The HVAC Factor: The Role Of Air Filtration

By Jeff Holt, CAFS, NCT
Published in the October 2008 issue of Today’s Facility Manager

HVAC equipment undergoes continuous degradation, and facility managers (fms) charged with maintaining the equipment should focus on keeping system components as close as possible to the original design intent. Preventing contaminants from entering the system is easier than removing them. An effective way to do this is through proper air filtration.

Fms should look at three factors when evaluating air filters for their HVAC systems: pressure drop, efficiency, and dust holding capacity.

Pressure drop is the resistance to the flow of air through a filter in its clean configuration. This factor is built into an HVAC system; if a filter with too high a pressure drop for the system is used, performance will suffer (along with the possibility of a malfunction).

Efficiency relates to how many of a specific size particle need to be removed to keep the equipment and facility clean. This is referred to as the minimum efficiency reporting value (MERV). If an HVAC system will accommodate MERV 13 or better (most should, since many filters are now available in lower pressure drop with higher efficiencies), fms should choose filters at a level that offers maximum contaminant removal.

Dust holding capacity refers to how much contaminant a filter will hold to its final recommended pressure drop. This factor helps to determine how long the filter will last. For example, there are some MERV 13 filters that hold 100 grams of dust and others that hold 200 grams of dust. All conditions being equal, the 200 gram capacity filter will last twice as long.

Assess The Current System

Fms can determine which air filters will maximize performance of their systems by examining the filters currently in use and assessing their effectiveness with other HVAC components. No matter how good the filter, if a system has leaks, bypass, or other problems, it will not be effective. The assessment should involve looking where pipes enter the unit for the coil, around ductwork and vibration collars, and where unit doors may be bent or cracked.

Does the filter bank need replacing because of age or condition? Are dampers working properly? Are drain pans and coils clean? Are motors and blowers operating at their best efficiency without vibration and with good power transfer through the belts? Are filter and door gaskets in working order?

Even the best systems develop leaks over time (especially true for rooftop units). This allows dirty air to bypass filter banks and get into component parts.

Beyond HVAC components, it is important for fms to acknowledge that certain environments generate different types of particulates. In office buildings, for example, laser printers and copy machines emit micro-sized carbon particles. Facilities with kitchens are generators of particles from cooking surfaces. Record storage and handling areas can generate paper dust and other contaminants. Also, vacuum cleaners should be checked to ensure they have fine particle filtration, so particles will not be re-entrained from this source.

Fms may want to conduct a particle count test to determine if particles of a specific size are causing problems. Many filter companies offer this service.

Now Do The MERV

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed an air filter testing standard (ANSI/ASHRAE Standard 52.2), which has a single number air filter designation based on the filter’s MERV. This number corresponds to the efficiency of the filter in its clean (normally lowest efficiency) condition. A filter increases in efficiency as it loads (possibly one MERV level). It is most inefficient when clean; hence the term “minimum efficiency.” By matching the particle size removal efficiency of a filter based on the MERV, an fm can determine what percentage of particles of a specific size will be removed from the air.

ANSI/ASHRAE Standard 62.1, “Ventilation for Acceptable Indoor Air Quality,” recommends a minimum of MERV 6 filters for commercial buildings. The U.S. Green Building Council’s LEED rating system recommends MERV 8 filters during construction of new buildings and MERV 13 after completion.

MERV 13 are the lowest number filters that can remove particles in the 0.3 to 1.0 micrometer range (see chart). This level filter will help to keep internal components of an HVAC system clean, even after years of operation.

Pressure Drop

In general, as the MERV number increases, so does the pressure drop of the filter (resistance to air flow). All systems have a design operating range for filtration pressure, and fms should ensure that the installed filter does not exceed this range.

As contaminants load on the filter, pressure in the system rises and can adversely affect airflow, causing it to run longer in order to satisfy temperature settings. Fms may want to have a Certified Air Filter Specialist (CAFS) examine their systems to ensure correct filter application.

There are newer filters that employ larger amounts of media while maintaining a lower pressure drop. These work well and have the added advantage of holding more contaminant between changes, saving on energy, labor, and disposal costs. While these filters—mini-pleat V-cell—are not for every unit, they are available in different configurations and can be adapted to many systems.

Dust Holding Capacity

Filters with more media generally hold more contaminants and last longer. An fm should ascertain this amount (usually in grams) from a test report to be sure the filter, while having acceptable pressure drop and correct MERV, will not need to be changed so often that its good points are negated by its holding capacity.

Fms are charged with keeping HVAC systems clean and operating at peak efficiency. Proper air filtration can greatly reduce coil cleaning and other component cleaning by stopping dirt before it enters.

Holt, CAFS, NCT is a Certified Air Filter Specialist and NAFA Certified Technician. In the air filtration industry for 20 years, he is vice president of Airflow Products Company, Inc. of Selma, NC. Holt is currently president of the National Air Filtration Association (