Do HEPA filters require special maintenance?

Indeed, HEPA filters necessitate a program of inspection, pressure-drop and monitoring of performance, and replacement of the filters when defined criteria signal to do so. Washing or vacuuming the filters is typically not advisable for standard HEPA media. The program is to be documented as part of the overall plan approved for the site and to follow the manufacturer's recommendations.

Maintenance of HEPA filters is not just a task. HEPA filter maintenance is a program of inspection, monitoring, and replacement actions. EACH of these has a defined trigger, a defined responsibility, and a record.

Most HEPA filters are not washable, and attempts to clean or wash them can damage the media or disturb the seal. As a result, HEPA filter replacement is the best course of action for HEPA media performance degradation. Some filters and some applications can allow the use of washable HEPA media, but that is the exception and not the norm.

Understanding the maintenance program is affected by the application. For example, a cleanroom HEPA filter would follow a qualification protocol, and an HVAC HEPA filter would follow a defined condition-based schedule based on pressure drop and airflows.

This article will discuss HEPA filter maintenance needs, methods of inspection, prefilter practices, safe filter replacement, and how to establish replacement intervals.

What HEPA filter maintenance actually requires

HEPA filter maintenance blends two types of work: non-invasive observations and corrective action. Non-invasive observations are visual inspections, recording of pressure-drop values, and checking airflow. None of these tests are invasive and the filters remain undisturbed.

Corrective action is replacing a gasket that is torn or worn, repairing a defective housing, or replacing a HEPA filter. Corrective action will entail downtime, a containment strategy, and qualified personnel.

Handling and maintenance procedures intended for specific models may vary. Manufacturers' manuals should be consulted before any interventions are made with the filter. The general recommendations contained in this document should not be construed as a substitute for the manufacturer's manual.

What routine HEPA filter maintenance includes

Routine maintenance consists of documentation and observation, rather than actual intervention.

• Record differential pressures at various airflow levels.
• Inspect the filter for obvious damage.
• Analyze the gasket and housing interface and report any damage.
• Make sure that the airflow meets specifications.
• Keep a maintenance log that states the readings and dates.

Each record should contain a filter ID as well as other specified information that includes the operating fan speed or airflow, the location of the measurement, the name of the service technician, and the corrective procedure taken.

Table 1: Overview of HEPA Filter Maintenance Tasks

Table 1: Overview of HEPA Filter Maintenance Tasks.

Why HEPA filter cleaning is usually not recommended

Due to the fragile composition of HEPA filter media and the critical nature of seals, HEPA filter cleaning is generally not recommended. Vacuuming the material can scour the fibers, and compressed air can develop holes in the filter's media. Moisture brought on by wet wiping may contaminate and allow the dirt build-up process to begin.

Despite the cleanliness of a filter, there can still be a shift in the fibers, or the sealing process may have failed, leading to a poor seal. Therefore, the visual clean state of a filter cannot be an indication of its efficacy.

Cleaning cannot be relied on to restore compliance. If the manufacturer provides appropriate cleaning methods for particular filters, only those methods are acceptable. In all instances replacement is safer.

Inspection and monitoring for HEPA filters

There are two main activities for maintaining HEPA filter performance: HEPA filter inspection and HEPA pressure drop monitoring. Inspection reveals any physical damage done to the filter while pressure monitoring reveals its performance.

It is essential to document both HEPA filter inspection and HEPA pressure drop monitoring. This documentation will support your replacement decisions, and also serve as evidence in the event of an audit. The absence of documentation in a facility indicates that filtration is not kept in check.

The frequency of inspections should be determined through a risk assessment based on the operating history, requirements applicable to the specific facility, and the qualifications plan established by the site. There is no set frequency that can be applied to all instances.

Table 2: Inspection and troubleshooting matrix

Table 2: Inspection and troubleshooting matrix.

How to inspect the filter, frame, seals, and housing

Inspections should be done according to safety and contamination control requirements. External visual checks can be performed without stopping the unit. Any inspections requiring shutdown, containment, or physical contact with the filter must be done by qualified personnel.

The normal inspection sequence is as follows:

Inspection should not disturb either the filter media or the seals and should be performed as per the approved site procedure.

How to monitor pressure drop and system performance

Pressure drop should be recorded when airflow values are the same. Values from different airflow readings cannot be compared to the baseline. The initial observation marks the initial point of resistance based on airflow specifications needed. Every other reading taken afterward will be measured against this observed value.

Limits designated by manufacturers assist in ascertaining how much resistance in strict range can be accepted. According to the Deiiang catalog, final resistance in a unit utilizing Combined H13/H14 filters typically ranges from 400–600 Pa for standard airflow. This shows the catalog number of this equipment when working to its rated standard.

Besides this information, the design choice must also consider fan performance levels, forced airflow, and team design.

Prefilters and practices that extend HEPA filter life

Changing the HEPA prefilter also allows for lowering the impact on HEPA media. Here the prefilter captures more of the dust before it reaches the HEPA filter and thus reduces the rate with which dust enters the HEPA filter.

The prefilter material will usually be a bag or panel-type filter that can be changed more often as it costs less.

Whether a prefilter is efficient or not does not determine how much longer HEPA life will last, as a person has to consider other factors such as airflow, ambient particle load, and maintenance procedures.

How prefilters reduce loading on HEPA filters

Prefilters are utilized to decrease the amount of large particles that reach HEPA filters, thereby decreasing the amount of dust entering the HEPA filtration media.

The potential benefit of using a HEPA prefilter differs among sites. The use of prefilters brings out more benefits in an area with more dust than in a cleaner setting. The effects of prefilters should always be recorded since their contribution cannot simply be determined based on the efficiency alone, but rather based on site-specific data collection. They delay loading. However, a HEPA filter still needs to be monitored and ultimately replaced.

How to plan prefilter inspection and replacement

HEPA prefilter replacement should be determined according to pressure drop as well as visual condition.

• Monitor prefilter pressure drop at the specified frequency set forth in the plan.
• Replacement should occur when the resistance reaches the finish value.
• ｒｅｐｌａｃｅ with increased frequency in high dust conditions.
• Record each replacement in the maintenance record.

Deiiang catalogue specifications state the six-bag prefilter 592×592×381 mm has an airflow rating of 2,050 m³/h with a filter area of 3.18 m². The initial resistance rate is given as below 50 Pa for F5 and below 100 Pa for F8. The final resistance is listed as between 250 and 400 Pa. The above are catalogue values applicable under general conditions.

HEPA filter cleaning, handling, and replacement

The process of replacing a HEPA filter is a controlled operation. Filters are removed carefully, bagged if contaminated, and replaced with a verified filter. New filters are then performance tested according to the parameters and requirements of the project.

HEPA filter cleaning normally does not take place as part of standard operating procedure. Washing, vacuuming, or using compressed air can damage HEPA filter media and should only be performed if approved by the manufacturer for the specific model.

Handling precautions must be observed so the filters are unharmed and the technicians are safe. Gloves and respirators may be required. When there is concern about contamination, the filter should be bagged prior to handling.

Table 3: Replacement process and verification matrix

Table 3: Replacement process and verification matrix.

How to handle and ｒｅｐｌａｃｅ a HEPA filter safely

The filters must be replaced following the site-specific lock-out, containment, and contamination control procedures. No universal shutdown sequence is prescribed, as the site's approved procedure dictates the required steps.

The normal procedure is as follows:

• Follow the isolation and lock-out procedures of the site.
• Wear suitable PPE.
• Carefully remove the filter from its frame.
• Should there be concern for contamination, the filter must be bagged.
• Inspect the housing along with the gasket surface.
• Install the new filter properly seated.
• Properly tighten the clamping to the specifications indicated.
• Make a record of the installation and of its initial resistance.

It must also be noted that the media should not be touched. Only handle the filter using the frame, as contaminated filters may necessitate a specific site risk assessment.

What to verify after HEPA filter replacement

Following installation, the tests required will vary according to the project protocol. Tests may include installed filter leak testing, airflow verification, as well as room particle testing.

It is important to note that installed filter leak testing and room classification testing are completely different verifications and cannot be interchanged. The appropriate acceptance criteria and procedure must be indicated by the project.

The pressure drop should be recorded as the new baseline. It is necessary to validate airflow at its design capacity.

Setting HEPA filter replacement intervals

HEPA filter replacement intervals must be established based on operational data as well as risk assessment. There are ways to establish a fixed calendar date for replacement, but this will need to be verified as appropriate in accordance with the site's validation plan or risk assessment process. In some situations, condition monitoring practices may be utilized to determine replacements without using a calendar schedule.

Determination for replacement must be made based upon one or more predetermined criteria. The primary indicators for HEPA filter replacement include pressure drop, airflow, and integrity testing results.

Which conditions determine HEPA filter replacement timing

Determination for replacement may need to occur when any one of the following conditions is present:

• Pressure drop has reached an action limit set by the facility.
• Airflow is below the design value.
• Integrity test failed.
• Tangible evidence of physical damage identified.
• Evidence of wetness or contamination exists.

In situations where evidence of wetness or contamination exists, replacement of the filter may be required if integrity verification has not been completed. The manufacturer and the site's contamination procedure should be consulted.

How to establish a documented replacement schedule

Knowing how to establish a documented replacement schedule means defining when each trigger occurs, knowing who is responsible for the actions associated with the trigger, and outlining verification steps.

The documented replacement schedule must detail:

• Action limit for pressure drop.
• Baseline at commissioning of facility.
• Inspection and monitoring frequency.
• Replacement procedure.
• Verification after replacement.
• Recording of all maintenance activities within a maintenance log.

The schedule should include a review trigger for changes in airflow, process conditions, prefilter configuration, or recurring failures. A new baseline is established only after the required installation and verification checks are complete.

Cleanroom-specific HEPA maintenance and testing

Cleanroom HEPA filters undergo prescribed maintenance through an established qualification protocol. This qualification protocol has defined time periods for testing, guidelines for how the tests must be conducted, and benchmarks for acceptable results.

Cleanroom air cleanliness is classified according to the concentration of airborne particles, as stipulated in iso 14644-1 [1]. iso 14644-3 provides the testing procedures to follow [2]. The two standards facilitate the qualification procedure, but neither offers a universal maintenance frequency.

The specific testing to be conducted and permitted frequency is dictated by standards applicable to the situation, regulations or customer requirements, risk assessment, and the qualification protocol approved for that site. There is no guarantee that this information is determined solely by the room class number.

How cleanroom classification affects maintenance planning

While the room's classification does influence maintenance planning, it is not the ultimate determining factor. The maintenance plan is created through consideration of the classification but only in conjunction with how the risk of contamination is assessed, the qualification protocol, and the historical operations of the site itself.

Generally, cleanrooms classified at a higher class level may experience the need for more frequent testing or more stringent acceptance criteria. However, the requirements for maintenance depend on the standards required by that site.

The IEST-RP-CC001 specification provides information on recommended practices for HEPA and ULPA maintenance [3]. The most recent edition should be adhered to for further clarification; nevertheless, it does not provide definitive standards for maintenance frequency.

What cleanroom checks follow filter installation or replacement

The correct procedure for post-installation testing is dependent on the qualification protocol. Standardly, however, these activities include:

• Testing the installed filter for leakages.
• Balancing airflow and/or flow velocity testing.
• Pressure checks for room pressure differences.
• Conducting a particle count for the room.

Confirming contaminants using airflow and pressure checks proves that the systems are balanced. Additionally, proving that the room passes the particle count measurements provides further proof of the integrity of the room classification. These tests are not interchangeable, nor will testing for one serve as testing for the other.

HVAC applications, standards, and filter selection

The principles of operating the HEPA filter in HVAC applications are similar to those employed in cleanroom maintenance. Although the qualification requirements are less stringent, HVAC HEPA filter care still requires monitoring of the pressure used for the prefilter and the timely replacement of HEPA.

With respect to resistance, the relationship between resistance and airflow and filter properties is not a simple universal relationship. Variables such as loading, media type, humidity, and temperature will affect the performance differently from one product to another. The rule of thumb regarding the limits for operation should always be tied to the specific manufacturer's specifications.

Standards EN 1822 and ISO 29463 establish HEPA and ULPA classification [4][5], while ASHRAE 52.2 allows for MERV ratings for general ventilation filters [6]. Legacy prefilter classification is set forth in EN 779 [7]. Finally, MIL-STD-282 covers historical testing specifications [8].

Which standards apply to HEPA filter performance and testing

The appropriate standard can depend on the region and the project.

• EN 1822: European HEPA and ULPA classification.
• ISO 29463: International HEPA and ULPA classification.
• ISO 14644-1: cleanroom classification by particle concentration.
• ISO 14644-3: cleanroom test methods.
• IEST-RP-CC001: Recommended Practice for HEPA and ULPA in the United States.
• ASHRAE 52.2: Test methods for HVAC filters and MERV rating.
• EN 779: Legacy prefilter classification.
• MIL-STD-282: Historical HEPA test specification.

Make sure to verify the applicability of the current edition prior to specifying the standard.

How HVAC operating conditions affect filter maintenance

The loading rate and filter life of an HVAC system are influenced by the operating conditions. The dust concentration in the air affects the loading rate; the higher the dust concentration, the higher the loading rate of the filter. Higher humidity levels contribute to the deterioration of specific types of media. The higher the temperature, the greater the impact on filter life when referenced to the type of filter media used.

Deiiang catalog data states the maximum operating humidity level as 80% and maximum operating temperature as 70 degrees Celsius for combined H13/H14 filters. This holds for already defined types only. Systems working under conditions that exceed these defined boundaries will result in reduced filter life.

Maintenance records, troubleshooting, and project example

The documentation of maintenance records is essential when it comes to troubleshooting and audit purposes. The maintenance records provide the documentation on how the filter has performed from the date of commissioning until replaced.

The maintenance record will include information such as model of the filter, installation date, initial resistance, monitoring readings, inspection notes, and date of replacement. This information should be preserved for the life of the installation.

The tracking of the troubleshooting path is another aspect of documenting the troubleshooting efforts, where the tracking process follows through the symptom observed, inspection and testing, and the corrective actions taken.

What HEPA maintenance records should facility teams keep

The records that should be maintained are:

• The serial number of the filter/model whenever applicable.
• Installation date and initial resistance.
• Monitoring readings with the date and airflow.
• Inspection notes and photographs.
• Date and reason for the replacement.
• Post-replacement test results.

What a verified Deiiang project case study should demonstrate

A verified case study must provide the challenge, solution, and outcome associated with the project. The project date, project location (if anonymity is required), test method, measurement conditions, and before-and-after data must be included.

The following is an illustration only, not a verified case study. It uses a project record but is missing the published measurement information.

Deiiang provided H14 combined filters at 592×592×292 mm rated for 2,500 m³/h with 20.04 m² of media area and an initial resistance of ≤220 Pa (according to the catalog). Silicone-free gaskets and F7 prefilters were included. DOP scan tests were performed on a per-unit basis prior to shipping.

According to the project outcome report, the particle counts moved from iso 7 baseline to stable iso 6. The measurement method, sampling conditions, and baseline data need to be verified back to original records before the data can be published.

Frequently Asked Questions

Do HEPA filters require special maintenance?

Yes, but the maintenance is specific. It involves inspection, pressure-drop monitoring, prefilter replacement, and eventual HEPA replacement. Cleaning is not part of standard HEPA filter maintenance for most models. The program should be documented and tied to the site's approved plan.

Can you wash or vacuum a HEPA filter?

No, not unless the manufacturer explicitly approves it for the specific model. Washing and vacuuming may damage the filter media or seal. Most HEPA filter manufacturers recommend that the filter be replaced when performance deteriorates.

How often should a HEPA filter be replaced?

Replacement should be justified by operating data and risk. Most often, it is a result of pressure drop, airflow, or integrity testing results. A predetermined time period may be acceptable as long as it is supported by the validated system plan or risk assessment. The Deiiang catalogue gives the final resistance values as 400–600 Pa for Combined H13/H14 filters at rated airflow.

How can I tell when a HEPA filter needs replacement?

The indicators that warrant replacement are increased pressure drop, reduced airflow, failure of integrity tests, damage to the filter media, wet filters, or contaminated filters. Each site must establish its action level at start-up and periodically review the action level, especially when operating conditions change.

Do prefilters help extend HEPA filter life?

Prefilters eliminate coarse particles prior to entering the HEPA filter media and may reduce the loading rate of the HEPA filter. It depends on the environmental airborne particulate level, airflow conditions, and maintenance practice. An efficiency rating for a prefilter may not by itself provide a basis for asserting that HEPA filter life is extended.

Should HEPA filter replacement be tested afterward?

The verification required is dependent upon the project-specific protocol. Depending on the project, leak testing of the installed filter, airflow verification, room particle testing, or both may be required. The testing of the installed filter is distinct from room classification testing and one test cannot substitute for the other.

References

• [1] iso 14644-1:2015 — Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration. International Organization for Standardization.
• [2] iso 14644-3:2019 — Cleanrooms and associated controlled environments — Part 3: Test methods. International Organization for Standardization.
• [3] IEST-RP-CC001 — HEPA and ULPA Filters. Institute of Environmental Sciences and Technology. Ensure the current edition and title.
• [4] EN 1822-1:2019 — High efficiency air filters (EPA, HEPA, and ULPA) — Part 1: Classification, performance testing, marking. European Committee for Standardization.
• [5] ISO 29463-1 — High efficiency filters and filter media for removing particles in air — Part 1: Classification, performance, testing, and marking. International Organization for Standardization.
• [6] ASHRAE Standard 52.2 — Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. ASHRAE.
• [7] EN 779:2012 — Particulate air filters for general ventilation — Determination of the filtration performance. European Committee for Standardization. Historical classification.
• [8] US MIL-STD-282 — Method of Test for Filter Units, Protective Clothing, Gas-Mask Components, and Related Products. United States Department of Defense.
• [9] Deiiang Combined High Efficiency Filter catalogue, DOP Integrated Filter catalogue, Super High Efficiency Baffle-Free Filter catalogue, and F5–F9 Pre-filter catalogue. Product data provided by Deiiang.