Building Automation Systems – HVAC ControlMany people wonder what Building Automation Systems (BAS) do. BAS is mainly used in commercial HVAC control systems and energy management system applications. Building Automation itself is an energy management system which saves management companies and building owners by efficiently controlling air conditioning and heating comfort systems. 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 stand alone 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. A set-up in a multi-story automated building would have many building automation controllers serving different types of air conditioning and heating equipment (BAS is not limited to just HVAC applications). Every building is different and it is important for the BAS engineer to select the proper BAS HVAC control system and programs to control the various types of HVAC systems in a particular automated building. For building automation systems to be effective, it is important that the BAS system is installed and tuned properly. Some advantages of a good HVAC BAS building automation system are:
  • Building Automation System (BAS) should allow the owner to set up schedules of operation for the equipment and lighting systems so that energy savings can be realized when the building or spaces in the building are unoccupied.
  • Building Automation System (BAS) should allow the equipment optimal start with adaptive learning. Optimal start is allowing the equipment to be brought on in an ordered and sequential manner automatically on a schedule before the building is reoccupied so that space set points can be realized before occupation. Adaptive learning allows the system to compare space temperature, outside air conditions, and equipment capabilities so that the equipment can be turned on at an appropriate time to ensure space set points are achieved before occupation.
  • Building Automation System (BAS)should have trim and respond capabilities. Based on zone demand the set point for various heating and cooling sources will change according to demand from the zones. In a VAV system all the VAV boxes are served from a central air handling unit. If all the zones are at set point then the supply air temperature set point of the air handler is automatically changed to prevent mechanical cooling from occurring when it is unnecessary. When the zones grow warmer the supply air temperature set point is automatically lowered to allow mechanical cooling to satisfy demand. Older systems have a single supply air temperature set point of 55° Fahrenheit which requires the compressors to cycle even when it is not necessary.
  • Building Automation System (BAS) should have the ability to monitor energy usage including the ability to meter electric, gas, water, steam, hot water, chilled water, and fuel oil services.
  • Building Automation System (BAS) in conjunction with the appropriate mechanical system set-up should offer economizing based on enthalpy calculations and/or CO2 set point control.
  • Building Automation System (BAS) should have such BAS control algorithms as reset schedules for heating plants, static pressure control, and other systems where energy savings can be realized through these predictive programs.
  • Building Automation System (BAS) should offer load shedding when power companies are at peak demand and need business and industry to cut back on power usage to prevent brown outs. Building Automation Systems allow the owner to cycle various things off like water heaters or drinking fountains where use of these things will not be noticed even though they are off.
  • Building Automation System (BAS) should offer the ability to send alarms via email, pager, or telephone to alert building managers and/or technicians of developing problems and system failures.
  • Management companies who acquire a good building automation system (BAS) can have BAS set up to bill tenants for energy usage.
  • Building Automation Systems (BAS) should have the communications abilities to be integrated with other building automation control systems and TCP/IP. BACnet compatible or other open source communication protocol is a plus.

The Basic DDC Control Loop

To understand Direct Digital Control (DDC) we must understand the basic control loop. Even the most adept HVAC controls technicians benefit in their work from going back to the basic control loop to solve problems or break down and understand complex DDC control algorithms. For a basic DDC Controls or building automation control loop we need three things: 1. Input from a sensor or device. This can be analog or digital. (See DDC/Building Automation page two for descriptions of analog and digital). In this step we are Measuring temperature or C.F.M.’s (any variables) and collecting data. DDC building automation inputs basically measure a medium or monitor the HVAC systems such as smoke detectors and high/low limit switches. DDC inputs measure temperature, humidity, pressure, current, wattage, and air and water flow among other things. 2. A DDC or building automation controller to process information and which holds the logic or programming. In this step the DDC or building automation controller is processing the information from the input device(s) and based on the algorithm, possibly sending an output signal to a device to take appropriate action if necessary. The input device(s) does not need to be hard wired to the local equipment controller nor does an output response from the local equipment DDC building automation controller going to effect the DDC building automation controllers local equipment. Over a communication trunk the DDC or building automation controller can receive input signals from distant automation controllers and issue output commands to those same or other distant building automation or DDC automation controllers. It really depends on the program and set-up of the entire system in the algorithms of the building automation system as a whole. 3. The actual device being controlled based on what the input is feeding to the DDC controls controller. In this step the controlled device is taking action to maintain the program based on program variables. Output devices can be damper actuators, valve actuators, relays (electrical and or pneumatics (p-e or e-p), variable frequency or speed drives, compressors, blowers, and pumps. Building Automation Systems - HVAC ControlExample of a DDC Controls Loop: A VAV box is reading 600 C.F.M.’s and the zone temperature is 68 degrees F. The controller takes this input information and processes it based on pre-programmed set points. In this situation the zone is too cold so the DDC controller sends an output signal to the damper actuator to close it (some to maintain a minimum heating C.F.M. set point), energize a heating relay for heat and (with a parallel box) turn the fan on. (For more on VAV boxes see DDC/Building Automation (BAS) page 3). Because the output device is taking an action it is reasonable to assume that our input sensor variables are going to change. The process starts all over again until all set points in the DDC building automation algorithm are satisfied.

DDC Control Responses

Building Automation Systems - HVAC ControlDDC building automation algorithms contain what is called control responses. This is built into the logic and is responsible for the output to the controlled device. Some of these control responses prevent overshooting and/or undershooting. In these control responses we have five different responses:
  • two-position control – either open or closed (can utilize upper and lower limits).
  • floating control – example: an actuator controlled vortex damper that moves within a setpoint range to control static pressure. Floating control commonly uses deadbands. In the example above, when the static pressure is in the deadband, the damper does not move.
  • proportional control (p)*
  • proportional plus integral (pi)*
  • proportional plus integral plus derivative (pid) *
*Proportional (P), proportional plus integral (PI), and proportional plus integral plus derivative (PID) control responses are very complex algorithms. To read a detailed report on these algorithms and how they work see our DDC programming pages

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