When should an air handling unit filter be changed?
An air handling unit filter should be changed when the differential pressure reaches the final pressure drop given by the manufacturer, when the recommended maximum service life expires, or when the filter is damaged. Whichever of these three criteria occurs first, the replacement is done then.
The most common mistake in the field is changing the filter only by the calendar. In a unit in a dusty industrial area, a G4 pre-filter can fill up in a few weeks, while in a clean office building the same filter can run for months. The calendar should be used as an upper limit, and the real decision should be made by measurement.
What are G4, F7 and HEPA filters for?
G4, F7 and HEPA are, respectively, coarse, fine and very high efficiency filter classes used in stages in air handling units. Each stage captures particles of a different size and protects the next stage.
- G4 (coarse filter): Used at the fresh air inlet or as the pre-stage. It captures insects, leaves, fibers and coarse dust. Its job is to protect the fine filter and the coil behind it.
- F7 (fine filter): Usually of the bag or compact type. It captures pollen, fine dust and a significant part of particulate matter. It is the common main filter in office, school, hotel and shopping mall units.
- F9: Used as the second fine filter stage in hospitals, laboratories and sensitive production areas.
- HEPA (H13, H14): Used as a terminal filter in areas such as operating rooms, clean rooms and pharmaceutical production. It has very high capture efficiency even for very small particles.
Difference between EN 779 and ISO 16890
The names G4 and F7 come from the old EN 779 standard. This standard has been replaced by ISO 16890; the new standard classifies filters in the ISO Coarse, ePM10, ePM2.5 and ePM1 groups. Because the old names are still widely used in the field, the two systems are mentioned together.
| Old class (EN 779) | Approximate ISO 16890 equivalent | Typical use |
|---|---|---|
| G4 | ISO Coarse (approximately 60% and above) | Pre-filter, fresh air inlet |
| M5 | ePM10 approximately 50-60% | Pre-stage or intermediate stage |
| F7 | ePM1 approximately 50-70% | Main fine filter |
| F9 | ePM1 approximately 80% and above | Hospital, laboratory second stage |
| H13, H14 | Under EN 1822, outside ISO 16890 | Terminal HEPA filter |
The equivalents in the table are approximate because the two standards use different test methods. When ordering filters, base your order on the manufacturer's ISO 16890 class declaration and the dimensions.
How is the replacement time determined by differential pressure?
Differential pressure is the pressure difference between the front and back of the filter and is the most reliable measure of how full the filter is. As the filter gets dirty, air passage becomes harder and the differential pressure rises.
Every filter has an initial resistance when clean and a final pressure drop recommended by the manufacturer. Replacement should be done when the measured value reaches this final value. As a practical rule, when the value rises to about two to three times the initial resistance, the filter is considered to be nearing the end of its economic life. The exact limit should always be set according to manufacturer data.
- A separate differential pressure gauge or sensor should be installed on each filter stage.
- When a new filter is installed, the initial value should be noted; later readings are interpreted against this value.
- If the differential pressure suddenly drops, the filter may be torn or may have slipped out of its seat. This does not mean the filter has been cleaned.
- In units connected to building automation, generating an alarm when the final value is approached makes replacement easier to plan.
Time limits and visual criteria
Even if the differential pressure stays low, it is recommended not to use filters beyond a certain time. The reason is hygiene: microorganisms can multiply in filter media that stay in contact with humid air for a long time, and odor can develop.
Hygiene guidelines recommend maximum service lives of about one year, especially for the first filter stage, and longer for later stages. Typical replacement intervals in the field are as follows:
| Filter | Typical replacement interval | Note |
|---|---|---|
| G4 pre-filter | 1-3 months | More often in dusty environments |
| F7 bag or compact filter | 6-12 months | If the pre-filter is changed regularly |
| F9 second stage | 12-24 months | Per guidelines in hospital applications |
| HEPA terminal filter | Based on differential pressure and test results, often several years | Leak test after replacement |
Base filter replacement on measurement
Let us check the filter stages, differential pressures and sealing of your units on site and prepare a replacement plan.
In a visual inspection, if any of the following is present, the filter should be replaced regardless of measurement and time: a tear or hole, wet or water-soaked filter media, mold or odor, deformation of the frame and gasket damage.
What does late or early replacement cost?
Changing a filter late is a loss in terms of energy and air quality, and changing it early is a loss in terms of material cost. The right timing is finding the balance between the two.
A filter changed late causes airflow to drop in constant-speed fans, and in variable-speed and EC fans the fan consumes more energy. An overloaded bag filter can tear and release the dust it has captured onto the coil and duct. Early replacement, on the other hand, means throwing away a filter that has not yet reached the end of its life. Changing the pre-filter regularly is the most economical way to extend the life of expensive fine filters and HEPA filters.
For the effect of filter media selection on fan energy, see our nanofiber filters and energy consumption article.
Who should change filters and how?
Air handling unit filter replacement should be done by trained staff or a qualified technician after the unit has been stopped and the fan has come to a complete stop. Opening the filter cover on a running unit is both dangerous and causes dust to spread into the system.
- The dirty filter should be removed in a bag, and dust must not be allowed to spread into the environment.
- The direction (airflow arrow), class and size of the new filter should be checked.
- There must be no gaps in the filter frame and gaskets.
- The replacement date, filter class and new initial differential pressure should be recorded.
- After replacement, a leak test should be done on HEPA filters.
Filter replacement is part of air handling unit maintenance. For other checks, take a look at our air handling unit maintenance checklist article. At YOİ Mühendislik, in our air handling unit service we plan filter replacements according to differential pressure records and report every replacement. The scope is determined at the site survey according to the number, class and dimensions of the filters.
Frequently asked questions
How often is a G4 filter changed?
Depending on the dust level of the environment and operating hours, it is generally changed every 1-3 months. The most accurate criterion is the differential pressure reaching the final value given by the manufacturer.
When should an F7 bag filter be replaced?
If the pre-filter is changed regularly, an F7 bag filter generally lasts 6-12 months. It should be replaced regardless of time when the differential pressure reaches the final value or when tears, wetness or odor are seen in the bag.
Can a HEPA filter be cleaned and reused?
No. HEPA filters are not washed, vacuumed or reused; these operations damage the filter media. A HEPA filter that has reached the end of its life is replaced with a new one of the same class and size, and a leak test is performed.
What does an F7 filter correspond to in ISO 16890?
There is no one-to-one equivalent because the two standards use different test methods. Roughly, F7 filters correspond to filters in the ePM1 50-70% range in ISO 16890; the manufacturer's ISO 16890 class declaration should be the basis for selection.
What happens if a filter is changed late?
The fan uses more energy or the airflow drops, and spaces are not ventilated and cooled enough. An overloaded filter can tear and let the captured dust pass to the coil and ducts.
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Detailed guide: Air handling unit maintenance: intervals, checklist, and maintenance form
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