The HVAC filter system for a building is a point of defense in protecting against contamination. Accidental contamination (Legionnaires' disease, mold) or deliberate terrorist contamination (anthrax, ricin, sarin) can spread havoc by introducing a toxic substance into an HVAC system. Filters cannot eliminate all toxic materials from the air, but they can filter out some of the contaminants that are accidentally or deliberately introduced into the air stream.

A properly functioning air filter system provides cleaner and safer air to building occupants. It also makes the HVAC system operate more efficiently and extends equipment life by eliminating clogged coils, dirt buildup on fan blades, and dirt deposits in the ductwork. Building owners should recognize that improving the HVAC filter system makes sense economically as well as making the building safer.

Any survey of an HVAC system by building personnel or an HVAC contractor (IAQ, TAB, energy management, etc.) should determine the efficiency of the installed filter cells, check for proper installation, and look for a filter cell replacement schedule and a record that cells are being replaced regularly.

Filter Facts

Filters are often selected for their first cost rather than their effect on indoor air quality or long-term costs. The result is that many filters are of lower efficiency than they should be. Sometimes cells are not replaced as often as they should be.

The frame should hold the filter cells firmly in place and should have gaskets to prevent any air from bypassing the cells. There should be good caulking between frames and between the frame and the plenum wall.

The filter delta P - the difference in pressure between the entering side and the leaving side of the filter rack - does not remain constant. A clean filter cell has the least pressure drop. As the filter loads up with particles, the pressure drop increases. A filter bank should have a permanently installed differential pressure gauge to show the delta P across the filters.

A filter is generally not at its peak efficiency when it is clean. It requires some adhesion of air particles before it operates at its maximum efficiency. However, after a filter becomes fully loaded, it could break down and no longer stop all the particles that it should. As a result, the supply air quantity is reduced and the HVAC equipment, such as the coils and fans, can load up with dirt and require more frequent maintenance or even replacement.

The effectiveness of any filter depends upon the velocity of the air as it passes through the filter. The standard face velocity - the velocity of the air at the entering side of the filter - is 500 fpm (feet per minute) for group 1, 2, and 3 filters. Face velocity is generally 250 fpm for Group 4 filters.

Figure 1. Size of common pollutants and the filters that can capture them.

Types Of Filters

Air that is filtered is not necessarily perfectly clean. There are many different sizes and types of particles in the air. Some filters capture only the larger particles.

Particulates in the air are measured in microns. A micron is one millionth of a meter - that's 0.00000039 of an inch. The diameter of a human hair is about 100 microns.

Filters are described by the size in microns of the particles they can capture. Figure 1 shows the size of some common pollutants and the general type of filter that removes them.

Filters are divided into four general groups: Group 1, Low Efficiency; Group 2, Medium Efficiency; Group 3, High Efficiency; and Group 4, Very High Efficiency.

Group 1 - Low Efficiency Filters
Low efficiency filters generally capture up to 80 percent of particles that are 50 microns or larger - the range of dust and larger-size pollens. They include throw-away residential filters, and pre-filters that keep large particles of dust from reaching more expensive, higher efficiency filters. Low efficiency filters do little to remove contaminants from the air. Their main purpose is to protect the fan, coils, and duct from dirt buildup.

The resistance of clean Group 1 filters is around 0.3 inches wg.

Group 2 - Medium Efficiency Filters
Medium efficiency filters capture 80 percent to 95 percent of particles that are 5 microns or larger - the range of small pollens and large bacteria. Most commercial building filters are in this group. Most Group 2 filters are pleated filters and pocket filters, which are designed to provide a larger filtering surface than a filter that is flat to the air stream. This larger surface also keeps the resistance relatively low even though the filtering media is dense. Pocket filters may have a wire grid or some other method of supporting them in addition to the filter frame.

The resistance of clean Group 2 filters is about 0.25 inches wg to 0.5 inches wg.

Group 3 - High Efficiency Filters
High efficiency filters capture up to 99 percent of particulates that are 0.3 microns or larger. This range includes all bacteria, about 50 percent of the viruses, and most fumes. These filters do a good job of capturing anthrax bacteria. Common types are bag filters and rigid cell filters.

The resistance of clean Group 3 filters can range from 0.4 inches wg to 0.7 inches wg.

Group 4 - Very High Efficiency Filters
Very high efficiency filters are used for special applications such as clean rooms, hospital operating rooms, and laboratories. Resistance of clean filters is very high - from 0.95 to 1.5 inches wg. Group 4 filters are either HEPA (high efficiency particulate air) filters or ULPA (ultra low penetration air) filters:

  • HEPA filters: The most commonly used very high efficiency filters. They capture from 99.7 percent of the particulates that are 0.3 microns or larger.

  • ULPA filters: For very special applications. They capture 99.999 percent of the particulates that are 0.12 microns or larger.

    Because of their high resistance and relatively high cost, Group 4 filters are not practical for most commercial or industrial buildings. HVAC systems that use these filters require special design for air volume, air velocity, airflow patterns, and the fan and system static pressure. They require a larger fan motor, special treatment of the duct to minimize leakage, and a special filter rack to eliminate bypass leakage.

    MERV Rating
    A filter rating called MERV (minimum efficiency reporting value) has been established by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). The higher the MERV rating, the more efficient the filter:

  • A MERV rating of 1, 2, 3, or 4 indicates low efficiency filters such as those used in most residential units. These filters trap 80 percent of the particles larger than 10 microns. These are Group 1 filters.

  • A MERV rating of 5, 6, 7, or 8 indicates medium efficiency filters. These are Group 2 filters used in most commercial buildings.

  • A MERV rating of 9, 10, 11, or 12 indicates high efficiency Group 3 filters.

  • A MERV rating of 13, 14, 15, or 16 indicates a very high efficiency Group 4 filter. These filters trap 99 percent of the particles 0.3 microns and larger.

    Effectiveness Of Installed Filters

    The effectiveness of an HVAC filter system depends on these factors:

  • The rated efficiency of the filter.

  • Resistance created by the filter to the air passing through it.

  • Air bypass in the filter rack.

  • The maintenance of the filters.

    Rated Efficiency
    The efficiency rating of a filter is based on the size of smallest particles it will trap and what percentage of these particles it will trap. For example, a low efficiency filter may be rated as able to trap 80 percent of particles that are 50 microns or larger. In contrast, a high efficiency filter may be rated as able to trap 99 percent of particles that are 0.3 microns or larger.

    Filter Resistance
    Every filter bank imposes resistance on the fan, which in turn must produce sufficient pressure to overcome that resistance. The fan and fan motor for the HVAC system were selected by the system designer to overcome the overall system resistance - the resistance of the filters and other components (coils, ductwork, outlets, etc.).

    Changing to another type of filter can create more resistance than the system was designed for. This could lead to system problems, such as less air delivery, overloaded fan motor, and possible damage to the fan due to exceeding its construction limitations.

    Air Bypass
    Because of the resistance created by filters, if the rack that holds the filters has gaps, air will leak through and bypass the filters. As a result, some unfiltered air will enter the system. As the filters load up and their resistance increases, a greater amount of air will bypass the filters.

    Filter Maintenance
    Filters must be inspected regularly. But they should not be replaced by the calendar (such as every three months). Every filter situation is different. One location may pick up a lot of contaminants, while another location may pick up very little. Also, for any location, one season of the year may create more contaminants than another.

    Filters should be replaced when the delta P across the filter bank reaches the loaded condition indicated by the filter manufacturer. Ratings for different types of filters vary. Differential pressure gauges should be installed so that pressure drop can be determined easily. Some differential pressure gauges send a signal when the filter needs to be replaced.

    All inspections, with the dates and filter delta P, must be logged on a report sheet. After a year or two, this history of the filter delta P and date of service will indicate when replacement is needed.

    If the HVAC system has been exposed to anthrax or any other biological hazard, building personnel must not handle the filters. Contact local health authorities to find out how to proceed.

    Figure 2. Slide-in filter cells taped together accordion fashion.

    Replacing Filters

    The efficiency of a certain type of filter doesn't mean much if the filter cells are not replaced correctly.

    Filters should be changed when the building is not occupied. For safety, the fan should be turned off and the disconnect switch locked out so that it cannot be turned on while the work is in progress. There should be a service light in the plenum so that the work can be performed safely.

    A dirty cell that is removed should immediately be placed in the empty box that the new filters came in. When the dirty filters have been removed, the plenum should be vacuumed.

    Be certain that the filter cells are installed so that the arrow on the side matches the airflow direction.

    Each drop-in filter should have gaskets and be in a locking frame. These filter cells should be installed so that the airflow presses the cells to the gaskets of the frame. Sometimes the frame has been installed backward. This will cause the cell to pull away from the frame. If this is the case, the frame must be changed.

    Slide-in cells should be taped together to prevent air from leaking between them. Stack them and tape them together on alternating sides of the stack (Figure 2) to create an accordion pattern. Open the group flat and slide it into the filter rack. At each end of the row, install an end block with foam gasket (Figure 3).

    Figure 3. End block for slide-in filter cells made of corrugated sheet metal with foam gaskets.

    Danger Signs

    Visible dirt in the coils is a sign that the filters are not performing as they should, perhaps because of improper maintenance or the wrong type of filter. Look for any of these additional signs that the filters may not be functioning properly:

  • A permanent differential pressure gauge is not installed.

  • Differential pressure is greater than the manufacturer's rating.

  • There is no log of filter inspection and replacement.

  • The maintenance log does not show the date of inspection and the differential pressure across the filters for each inspection.

  • The maintenance schedule is on a time basis, not a differential pressure basis.

  • The filter rack allows air to bypass:

    - Cells are not securely held in place.

    - Gaskets are torn or broken.

    - Cells are buckled from excessive resistance.

    - Gaps and cracks allow air to leak past the filters.

    - Spaces between cells are not caulked.

    Possible Retrofit Measures

    If any of the danger signs are found, some retrofit measures may be needed. Consider these:

  • Install a differential pressure gauge that indicates when the filters are loaded.

  • Set up a schedule for reading and logging the differential pressure.

  • Retrofit the filter rack to minimize bypass. Replace the rack or seal all possible air bypass paths by taping or caulking.

    Consider installing the most efficient filters permitted by the total pressure of the HVAC system. An HVAC professional must determine the highest practical resistance that the filters can impose on the system. However, there may not be adequate room for different filters. Allow enough room between the filter and the coil to clean the coil.

    Installing filters with a higher resistance usually requires increasing the fan speed so that the fan will deliver the volume of air required by the system. However, increasing fan speed can result in other problems:

  • A larger fan motor may be needed.

  • Energy costs for the system may be higher.

  • The fan may be running beyond its structural limits and may fail.

  • Higher system pressures can increase leakage in the duct system.

    A qualified person must consider these possible problems and make sure they are avoided.

    An HVAC filter system that is properly selected, installed, and maintained is cost effective. It can improve heat transfer, reduce housekeeping, increase the life of the equipment, and provide a healthful environment for the occupants.

    Adapted from HVAC Security and Safety, by Leo A. Meyer, one of the books in the Indoor Environment Technician's Library series published by LAMA Books. For over 30 years, Meyer has been writing and publishing training materials for the HVAC industry. His books cover a wide range of topics, including heating and cooling, indoor air quality, sheet metal work, electricity basics, safety, and others. For more information, visit

    Publication date: 11/14/2005