Chapter 4: Filtration

4.1. Theory and Applications. 1

4.2. Commissioning the Filtration System.. 2

4.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

Key Preparations and Cautions

Time Required to Test 

4.1. Theory and Applications

Nearly all HVAC systems employ a filtration system. The level of filtration can vary widely. A bird screen prevents the entry of animals and small objects, low efficiency roughing filters protect the heat transfer elements and maintain a basic level of cleanliness in the system, and Ultra Low Penetration Air filters (ULPA) can have efficiencies of 99.999% on 0.3 micron test particles.[1] The level of filtration selected by the designer is related to the requirements of the process. These requirements are driven by the need to maintain indoor air quality (IAQ), protect the occupants from airborne hazards and contaminants, or maintain cleanliness in an occupied zone or production area. The requirements can be set by the owner’s staff, a health care facilities infections control department, building codes, health care licensing requirements, industry standards, or Environmental Protection Agency requirements for effluents. For hazardous exhausts, scrubbers may also be employed in the exhaust stream. (Refer to Chapter 13: Return Relief, and Exhaust and Chapter 14: Scrubbers, respectively, for a discussion of hazardous exhaust and scrubbers.)

Most filters use a mechanical process that physically captures the contaminants by adhering to the filter media. However, there are other technologies employed, including electrostatic attraction and air washing. The air washing approach often yields other benefits in the HVAC process such as humidification and cooling on the supply side, and neutralization of hazardous vapors in the scrubber on the exhaust side. Some systems employ chemical filters that are treated with a catalyst designed to react with the air stream and remove odors and other gaseous contaminants.

Due to the nature of their construction, some filters are considered flammable. As such, they may require U.L. classification for application in certain systems depending on the requirements of the local code authority and the insurance underwriter. In rare instances, fire suppression is required for large filter banks.

In addition to having a major impact on IAQ, filters can have a significant impact on energy consumption in the system due to the pressure drops associated with them. Both of these factors make commissioning the filters and their related framing and monitoring systems critical for ensuring a system’s IAQ performance and energy efficiency. Proper monitoring and change out procedures combined with creative approaches to achieving the desired filtration efficiency at low pressure drop can significantly reduce the operating cost and waste streams associated with the HVAC equipment.

4.2. Commissioning the Filtration System

The following tables outline the benefits and background information associated with testing filtration systems. These tables are linked to related information throughout the Guide. For additional functional testing information, refer to Functional Testing Basics for guidance related to all functional testing activities, regardless of the component or system being tested.

4.2.1. Functional Testing Field Tips

Key Commissioning Test Requirements

All filters induce an air pressure drop that is related to the efficiency level of the filter and the design of the filtration media. Because of this pressure drop, filters create an ongoing energy burden in the air handling system. By working with the designer, owner, and operator, an informed commissioning provider can help the operating staff find the right balance between the frequency of filter change-outs (labor costs) and the filter pressure drop (energy cost). Appendix C.2 contains sample calculations for the horsepower savings associated with reduced pressure drops as well as the operating cost savings associated with these pressure drops over the life of the filters. Some filter manufacturers offer software that can perform this calculation.

Commissioning of the filter pressure drop monitoring system ensures that the filter change-out cycles are optimized and that excessive pressure drops are not introduced into the fan system.

Most filter functional testing has the following goals:

1 Verify that the filters and frames installation provides the intended level of filtration.

2 Verify that the monitoring and indication systems are in place to allow proper filter maintenance.

3 Verify that the installed system provides the required level of filtration by directly measuring the performance based on particle counts, air patterns, and effluent analysis.

The commissioning provider can also be an advocate for requiring filtering during the temporary operation of the air handling systems. See Section 11.4.6: Indoor Air Quality for more information.

The acceptance criteria associated with filter testing will vary with the rigor of the test. For pressure drop indicator calibrations, the acceptance criteria will be related to the accuracy of the instrument being calibrated as well as the accuracy of the test instrument. Generally, instruments rated for 5%- 10% of full-scale accuracy will be sufficient as long as the full-scale pressure drop is matched to the dirty pressure drop of the filter. An instrument rated for 0 to 1 inches w.c. ±0.05 inches w.c. monitoring a filter with a change-out requirement of 0.9 inches w.c. is adequate. An instrument rated for 1 to 10 inches w.c. ±0.5 inches w.c. monitoring this filter would not be satisfactory.

The acceptance criteria for qualification-based testing is usually set by the owner's requirements, by Health Care Facility Licensing standards, or by environmental or code compliance requirements for the effluent.

Key Preparations and Cautions


Inspection of filter banks in operating machinery requires the cautions normally associated with working in closed proximity to rotating equipment, exposed wiring, steam injection systems associated with humidity control systems, active control elements, and hot and cold surfaces. Of particular concern is the need to pass through doors that will have significant opening and closing forces generated on them by the static pressure differences between the unit's interior and exterior.

When working down stream of any filter bank, remember that you, the commissioning provider, are basically a contaminant. The severity of contaminant you represent depends on the level of filtration upstream of your location. You should not enter any portion of the air handling system that is down stream of filters with dirty shoes. The further into the system you go, the cleaner your attire needs to be. You may want to carry disposable booties or a clean pair of shoes for the clean sections of an air handling system on an active construction site.

Proper attire is critical when working down stream of HEPA or ULPA filters. In many locations, the owner will require full clean room or surgical garb. Even if the owner does not require this, it is considered good practice in may situations. Given the high cost of the filters ($300 or more per module) and the penalties associated with downstream contamination, take these simple steps to ensure cleanliness. Similar considerations apply when changing out the filters, even when working in the upstream compartment.

Test Conditions

Some filter commissioning functions are passive, relying primarily on visual inspection and procedural controls to verify performance. Other functions require that the system be operating at its rated air flow to allow clean filter pressure drop to be verified and filter to frame, frame to frame, and frame to casing leakage to be checked. For qualification tests, the air handling and scrubbing equipment will need to be fully functional.

Instrumentation Required

Instrumentation requirements will vary from test to test, but typically includes the following instrumentation in addition to the standard tool kit listed in Functional Testing Basics:

1 Inclined manometers, Magnehelics., Shortridge Air Data Multimeters., and other instruments capable of measuring and documenting low air static and velocity pressures.

2 A Borozine gun or smoke sticks to allow the airflow patterns near the filter holding frames to be viewed and analyzed.

3 For situations where the areas downstream of a HEPA or ULPA filter must be entered, clean suits and/or clean room gowns, gloves, boots, hair covers, and masks will be required.

4 For applications where filters or scrubbers must be qualified based on performance, special equipment including particle generators, particle counters, and chemical analyzers will be required depending on the test protocol. In many instances, retaining the services of a firm specializing in this type of work will be desirable. These costs should be taken into consideration when establishing the commissioning budget for systems with these requirements.

Time Required to Test

Visual inspections and pressure drop monitor calibrations and verifications typically take 15 to 30 minutes per filter bank unless the filter bank is unusually large.

Operating inspections for pressure drop and leakage can take 30-60 minutes depending on the rigor of the test and the size of the filter bank. Qualification testing will take at least 4 to 8 hours for two people per filter bank in an air handling unit application, with larger filter banks requiring more effort than smaller banks.

Qualification testing of ceiling filters serving large areas such as clean rooms and surgeries can take several days to several weeks for a crew of workers. The length of time required will be a function of the size of the room, the rigor of the test, and the ceiling height. In addition, working in ultra clean environments requires special dress and site-specific training which can add to the time required to perform day-to-day tasks.


[1]   As a general frame of reference, a human hair is approximately 100 microns in diameter.