How to combine and configure G4, F7, F9 and H13 filters?

1. What Roles Do G4, F7, F9, and H13 Filters Perform?

These four filter grades perform different functions within an air-filtration system.

The basic logic of multistage filtration is therefore:

Coarse Particles → Medium Particles → Finer Particles → Final HEPA Filtration

The H13 filter should not be expected to handle the entire particulate load on its own.

2. Why Is Pre-Filtration Required Before an H13 Filter?

H13 HEPA filters generally cost more than primary and medium-efficiency filters, and their filter media have a denser structure.

If outdoor air passes directly through an H13 filter, large quantities of dust may quickly accumulate on the HEPA media, causing a rapid increase in pressure drop.

This may result in:

• Shorter HEPA replacement intervals;

• Rapidly increasing system resistance;

• Higher fan energy consumption;

• Reduced actual supply airflow;

• Higher operating and maintenance costs.

The main purpose of installing G4, F7, or F9 filters before an H13 filter is not simply to increase the total filtration grade. It is to progressively reduce the particulate load reaching the HEPA filter and extend the service life of the terminal filter.

3. Which Projects Are Suitable for G4 + F7 + H13?

This is a common three-stage filtration configuration:

G4 Primary Filter → F7 Medium-Efficiency Filter → H13 HEPA Filter

The G4 filter first captures larger airborne particles. The F7 filter then controls finer particles, while the H13 filter performs the final high-efficiency filtration.

This configuration is suitable for:

• General clean manufacturing facilities;

• Laboratory supply-air systems;

• Medical air-handling systems;

• Electronics production environments;

• Industrial AHUs requiring controlled air cleanliness.

Its advantages include a relatively simple system structure, manageable pressure drop, and reasonable equipment and replacement costs.

If the project does not have particularly strict fine-particle control requirements, it may not be necessary to add an F9 stage between the F7 and H13 filters.

4. When Is G4 + F7 + F9 + H13 Required?

A four-stage filtration configuration may be used in projects with high contamination loads, strict cleanliness requirements, or a need to extend HEPA service life:

G4 → F7 → F9 → H13

In this configuration, the F9 filter serves as a fine pre-filtration stage immediately before the H13 filter.

It is more suitable for:

• Pharmaceutical cleanrooms;

• Sterile production areas;

• High-grade laboratories;

• Semiconductor and precision electronics manufacturing;

• Systems with high HEPA replacement costs;

• Regions with severe outdoor air pollution.

However, adding an F9 filter introduces an additional pressure drop into the system.

The design must therefore verify:

Whether the AHU fan has sufficient available external static pressure.

If the fan capacity is insufficient, adding an F9 filter may improve upstream particle control but reduce the supply airflow.

5. Can F7 Be Omitted in a G4 + F9 + H13 System?

Some projects may use:

G4 → F9 → H13

Whether this configuration is suitable depends on the dust-holding capacity of the F9 filter, the system’s particulate load, and the design pressure drop.

This arrangement reduces one filtration stage, but the F9 filter must handle a greater particulate load, which may cause its resistance to rise more quickly.

If the outdoor dust concentration is high or the system operates continuously for long periods, using G4 + F7 + F9 generally distributes the particulate load more effectively.

F7 should not be removed simply to reduce the number of filtration stages. The decision should be based on lifecycle cost.

6. How Should Different Engineering Projects Be Configured?

General Commercial HVAC Systems

If there is no cleanroom requirement, an H13 filter is generally unnecessary.

A suitable configuration may be:

G4 → F7

This controls outdoor dust and PM particles while maintaining reasonable system resistance.

General Clean Environments

A suitable configuration may be:

G4 → F7 → H13

This provides a practical balance among filtration performance, equipment cost, and operating resistance.

High-Grade Cleanrooms

The following configuration may be considered:

G4 → F7 → F9 → H13

Multistage filtration reduces the particulate load reaching the HEPA filter and improves the operating stability of the terminal filtration system.

FFU Cleanroom Systems

The central AHU may first be equipped with G4/F7 or F9 filtration, while the cleanroom terminals use:

FFU + H13/H14

This separates pre-filtration from final high-efficiency filtration, making maintenance and pressure-differential control easier.

7. System Pressure Drop Must Not Be Ignored

The more filtration stages a system has, the higher its total resistance will generally be.

The total filtration resistance can be understood as the accumulated pressure drop across all filter stages. Purchasers should therefore confirm each individual value:

G4 Initial Pressure Drop + F7 Initial Pressure Drop + F9 Initial Pressure Drop + H13 Initial Pressure Drop

The increase in resistance caused by dust accumulation must also be considered.

If the fan is selected only according to the initial resistance of new filters, without allowing for resistance growth during operation, the system may experience insufficient supply airflow after running for some time.

8. Which Parameters Should Be Confirmed During Procurement?

For a G4, F7, F9, and H13 multistage system, purchasers should provide the supplier with at least:

1. AHU design airflow;

2. Filter installation dimensions;

3. Target filtration grades;

4. Allowable initial pressure drop for each stage;

5. Allowable final system resistance;

6. Outdoor air quality and particulate load;

7. Cleanroom class or terminal cleanliness requirement;

8. H13 installation position and sealing method.

For new projects, it is also necessary to confirm whether the specification uses traditional EN 779 grades or ISO 16890 classifications.

G4, F7, and F9 remain common traditional engineering classifications. For new projects, the corresponding ISO ePM class and actual tested efficiency should also be specified.

9. Frequently Asked Questions

Is an F9 Filter Always Required Before H13?

No. General clean-air systems may use G4 + F7 + H13. An F9 stage can be added when cleanliness requirements are higher, the particulate load is heavier, or stronger HEPA protection is required.

Can F7 and F9 Be Installed Together?

Yes. These two filtration stages can progressively reduce the particulate load. However, the fan’s available static pressure and the total system resistance must be checked.

Can G4 Be Directly Combined with H13?

It can be designed technically, but it is generally not recommended for long-term systems with high dust loads. The efficiency gap between G4 and H13 is large, which may cause the HEPA filter to handle a greater fine-particle load.

Are More Filtration Grades Always Better?

No. Additional filtration stages can improve particle control, but they also increase equipment cost, pressure drop, and maintenance requirements. A properly balanced configuration is more important than simply adding more filter stages.

10. Boyang Multistage Air-Filtration Solutions

For AHUs, fresh-air systems, cleanrooms, hospitals, pharmaceutical facilities, and electronics manufacturing projects, Boyang can provide different multistage filtration configurations according to the design airflow, cleanliness requirements, and allowable system pressure drop, including:

G4 Primary Panel Filter → F7/F9 Bag Filter → H13/H14 HEPA Filter

For general air-handling systems, the number of filtration stages can be reduced according to actual requirements. For high-cleanliness projects, F7 and F9 filters can progressively reduce the particulate load, while H13 or H14 terminal filtration provides stable clean-air control.

Purchasers should not provide only the grades “G4 + F7 + F9 + H13.” They should also specify the design airflow, filter dimensions, allowable pressure drop, cleanliness requirements, and operating environment.

A properly designed filtration system is not necessarily the one with the greatest number of stages. It is the one that achieves the right balance among filtration efficiency, HEPA protection, system pressure drop, fan energy consumption, and filter replacement cost.