For many of us with a touch of gray in our hair, (or for that matter, a touch of hair on our heads), HVAC control systems were traditionally made up of electro-mechanical or pneumatic components. Packaged equipment came equipped with two-position operating and safety controls, and any "modulation" was accomplished through a series of step controllers. Larger built-up systems were controlled by a pneumatic control system, which could reasonably mimic a sequence of operation through a complicated series of sensors, receiver-controllers, sub-masters, amplifiers, and enough other gadgets and gizmos to create a jumbled mass of tubes and devices that was difficult to understand, nearly impossible to calibrate, and would require an act of god to modify.

Electronic controls were offered as an alternative to pneumatics, but these were also difficult to setup properly, even more difficult to modify, and were considerably more expensive on larger jobs. With the advent of the microprocessor in the early 1980's, the control manufacturers began offering centralized control systems, which had the ability to monitor and control the operation of system components from a central location. These early systems were expensive to install, were often overlaid on the existing pneumatic systems, and usually ended up acting as glorified time clocks. As the price of microprocessors continued to drop, the availability of inexpensive personal computers and the advent of distributed DDC controllers revolutionized the HVAC control industry.

Today, almost all HVAC components are equipped with smart controllers, which can carry out sophisticated control sequences based on feedback from multiple sensors and control components. Many of these controllers have communication interfaces, which allow information to be exchanged with a central control panel over a dedicated communication line. In larger systems, multiple control panels are often networked together on Local Area Networks (LAN's) using commercial standards such as Ethernet. The languages that are used for communication on the LAN's and between the control panel and the control devices have traditionally been proprietary languages developed by the manufacturers. However, even that is beginning to change, as owners and operators have been leading the charge to develop common open protocol languages such as BACNET for the LAN-based systems and Echelon for the lower level communications between the control devices and the central control panels.

Part of this revolution in communication has been the ability to remotely access the control system via a telephone modem interface. With this capability, a single operator can monitor, control, and troubleshoot multiple sites from a single location. These control systems can store operating information from a variety of individual points in the memories of their systems. These trend points can be configured to collect data from once per minute to once a month to correspond to a billing period. However, there are limitations to the amount of data that a trend can store, and how many trends can be stored at one time. For example, a control panel may only have the ability to trend a maximum of 30 points, with 200 pieces of information from each trend. Obviously, if you are trending data once per minute, you will only be able to store less than four hours of operating data before the oldest data begins to roll off. Similarly, if you were to collect the data once per day, you would collect over half a year before starting to lose information.

These are some of the technical limitations that need to be considered when determining how to take full advantage of the operating information that has become available to the operators and building managers. In the age of overlaid systems, three or four critical points of information were available from each component. Now, with the integrated controllers, you may have 100 times that amount of information available. The configuration of your trending system is determined by defining who will be using the information, and which trend points provide the most critical information.

In most of the facilities that we have surveyed, it has been the plant operator who defines the trending requirements of the control system. They use the trending function as a troubleshooting tool, trending critical operating points every five minutes to use as a diagnostic tool in case of a component or plant failure. They are generally concerned with instantaneous analysis, and allow operating data to roll off without regard to longer term issues. This short term maintenance focus ignores the significant opportunities that are available to improve the overall operation of the HVAC system through a well structured long term trending configuration.

The structure of a trending system will be determined by the type of HVAC system that you have in your facility. In general, the goal is to provide a measurement of system capacity and a measurement of the energy required to achieve that capacity. For a typical office building with rooftop VAV systems and perimeter radiation, this would include a measurement of rooftop kw, cooling coil delta T, and supply air flow, along with the total kw for the perimeter radiation. Using this information in conjunction with your electric bill rate structure would allow you to calculate an instantaneous cost per ton for cooling or cost per million btu's for heating. Although this information would be useful to a building manager, it is only the start of the benefits available through trending. Take this information and trend it on an hourly basis, and you can see the peak load requirements on a monthly basis. You may also identify times when both perimeter radiation and mechanical cooling are on simultaneously, a condition that can dramatically increase your electric bills. Add a point for overall building kw, and you can see how your HVAC system impacts your monthly peak demand. Collecting this operating data provides benchmarking, which may be used to measure the true impact of future energy projects in the facility.

This information can be used to measure what is happening in your building. By identifying other critical operating information, a trained observer may be able to tell how to improve the operation of your systems. Add outside temperature and you can determine how your heating and cooling loads vary with outside conditions. You can also determine the part load efficiency of your rooftop equipment, and it's monthly part-load profile. Newer rooftop units may also provide the ability to monitor and control the amount of fresh air being introduced into the building, a significant factor in overall system energy use.

These same techniques can be applied in larger, more sophisticated central systems. In large chiller plants, the key to effective control is the ability to define an instantaneous kw per ton for the plant. This information provides important feedback as changes are made to improve overall system operation. Once again, information on current plant operation can be used to calculate the energy savings potential for retrofits like the addition of variable speed drives on tower fans and condenser water pumps, free-cooling heat exchangers for plants with extensive winter operation, and modifications to current sequences of operation. The ability to monitor and verify energy savings is often a critical component in obtaining funding from financial officers.

Better utilization of the trending functions of your building's control system can provide significant information on the operation of the building's HVAC systems. This information can be used to continuously fine tune the operation, and may be used to justify funding for energy-based improvements and upgrades. Outside contractors are available to help you collect and analyze your operating information and to provide monthly summary reports. These services typically cost $300 to $500 dollars per month, depending on the complexity of your system.