Lags Behind Advances in HVAC Technology for Automated Buildings

Ken Bloom, President Precision Air Technology, Inc.

The last decade has seen great advances in the HVAC industry, particularly in the areas of building controls. While great strides have been made in building automation, air filtration monitoring relies upon the same methods employed for decades. The most common means of monitoring air filters continues to be by measuring the differential pressure across the filter bank. While much of the industry relies on this method, practice indicates that it isn't the most accurate means to measure the relative obstruction/impedance of a particulate air filters.

Building owners and managers will readily admit the replacement air filters is left to varying degrees of estimation - often guesswork. The real question many ask is when do we really know when to change a filter.

The commonly accepted industry "rule of thumb" is that a filter is ready for replacement once it has reached twice the initial pressure drop across the filter bank. In practice, achieving a doubling of the differential pressure is problematical. The statement is true when the air velocity remains constant for both the clean and the dirty filter measurements. However, in constant volume system (constant motor speed) the fan often begins to push less air as the filter becomes impeded. As the obstruction increases there is no commensurate increase in pressure. Thus in the worst case, achieving a recommended doubling of the pressure many not be reached until the system becomes excessively impeded/obstructed resulting in system not functioning as designed. Similarly, in a variable speed fan system using differential pressure to monitor filter impedance can lead to inaccurate results. As the velocities are changed the pressure also changes resulting in erroneous measurement of the filter status.

The Need for Better Filtration Monitoring

With ever increasing development of building automation systems, few areas of operation are left to varying degrees of guesswork. Modern building controls systems accurately monitor a whole host of environmental factors. Unfortunately, when it comes to air filtration maintenance, the HVAC industry for the most part lacks a reliable tool. This being the condition, it can only be estimated how many air filters are discarded prematurely or in the alternative how often the "forgotten" filter is left to stress the system and compromise indoor air quality.

Many facilities such as hospitals, pharmaceuticals, and electronic manufacture rely on high standards of indoor air quality. These critical environments employ sophisticated indoor air quality equipment to warn and detect of environmental anomalies. Ironically, the first line of defense against air contamination, the air filter bank, is left to relatively inaccurate means of measurement and control.

The cost of filters and their replacement is substantial. While the cost varies with the filter efficiency, consider that R.S. Means Cost Data estimates that the material cost of a 500 CFM air cleaning filter bank amounts to $3625. An even greater cost is born by those who prematurely replace filters that are still providing indoor air quality and a properly functioning HVAC system.

For many companies energy is the second highest annual cost. Installing higher efficiency filters to keep heat exchange elements clean can save 10 percent to 25 percent on building energy usage studies have shown. Proper filtration monitoring is the key to making maximum use of these higher efficiency filters.

Many industries both commercial and industrial rely on a clean environment. All the expensive environmental controls are only as good as the first line of defense, i.e. the air filter system. While building automation is wired for accurate monitoring of the air filtration system, there is lack of accurate monitoring tools to fully realize these capabilities. The financial costs of inadequate air filtration translate into lost building efficiency, increased energy usage, and reduced indoor air quality.

New Technology Satisfies the Needs of Building Automation

Precision Air Technology Inc. developed a new technology that answers the question is it time to change my air filters. This innovation in building controls is a patented electronic air filter monitor that accurately senses the status of particulate filters in both constant and variable airflows.

The air filter monitor technology is a thermal sensing system consisting of sensing probe and controls unit. The sensing element, in standard HVAC applications, is placed between the fan and the filter bank. Two signals are generated and sent to the controls unit, one related to the velocity of the air through the filter bank and other signal measures the ambient air conditions. The monitor requires a one-time calibration of the filter type (efficiency) to the air handler unit. As the filter bank becomes obstructed with particulates the difference between the two signals changes resulting in the dirty filter status. Perhaps the most unique ability of the system is its ability to accurately monitor air filters in variable air velocity. The relationship between the signals does not change with velocity changes. The electronics is designed to give virtually the same output whether the velocity is 200-fpm or 1200-fpm. The signal output changes only when the resistance in the filter bank changes as a result of particulates obstructing the airflow.

The air filter monitor features a local LED display as well as a 0-24 volt dry contact relay for remote monitoring. Options include and analog output available for data gathering as well as graphing of the filter status.

The air filter monitor operates accurately over a wide range of air velocities. Tests of the performance characteristics by Systima Technologies Inc., Woodinville, WA, report: "By using differential air velocity measurements, the Air Filter Monitor was able to distinguish an increase restriction in the air flow representative of a dirty filter in the HVAC system. The air filter monitor provided an appropriate higher signal voltage for the "dirty" configuration throughout a range (200-1000 fpm) of airflow velocities. The advantage is that increased airflow restriction, such as a dirty filter, can be detected long before increases in static pressure reveal a significant blockage. Thus, the tests show the air filter monitor provides a means to maximize the overall efficiency of the system though ease of monitoring…"

The advantages of the air filter monitor as a tool for building automation are several. First, it brings greater maintenance efficiency eliminating the guesswork and premature replacement of air filter, optimum replacement means substantial savings. Secondly, automated maintenance reduces the unnecessary service calls to monitor, detect, and maintain the filtration system. Eliminating the "forgotten filter" reduces drag contributing to unnecessary wear on the equipment. Excessively obstructed filters can consume more energy because it takes longer for the space to be heated or cooled. Thirdly, it is an essential element in implementing what can be called "clean coil technology". Installation of higher efficiency filters that keep heat exchange elements clean can save 10% to 25% on energy usage studies have shown. (Krafthefer, ASHRAE, Tran, 1987, Vol 93, Part 1458-1473.). Finally, the overall performance of the air filter monitor establishes new standards of indoor air quality by providing the ability to monitor and record data for proper building hygiene.

As building automation advances, the air filter monitor will become the new way to bring greater efficiency and lower operating costs in the maintenance and energy usage. Precision Air Technology Inc. provides owners and engineers with the new and innovative advance in building automation.

About the Author

Ken Bloom is a founding developer of the air filter monitor technology. He has 19 years experience in project management and design since receiving a Bachelor of Science from the University of Washington.

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