How to Choose Filters for an Air Handling Unit

Define the application

Air-cleanliness requirements and contaminant types vary greatly among applications, so begin by defining the primary need.

Ordinary residential and commercial applications, such as offices, shopping malls and homes: The main task is to remove dust and pollen and prevent dust buildup and mold growth inside the air-conditioning system. A coarse-filter plus medium-efficiency-filter combination is normally sufficient to meet the indoor-air requirements of GB/T 18883.

Sensitive applications, such as hospital outpatient areas and hotel rooms: PM10 and some bacteria must be removed. A combination of coarse, medium-efficiency and fine filters, or medium-efficiency and fine filters, is recommended to reduce respirable particles indoors.

High-cleanliness applications, such as operating rooms, electronics clean workshops and laboratories: Terminal HEPA filters are also required. The AHU normally contains coarse, medium-efficiency and sub-HEPA stages as prefiltration to keep larger particles from loading the downstream HEPA filters.

Special environments, such as textile mills, flour mills and chemical workshops: The air may contain large quantities of fibers, dust or corrosive gases. Give priority to wear-resistant coarse filters with high dust-holding capacity, such as metal-mesh media, or to corrosion-resistant materials such as PVC frames.

Filtration efficiency class

Select AHU filter efficiency according to national standards or industry requirements. The principal references include:

GB/T 14295-2019, Air Filters, which is widely used in China and classifies filters as coarse (G1-G4), medium-efficiency (M5-M6) and fine (F7-F9).

ASHRAE 52.2-2017, commonly used internationally, which classifies filters according to particle-size efficiency, including coarse filters with 40%-80% efficiency for particles from 0.3 to 10 micrometers and medium-efficiency filters with 80%-95% efficiency for particles from 1 to 3 micrometers.

Typical efficiency grades, target contaminants, AHU functions and applications:

Coarse filters (G1-G4)

Target contaminants: Large particles 5 micrometers or larger, including dust, hair and fibers.

Function in the AHU: First-stage filtration that captures large particles and protects downstream filters and air-conditioning equipment.

Typical application: Mandatory prefiltration in all air-conditioning systems.

Medium-efficiency filters (M5-M6)

Target contaminants: Particles 1 micrometer or larger, including pollen and fine dust.

Function in the AHU: Further removes fine particles and reduces deposits on cooling coils and heaters.

Typical application: Final filtration in ordinary residential and commercial systems, or second-stage filtration in high-cleanliness systems.

Fine filters (F7-F9)

Target contaminants: Particles 0.5 micrometers or larger, including smoke and some bacteria.

Function in the AHU: Deep filtration that reduces indoor PM2.5 and microbial concentrations.

Typical application: Sensitive areas such as hospitals and hotels, or third-stage filtration in high-cleanliness systems.

Note: An AHU normally uses multiple filtration stages, such as coarse plus medium-efficiency filtration, instead of a single filter. A coarse pre-filter extends the service life of medium-efficiency and fine filters by preventing rapid blockage by large particles, thereby reducing long-term maintenance costs.

Match the AHU parameters

Rated airflow must match the AHU design airflow

Rated airflow is the airflow at which a filter achieves its specified efficiency and resistance, as stated by the manufacturer. Select a filter so that actual operating airflow does not exceed rated airflow, with a recommended deviation of no more than 10%. Excessive airflow can cause:

Reduced efficiency because air velocity is too high for particles to be captured effectively.

A sharp increase in resistance, raising fan energy consumption and potentially overloading the fan.

Example: For an AHU designed for 10,000 cubic meters per hour, choose a filter rated for at least 10,000 cubic meters per hour. A slightly higher rating, such as 12,000 cubic meters per hour, provides useful reserve capacity.

Resistance must match fan capability

Filter resistance, both initial and final, directly affects energy use and operating stability.

Initial resistance: The resistance of a new filter. It should be no more than 30% of the air-conditioning system's design resistance so that fan load does not increase excessively.

Final resistance: The resistance at the filter replacement point, normally two to three times initial resistance. It must not exceed the maximum resistance the fan can handle, or system airflow will fall.

Example: If the fan's maximum design resistance is 200 Pa and the selected filter has an initial resistance of 50 Pa, a recommended final resistance is no more than 150 Pa (50 x 3), allowing the fan to operate normally until replacement.

Filter type and construction

Choose the filter construction according to installation space, dust-holding capacity, which affects replacement intervals, and ease of maintenance. Common types include:

Panel filters

Construction: Flat media in a compact, thin structure, normally 20-50 mm thick.

Dust-holding capacity: Low.

Application: AHUs with limited space, such as wall-mounted units and small cabinet units.

Maintenance: Washable when metal mesh is used, or disposable. Installation is simple.

Bag filters

Construction: Media formed into multiple parallel pockets, with a total depth of 60-300 mm.

Dust-holding capacity: High.

Application: Medium and large AHUs, including modular air-handling units.

Maintenance: Disposable. More pockets provide higher dust-holding capacity, but sufficient replacement space must be reserved.

Box filters

Construction: Media pleated into a V-bank inside a rigid box frame.

Dust-holding capacity: Medium to high.

Application: Systems requiring higher efficiency, including fine filtration.

Maintenance: Removed and replaced as one unit; best suited to AHUs with sufficient space.

Selection recommendations:

For restricted spaces such as fan-coil units, give priority to a G3-G4 coarse panel filter.

For medium and large AHUs requiring medium-efficiency filtration, choose an M5-M6 bag filter. More pockets, such as four or six, provide greater dust-holding capacity and a longer replacement interval.

For fine filtration in applications such as hospitals, choose an F7-F9 box filter. Its stable structure makes rated efficiency easier to maintain.

Filter media and frame materials

Materials must withstand the humidity, temperature and any corrosive conditions inside the AHU without deforming, failing or causing secondary contamination.

Filter media

Coarse filters: Common media include low-cost disposable nonwoven fabric, washable metal mesh for high-dust environments such as workshops, and glass fiber, which offers somewhat higher dust-holding capacity and good temperature resistance.

Medium-efficiency and fine filters: Synthetic fibers such as polyester are moisture resistant and low-lint. Glass fiber offers good heat resistance and is suitable when the AHU includes a heating section.

Special requirements: In humid environments, such as swimming pools or areas affected by a long rainy season, choose moisture-resistant media such as polyester to prevent water absorption and mold. For high-temperature conditions, including AHUs with high-temperature heating sections, choose heat-resistant media such as glass fiber rated for at least 120 degrees Celsius.

Frame materials

Ordinary environments: Galvanized steel provides a low-cost, rust-resistant option; aluminum alloy is lightweight and well suited to panel and bag filters.

Humid or corrosive environments, such as food-processing workshops or coastal sites: Choose a 304 stainless-steel frame for corrosion resistance.

Avoid cardboard frames, which can deform when damp, and ordinary plastic frames, which may become brittle at low temperatures.

Maintenance and cost

Replacement interval: Greater dust-holding capacity extends the replacement interval and reduces shutdowns for maintenance. At the same efficiency, a bag filter has greater dust capacity than a panel filter and is therefore better suited to dusty environments such as roadside buildings and industrial areas.

Ease of replacement: Choose quick-install filters with features such as clips or hook-and-loop fasteners to reduce replacement time. Confirm that the filter's length, width, height and flange connection exactly match the space reserved in the AHU, with a dimensional deviation of no more than 5 mm, to prevent air leakage.

Cost balance: Evaluate initial purchase price together with replacement frequency to determine life-cycle cost. For example, a washable metal-mesh coarse filter costs more initially but lasts longer, making it economical in dusty applications that otherwise require frequent replacement. A disposable nonwoven filter has a lower initial cost and is appropriate for low-dust environments.

Standards and certification

Required standards: The product should comply with GB/T 14295-2019 in China or ASHRAE 52.2 internationally so that efficiency, resistance and other parameters are reliable.

Test reports: Ask the manufacturer for third-party test reports, including media-efficiency and resistance data, to avoid products with overstated ratings.

Hygiene requirements: For hospitals, food workshops and similar applications, choose antimicrobial media, such as media containing silver ions, and confirm that it meets applicable hygiene requirements such as GB 15979 and is free of pathogenic microorganisms.

Summary of the selection process

1. Define the application, such as an office building or hospital, and determine the required filtration-stage combination, such as coarse plus medium-efficiency filtration.

2. Use the AHU design airflow and fan resistance to identify filters with suitable rated airflow and acceptable resistance.

3. Choose the construction type, panel, bag or box, according to the available installation space.

4. Select media and frame materials according to environmental factors including humidity, temperature and corrosiveness.

5. Consider ease of maintenance and life-cycle cost, giving priority to products that are easy to replace and have high dust-holding capacity.

6. Verify compliance certificates and test reports to confirm performance.

Following these steps will help you select an economical, durable AHU filter that matches the system, meets air-purification requirements, extends equipment life and reduces operating energy consumption.