G4 vs F7 ventilation filters: they do different jobs

The usual G4 and F7 arrangement

Use G4 on the extract side and F7 on the supply side in the usual heat recovery ventilation (MVHR) arrangement. The G4 protects the heat exchanger from material carried out of the home, while the F7 retains much of the pollen and fine particles carried in with the outside air. If pollen or urban particles are your concern, the supply filter is therefore the important one.

Fitting F7 on both sides is possible, but a fine filter restricts the airflow more than a coarse one. A second F7 on the extract side does not provide extra filtration for the outside air entering your rooms, because that air passes through the supply filter instead. Two F7 filters should therefore be a deliberate choice, not a reflex because finer sounds better.

EN 779 and ISO 16890 labels

G4 and F7 come from the former EN 779 classification. G4 describes a coarse filter, while F7 describes a fine filter. You will still see both labels because they remain familiar and are printed on many filter frames, manuals and ventilation units. They are useful shorthand, provided you remember that they belong to an older standard.

ISO 16890 replaced EN 779 in 2018. A G4 filter corresponds approximately to ISO Coarse, while an F7 filter corresponds approximately to ePM1 50 to 65%. These are rough correspondences, not exact one-for-one conversions. A label such as ePM1 gives you the newer classification; it is not simply a fresh name stamped onto every former F7 filter.

Supply and extract determine filter placement

An MVHR unit has at least two filters. The supply filter sits in the path of fresh air entering from outside, sometimes marked “supply” or “toevoer”. The extract filter sits in the path of stale air leaving the rooms, sometimes marked “extract” or “afvoer”. On the common G4 and F7 setup, the F7 belongs on supply and the G4 on extract.

Do not decide by looking only at the left and right sides of the open unit. The position of supply and extract varies between models and sometimes between configurations of the same range. Check the markings inside the unit or the manufacturer's instructions. If the filter frame carries an arrow, it shows the direction of airflow. A filter installed backwards can filter poorly and may deform.

Two F7 filters in pollen and city conditions

For a home where pollen is a concern, an F7 on the incoming supply air is the relevant choice because it retains a large part of that pollen before the air enters the rooms. The same reasoning applies near a busy road: the supply-side F7 deals with fine particles in the outside air. This describes what the filter captures; it is not a claim that the filter treats or prevents an allergy or illness.

Using F7 on both sides does not double that effect. The second filter is handling outgoing extract air, not another stream of outside air. It also adds more airflow restriction than the usual G4 extract filter. If you want to run two F7 filters, check the arrangement allowed for your ventilation unit in its manual. There is no single setting or filter layout that can be assumed for every model.

Class alone does not identify the correct filter

Knowing that you need G4, F7, ISO Coarse or ePM1 is only half the job. Filters with the same class can have different widths, heights and thicknesses, so a class label does not tell you that the filter will fit your unit. The exact model name is normally shown on the ventilation unit's rating plate. You can also use the reference printed on the old frame or measure its width, height and thickness in millimetres.

On VMCDirect, you can find VMCDirect-brand filters by ventilation model, by the reference on the existing filter or by dimensions. Check all three measurements rather than judging by the front face alone. A filter that is the right height and width but the wrong thickness is still the wrong size for the slot.

Handling coarse and fine filters correctly

An F7 filter should not be washed or vacuumed. Its fine filtering medium can be damaged, even when it still looks tidy afterwards. A G4 can be vacuumed between replacements to remove some collected material, but that does not replace fitting a new filter. Most manufacturers advise replacement at around six months, with checks in between, although conditions around the home affect how quickly filters become dirty.

After fitting the filters, reset the filter counter using the unit's screen, controller or application, depending on the model. That message is often triggered by elapsed time rather than measured dirt, so it does not tell you which class is installed or how blocked the filter really is. The exact reset menu and the way the unit opens differ by model; use the manufacturer's instructions rather than forcing a panel that appears reluctant. Plastic usually wins that argument by breaking.

Frequently asked questions