Selecting Proper Air Filter Efficiencies for Commercial Buildings

Stephen Nicholas

The information provided here is intended to assist those responsible for making technical decisions to improve air filtration in commercial buildings. These would include offices, retail facilities, schools, churches, transportation terminals, and public arenas such as sports coliseums, and malls. The focus here will be on air filter selection concerning particulate contaminants.

Building owners, operators, managers, designers, service contractors and maintenance personnel need reliable and accurate information regarding air filtration and air cleaning options. The decision to enhance and upgrade air filtration in a specific building should be based on the building, occupants, its engineering, and architectural, feasibility and cost. The information learned will allow one to make a more knowledgeable and informed decision about selecting, installing and upgrading air filtration systems. Effective air filtration can also help improve overall Indoor Air Quality, (IAQ) and worker health and productivity.

Implementation

Cost is always an issue affected by implementing a filtration upgrade to the HVAC system. Total system costs should be evaluated by the decision makers regarding the enhanced filtration upgrade. Life cycle cost analysis should also be conducted. They should include the following:

  1. Initial cost of the materials to include shipping, warehousing and “shrinkage”
  2. Operating cost, the energy consumption allocated directly to the air filters
  3. Replacement cost which is the labor cost to replace filters when they have reached the end of their service life
  4. Disposal cost

Higher efficiency filters typically have a higher initial cost than commonly used low to medium efficiency products that are specified in most HVAC systems. Usually, HVAC systems are equipped with filters designed to keep equipment components such as coils, compressors, fans, and ductwork clean. Higher efficiency filters may have a higher resistance to airflow called pressure drop, and fans may have to be changed to handle this increased pressure drop. Although these systems improvements will normally come at a higher initial cost, the benefits achieved by this change can offset many of the operating costs just by delivering cleaner air throughout the building and keeping the system components operating at peak energy efficiency.

Operating Conditions

Building pressure must also be considered for an effective HVAC filter system upgrade. The building envelope should be as airtight as possible but, as with most construction, this is a very difficult parameter to achieve. Some outside building walls leak (infiltration) and significant amounts of unfiltered air can enter the building envelope. Field studies have shown that, unless specific measures have been taken to reduce infiltration, as much air can enter the building through infiltration (unfiltered) as through the HVAC mechanical (filtered) system. Therefore, one cannot expect the HVAC filtration system alone to improve overall IAQ. Instead, one must consider air filtration in combination with other steps, such as building envelope tightness, and building pressurization to, as much as possible, insure that the air entering the building only comes in through the outside air HVAC air intake. The building envelope should be maintained under a slight positive pressure to inhibit infiltration as recommended by the Department of Health and Human Services (NIOSH) in their publication No. 2002-139 “Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks”.

Particulate Air Filtration

Contaminants of concern should be carefully be evaluated to determine the level of filtration efficiency required for the contaminant size. The size of contaminants is measured in micrometers (microns). Once a comprehensive list of contaminants of concern has been identified one will be able to use the ANSI/ASHRAE Standard 52.2-1999 to select the proper filter with the appropriate Minimum Efficiency Reporting Value, (MERV). A MERV 6 filter for example is the minimum required to comply with ANSI/ASHRAE Ventilation Standard 62.1-2004 located in Section 5.9 Particulate Matter (PM).