EN 779 to ISO 16890: How to Convert G4 and F7 on Legacy Equipment

If you’re maintaining an older air handling unit, it’s common to see filter labels such as G4 or F7 still written on the frame, the housing door, or the old maintenance sheet. Those codes belong to the EN 779 system, which has been superseded by ISO 16890, so the challenge is not just replacing a physical filter, but choosing a modern equivalent that suits the equipment and the site.

The key is to interpret the old label as a starting point, then confirm the actual filter dimensions, airflow direction, available space, and the performance needs of the system before ordering a replacement.

What EN 779 labels on legacy equipment really tell you

EN 779 was widely used for general ventilation filters and is still commonly seen on older plant and maintenance records in Australia. The classes G3, G4, F5, F6, F7, F8 and F9 describe filter performance in the old system, but they do not map directly to ISO 16890 in a simple one-to-one way.

That matters because a label alone does not tell you everything about the filter that is actually installed. Two filters marked F7 may behave differently if they have different media, construction, depth, face velocity tolerance or dust-holding characteristics.

For that reason, a replacement decision should start with the old class, but not end there.

• •Check the existing filter label, frame and any tag on the AHU
• •Confirm whether the filter is a panel, bag, compact or custom size
• •Measure the actual frame size if the label is missing or unclear
• •Note the airflow direction and any sealing method used in the holder
• •Check whether the system has a differential pressure gauge fitted

If you need help confirming the size of a legacy filter, our guide on how to measure an air filter is a good place to start.

Why EN 779, ISO 16890 and MERV do not convert directly

A common mistake is to treat old and new ratings as if they are exact equivalents. They are not. EN 779, ISO 16890 and MERV use different test methods and describe filter performance in different ways, so there is no universal conversion table that can be applied safely to every air handling unit.

ISO 16890 classifies general ventilation filters by their efficiency against particulate size ranges, which gives a better picture of how the filter performs on real airborne particles. That is useful, but it also means the old shorthand of G4 or F7 should be treated as a reference point rather than a direct specification.

In practice, the right replacement depends on the system and the goal of the filter stage:

• •A coarser pre-filter stage may be enough where the main aim is to protect coils and internal equipment from heavier dust loading
• •A finer secondary stage may be needed where the system previously used F7 to support cleaner supply air
• •In some systems, the right answer is to match the original performance intent, not to chase a higher number on the new label
• •In other systems, an overly tight filter can increase resistance and place unnecessary strain on the fan and holder

That is why it helps to look at the whole air path, not just the label on the old filter.

How to interpret G4 on a legacy AHU

G4 was commonly used for general coarse filtration in older HVAC systems. If your equipment still calls for G4, the first question is what role that filter is playing in the system.

Often, a G4 filter is fitted as the first stage in a multi-stage arrangement, where the goal is to catch larger airborne dust and protect downstream components. In that case, the modern ISO 16890 replacement should usually remain a coarse general ventilation filter with similar practical function, rather than being upgraded blindly to a much finer product.

When checking a G4 replacement, look at:

• •The filter’s position in the system
• •Whether there is a second stage after it
• •The amount of dust visible at the intake
• •Whether the filter rack can physically handle the media type you plan to install
• •Whether the frame and seal are sound, because bypass can undermine performance

If the holder is worn, the right answer may involve more than swapping the media. A poor seal, distorted frame or damaged door can defeat even a correctly chosen filter.

How to interpret F7 on a legacy AHU

F7 was commonly used where finer filtration was needed than a G4 stage could provide. In legacy equipment, F7 is often found in units serving offices, retail spaces, schools, hospitals, food areas or other applications where a finer grade was originally specified by the design team.

When replacing an F7 filter, avoid assuming that a “higher” ISO 16890 class is automatically better. A finer filter may suit the duty, but it can also increase resistance if the AHU was not designed for it. That can affect fan operation, service access and overall system balance.

A sensible approach is to ask:

• •Was F7 used as a final filter or as part of a staged arrangement?
• •Is the AHU built to accept a deeper or pleated filter type, or only a shallow panel?
• •Has the system been altered since the original filter was specified?
• •Are there signs of dust bypass, poor sealing or uneven loading?

If you are upgrading a site and want a filter set matched to the equipment rather than a generic replacement, our products page is a useful starting point, or you can look at custom air filters where standard sizes no longer fit the holder correctly.

A practical selection process for replacement filters

The safest way to replace legacy EN 779 filters is to work through the job methodically. This avoids under-specifying the filter and also helps prevent unnecessary over-specification.

Start with the equipment, not the label

Confirm the AHU model, filter rack dimensions, available depth, airflow direction and access constraints. If the label is missing or unreadable, physical measurement is essential.

Match the function of the original filter stage

Decide whether the old G4 or F7 was acting as a pre-filter, a main filter or a final stage. The intended role often matters more than the code itself.

Check maintenance conditions

Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted.

Consider the site requirements

For ventilation and air quality design issues, site requirements should be confirmed with the mechanical services engineer or building owner. If the building is being altered or refurbished, AS 1668.2:2024 may also be relevant to the broader ventilation arrangement.

Verify the product style

A filter that fits the dimensions may still be wrong if the frame type, pocket depth or sealing arrangement does not suit the holder. Some filters are designed to be washable - check the product instructions.

Common mistakes when replacing old EN 779 filters

Legacy filter change-outs often go wrong for simple reasons. The most common is assuming that a code on an old schedule is enough to order from memory. Another is replacing a worn frame with a new filter that fits loosely and allows bypass around the edges.

Other mistakes include:

• •Choosing a finer filter without checking fan capacity or system design
• •Reusing damaged holding frames or door seals
• •Measuring the old filter incorrectly and ordering the wrong size
• •Assuming all F7 filters behave the same
• •Ignoring the pressure drop once the replacement is installed

If the system is critical or the housing is non-standard, commercial filter servicing can save time by checking the existing arrangement before replacement. See filter servicing or commercial filter servicing for more detail.

Getting the right replacement for legacy equipment

For facility managers and mechanical contractors, the goal is not to modernise the label for its own sake. It is to choose a replacement that fits the AHU, suits the airflow, and delivers the intended filtration stage without creating avoidable problems.

If you have an old G4 or F7 reference and are not sure how to translate it into a current ISO 16890 specification, Total Air Filtration can help assess the filter type, size and application. We are a family-owned air filter manufacturer and supplier based in Hillcrest, Brisbane QLD, and we can supply standard sizes or make the right filter for a non-standard holder.

Call 07 3800 3448 or request a quote when you are ready to match a legacy filter to the current equipment.