How a DDC System Operates

December 8, 2008
Figure 1. A three-tier DDC system.


A smaller building may have a single, computerized HVAC controller that operates the direct digital control (DDC) system. However, a large building normally requires a more complex system of controllers, divided into separate sections called tiers. This is called the architecture of the system. A schematic diagram of a three-tier system is shown in Figure 1.

First Tier
The first tier is a central workstation (Figure 2), consisting of a dedicated computer, monitor, keyboard, and printer. Sometimes the keyboard is omitted at the central workstation and a laptop computer is plugged in when instructions are given to the dedicated computer. The central workstation is also called an operator-machine interface (OMI) or user interface (UI). It communicates with the controllers on the second and third tiers. The central workstation can receive, process, store, send, and print data.

The technician can communicate with the central computer by using a keyboard, mouse, touch screen, monitor, and printer. By using these, the technician can:

Figure 2. Workstation for a DDC system. (Courtesy ASI Controls.)

• Check the status of each component and the status of the system.

• See trends that indicate potential system problems, such as a gradually increasing difference between a space temperature and the set point.

• Change settings:

- Some settings, such as set points and times of operation, can be changed by the building operator or HVAC technician.

- Other settings, those that would have major effects on the system operation, require a special password to access and change.

Second Tier
The second tier is a system controller (Figure 1).

Figure 3. Terminal controller for a boiler. (Courtesy ASI Controls.)

Third Tier
The third tier is a series of terminal controllers (Figure 1). Each control panel is programmed to control one major piece of equipment such as a VAV terminal, chiller, cooling tower, or boiler (Figure 3).

Tiers Communicate
The controllers on different tiers communicate with each other:

• The terminal controllers communicate with the second tier controller.

• The second tier controller communicates with the central workstation.

• The central workstation can display or print out operating data for any component of a system.

This means that you can get information about any component from the central computer in order to identify problems and their probable cause. This takes much less time than it would take to test individual components in the mechanical equipment room.

Figure 4. An HVAC controller in a mechanical room. (Courtesy ASI Controls.)


In a DDC system, a controller is a dedicated computer, which means it is designed to operate only one specific program. An HVAC controller receives information from sensors and sends signals to actuators. It is often located in the mechanical equipment room (Figure 4).

Figure 5 shows a typical single zone DDC system. The controller in Figure 5 has a program to control the air handler portion of an HVAC system. It receives digital or analog input data, processes it, and sends appropriate digital or analog output signals to the HVAC system.

In a DDC system:

• Temperature sensors are electronic. They are either:

Figure 5. Input and output signals for an HVAC controller for a single zone system.

- RTDs (resistance temperature devices): Electric resistance increases as temperature increases.

- Thermistors: Electric resistance decreases as temperature increases.

• Valve and damper actuators are electrically operated.

• Both the input and output signals to and from the controller are electric, instead of pneumatic. They are in volts, milliamps, or ohms. As shown in Figure 5, the signals can be:

- AI (analog input), such as signals from temperature sensors to the controller.

- AO (analog output), such as signals from the controller to damper or valve actuators.

- DI (digital input), such as a signal from the fan motor starter to the controller to indicate whether the fan is running.

- DO (digital output), such as a signal from the controller to start and stop a fan motor.

Figure 6. Desktop computer connecting to a DDC system. (Courtesy ASI Controls.)


Every manufacturer of an HVAC digital control system uses the DDC to perform pretty much the same functions. The catch is that each DDC system has different ways of letting you use these functions depending on the computer program being used. If you are used to operating computers, you have the basic knowledge required. Most systems use Windows - which is known by most computer users - as the operating system. But each system uses different commands, keystrokes, mouse clicks, and icons. You have to study the instructions for your particular program.

The technician communicates with controllers in the system by using some sort of operator-machine interface (OMI). Many different devices are used as operator-machine interfaces. The device to be used depends on how complex the system is and what the technician needs. The following are different OMIs.

Desktop Computer
Large systems, such as complete building automation systems, or networks that contain more than one building, usually have a desktop computer (Figure 6) at the central workstation. It allows a qualified operator with an access code to make system-wide changes in the DDC system. Technicians and building operators who do not have the access code are limited to receiving data, making changes in times and set points, and reading temperatures and airflow rates.

Figure 7. Laptop computer connecter to a controller. (Courtesy ASI Controls.)

Laptop Computer
Building managers and building operators often use a laptop computer plugged into the main terminal. Laptop computers are also mounted in a controller (Figure 7). The laptop can perform the same functions as the desktop computer.

Handheld Terminal
For small systems, a handheld terminal (Figure 8) is often used. This is a small computer designed for only this one purpose. It has a limited number of display keys and does not have a keyboard. It displays only two to four lines of data. The handheld terminal provides limited operating data and generally allows for changing only times and set points.

A keypad (Figure 9) may be permanently mounted on a control panel. Keypads usually perform the same functions as handheld terminals. A keypad has only a few keys and displays two to four lines of data.

Figure 8. Handheld terminal. (Courtesy ASI Controls.)

Dumb Terminals
A dumb terminal is a display and keyboard similar to a keypad or handheld terminal. However, it cannot be used to change settings. Dumb terminals usually display 20 lines or more of data.

Outside Access
Many DDC systems can be accessed from phone lines, cell phone connections, or Internet connections. For example, a technician who is called to take care of a problem in the system can use a cell phone or regular phone to dial into the system. Or the technician can use a desktop or laptop computer to access the computer through the Internet.

Outside access can be used to change such values as set points and time schedules, and to control some alarms. By pressing the proper sequence of numbers on the phone, the system can be accessed and changes can be made. Each system has a specific sequence of numbers to perform an operation. The instructions for these sequences may be by voice on the phone or can be on a printed card.

Figure 9. Keypad. (Courtesy ASI Controls.)

Alarm printers are standard computer printers that are operated by the DDC software program. They print out alarms when values in the DDC system exceed a preset amount. Often the phone number of the person to notify is printed with a description of the alarm. The software program may also allow printing preventive maintenance instructions.

Some systems have a pager system. When an alarm occurs on a system, an on-call technician is automatically contacted through a paging service. Some systems use a beeper; others show a short message on a small display. The on-call technician - either on-site or from an outside access - can use an OMI to enter the system and solve the problem.

Some systems use the phone system to communicate. For example, a system may be programmed to signal an alarm by dialing a technician or an office at an off-site location. The phone number called can vary according to the time of day and the person on duty. Typical off-site locations are:

• The office for a building manager for one building or for a number of buildings.

• The office of a service or controls contractor responsible for a number of small commercial establishments (such as restaurants or medical offices).

Figure 10. A display showing trends for outside air, relative humidity, and carbon dioxide. (Courtesy ASI Controls.)


Because a DDC system is computer controlled, it can perform complex calculations and store a vast amount of data. It can monitor trends (Figure 10) and make many kinds of adjustments. Some common DDC operations for HVAC are listed below. Depending on the size of the system there can be many more.

In addition to the ordinary schedule for the building’s occupied times, DDC systems can control many different time schedules, such as the following:

• Precooling or preheating to bring the building to comfort level by the time of occupancy. The time of starting will vary as the computer calculates different variables such as outside and inside air temperatures to determine the best start time.

• Stop control turns off the HVAC system a set time before the building becomes unoccupied. For an office building, stopping the system a half-hour early means reducing the operating time of the system by 130 hours per year.

• Different set points for unoccupied times.

• Variable time schedules for different days of the week. For example, in addition to the regular daytime occupancy, a building may be in use three nights a week between 7 and 10 p.m.

• Holiday scheduling.

• Daylight savings time adjustment.

• Temporary scheduling - A timed override switch can be used for areas that have variable schedules. Monthly or weekly events can be programmed through an interface.

Economizer Control
Economizer control sets the system to use outside air instead of the mechanical chiller to cool the building, when outside air temperature and humidity are within a specified range.

Sequence Control
Sequence control, also called lead/lag control, is used if two or more pieces of the same item of equipment (such as pumps, chillers, or compressors) are installed in a system. Sequence control puts one in operation if the other fails. For example, if there is a primary pump and a backup pump, if the primary pump fails, the backup is started. Another strategy is to alternate the use of the two pumps so that they receive equal wear.

Reset Control
Reset control is an energy-saving strategy that changes the set point of a controlled variable as another variable changes. For example, a typical hot water set point for a heating coil is 220ºF when the outside air is at 0º. When the outside air temperature reaches 60º, the hot water set point could be automatically reduced to 140º, because less energy is needed to maintain the air temperature.

Low Limit and High Limit Control
Low limit and high limit control adjusts the system to limit the low and high of a controlled variable. For example, if the outside air temperature drops very low, the system adjusts to prevent the mixed air temperature from dropping below a certain set point.

Electrical Demand Control
In addition to the regular usage rate, most utility companies impose a demand charge. This is an extra charge per kilowatt based on the highest rate of use for a given period (usually 15 minutes). The demand charge can amount to as much as half the utility bill. DDC programs limit the demand charge in a number of different ways:

• Duty cycling - This turns off different HVAC units during peak load times in order to reduce the demand charge. The system regulates off time according to such things as indoor temperature.

• Load shedding - This turns off various building electrical loads to reduce demand charges. Usually a schedule called a shed table is developed that sets priorities on which loads will be first to be turned off.

• Averaging control - This averages the input from several sensors. For example, building temperatures will vary in a building. The temperature in a lobby will be much different than the temperature of an inner office on one of the higher floors. The computer receives input from sensors located in different parts of the building, averages them, and acts on the average.

• Soft starting - This brings large equipment on line slowly to minimize the large onrush of current that occurs when starting large motors.

DDC and Building Automation
If you are responsible for a building HVAC system, you are primarily concerned with DDC for HVAC. However, the DDC system that controls HVAC may also control many other building systems. The trend in DDC is toward total automation of all building energized systems. In addition to controlling the HVAC system, one central computer often controls scheduling, data gathering, monitoring, and identifying trouble spots for all building functions such as:

• Life safety

• Fire protection

• Security

• Energy management

• Lighting schedules

• Equipment monitoring and maintenance

With a complete building DDC control system, a technician can manage all building functions from one workstation. On-off schedules for lighting and equipment can be set; daily schedules can be altered; and input data can be obtained. The computer may be able to analyze trends and automatically change set points for many systems and conditions.

Excerpted and reprinted from Digital Controls for HVAC Technicians by Leo A. Meyer, one of the books in the Indoor Environment Technician’s Library series published by LAMA Books.

Publication date: 12/08/2008