirements for the sensors, actuators, final control elements, controllers, workstations and other components that comprise the control system are detailed in the specifications. Control damper and control valve schedules are an important component of these specifications.

In the next Topic, I will explain Lifts and escalators loads. So, please keep following.

Note: these topics about HVAC in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about electrical HVAC Power loads. But in other levels of our electrical design courses, we will show and explain in detail the HVAC Loads Estimation and HVAC equipment Capacities with examples for different types of buildings. 


  1. Your article is written very articulately and gives a very good explanation about HVAC control system. Great post! This article should be read over and over again.


  2. This post was so helpful! Taught me a lot, thanks

  3. useful for advancing knowledge