Replacing filters in a Zehnder ComfoAir Q350, Q450 or Q600

The two filters you need

A Zehnder ComfoAir Q350, Q450 or Q600 uses two filters measuring 160 × 500 × 23 mm: one for the incoming supply air and one for the outgoing extract air. The three unit sizes take the same filter dimensions, so the Q number does not change the physical size you need.

The supply filter treats fresh air entering the building. The extract filter catches dust carried back from the rooms and helps protect the heat exchanger inside the ventilation unit. Both matter. Replacing only the one that looks worse leaves the other side of the system running through an old filter.

You may see the Dutch terms “toevoer” for supply and “afvoer” for extract on ventilation documentation or labels. In English, look for “supply” and “extract”. Note which filter comes from each position before you remove both of them.

Recognising the correct size and references

Start with the full model name on the ventilation unit’s identification plate. ComfoAir Q350, Q450 and Q600 are the relevant models here. You can then confirm the filter by measuring the old frame in millimetres: width × height × thickness should be 160 × 500 × 23 mm.

Filter-set references encountered for this ComfoAir Q range include 400100900, 400502012 and 400502013. A reference can help you identify a set, but these numbers alone should not be used to decide which filtration class it contains. Check the description and the markings on the filter frame.

On VMCDirect, you can find VMCDirect-brand filters for these units by model, by reference or by dimensions. The model name is usually the simplest route, while the dimensions are a useful final check before fitting anything.

Choosing the filter arrangement

A common arrangement is a coarse G4 filter on the extract side and a finer F7 filter on the supply side. The G4 protects the heat exchanger from dust in the extracted air. The F7 catches pollen and fine particles from the fresh air entering the unit. You may also use F7 filters on both sides, although finer filters restrict airflow more.

G4 and F7 are classes from the older EN 779 standard. Under ISO 16890, which replaced it in 2018, G4 corresponds roughly to ISO Coarse, while F7 is roughly ePM1 50 to 65%. These are approximate correspondences rather than exact matches, so read the stated class rather than relying on a familiar-looking filter colour or fabric.

Do not wash or vacuum an F7 filter, as this can damage its filter medium. Vacuuming a G4 between replacements may help temporarily, but it does not replace fitting a new filter.

Replacing the filters from the front

The filters come out through the front of the ComfoAir Q350, Q450 and Q600, behind their individual covers. The precise way those covers open is not covered here, so follow the manufacturer’s instructions for your unit rather than guessing at clips, catches or turning mechanisms.

Before removing anything, look at the positions of the supply and extract filters and check the direction arrows on their frames. An arrow shows the direction of airflow. Installing a filter backwards can reduce its performance and may cause the filter to deform.

1. Confirm that the identification plate says ComfoAir Q350, Q450 or Q600, and check that the replacement filters measure 160 × 500 × 23 mm.
2. Locate the filter covers at the front of the ventilation unit. Open them using the method shown in the manufacturer’s instructions.
3. Withdraw both old filters from the front. Keep track of which position is supply and which is extract, particularly if the two filters have different classes.
4. Insert one replacement filter in each position. Match the intended class to the supply or extract side and follow the airflow arrow on the frame.
5. Refit the filter covers as described in the unit instructions, then reset the filter counter.

Resetting the filter counter

After replacing both filters, reset the filter counter from the ventilation unit’s screen or through the Zehnder application. The exact menu path can vary, so use the instructions supplied with the unit or application for the individual button presses.

Resetting the counter does not change airflow or clean a filter; it tells the system that a new filter period has started. Many ventilation units base their filter message on elapsed time rather than measuring actual dirt in the filter, which means a reminder can appear before or after the filters genuinely need attention.

Do the reset only after the new filters are fitted. Otherwise, the next reminder will no longer reflect when the filters were last changed.

Checks after fitting

Make sure both filter covers are back in place and that each filter sits in the position from which the old one was removed. If the supply and extract filters use different classes, check once more that they have not been swapped.

Most manufacturers advise replacing ventilation filters roughly every six months and checking them in between, with more frequent attention near a busy road, during building work, in pollen season in the countryside or where animals add more dust. A dirty filter can reduce airflow, make the fans work more noisily, increase electricity use and allow the heat exchanger to become dirty.

Frequently asked questions