Common mistakes when changing heat recovery unit filters – check and avoid extra costs!
Replacing heat recovery unit filters is a simple task you can easily do yourself in just a few minutes.
However, many homeowners make a few common mistakes that can shorten the filter’s lifespan, reduce airflow, and lower the efficiency of the MVHR system.
Discover how to change your filters correctly and avoid unnecessary costs.
1️⃣ Incorrect airflow direction
Filters such as F7, M5, or F9 are made from several layers of material that capture particles of different sizes. If you insert the F7 filter the wrong way round, dust and dirt accumulate on the outer layer, which leads to:
- ⚠️ faster clogging and shorter filter lifespan;
- ⏳ up to 15% higher airflow resistance;
- ❌ possible air leaks in some units (for example, Zehnder ComfoAir models with “wing-type” gaskets).
How to insert the filter correctly:
- Always follow the airflow direction arrows printed on both the filter and the unit.
- If there are no arrows, the airflow should always point towards the heat exchanger (for both filters).
Exceptions:
- Zehnder ComfoAir Q 350/450/600 – marked with “This side up,” meaning the arrow must face upwards.
- Blauberg Komfort 250 and 230/270 – the efficient F7 filter is installed after the heat exchanger, so the arrow should point away from the exchanger.
2️⃣ Mixing up F7/F9 and M5/G4 filter positions
In many heat recovery units, both filters look identical in size but are made to different efficiency classes. Swapping the supply-air filter (F7, F9 or activated-carbon) with the extract-air filter (G4 or M5) can lead to:
- ⚠️ poorer indoor air quality;
- ⏳ lower heat-recovery efficiency;
- ❌ unbalanced airflow inside the MVHR system.
How to tell which filter goes where:
- The higher-efficiency F7 / F9 / activated-carbon filter is installed on the fresh-air (outdoor) side.
- The lower-efficiency G4 / M5 filter is placed on the exhaust-air (indoor) side.
If you’re unsure, check your heat-recovery unit’s installation and maintenance manual – it usually specifies which side is for supply air and which for extract air.
If the manufacturer hasn’t marked the airflow direction or filter type, label it yourself using a sticker or marker showing the airflow direction and filter class (e.g. F7, G4). This will make the next filter change quicker and help you avoid mistakes.
Additional tips:
- The outdoor-air duct is usually insulated, so that’s where the F7 filter belongs.
- If your unit includes a condensate drain or pre-heater, these are typically on the fresh-air side – fit the F7 filter there.
3️⃣ Changing filters without turning off the unit
Never replace the filters while the fans are still running! Doing so allows dust and debris to be drawn into the heat exchanger or onto the fan blades, which can cause several problems:
- ⚠️ Shortened lifespan of the heat recovery unit;
- ⏳ Increased fan noise over time;
- ❌ Reduced heat-exchange efficiency and higher energy use.
Before replacing your filters, always switch off the heat recovery unit using the controller or mobile app. This prevents dust from entering the system and ensures the fans and heat exchanger remain clean and efficient for longer.
4️⃣ Turning off the unit incorrectly (skipping the purge cycle)
Some users switch off their heat recovery unit by unplugging it from the mains instead of pressing STOP on the controller or app. This prevents the system from performing its automatic purge cycle, which can lead to overheating and damage.
- 🔥 The heating element can reach 200–300 °C locally if not cooled by airflow.
- ⚙️ Overheating can affect temperature sensors and cause error messages or automatic shutdowns.
- 💡 In some cases, the unit won’t restart until the safety thermostat has cooled down completely.
Recommendation: always stop the unit using the controller or mobile app. This triggers the purge mode, which safely cools the heating element before powering down the system.
5️⃣ Forgetting to reset the filter timer
Most heat recovery units come with a filter timer that tracks the operating time between filter changes. After replacing your filters, it’s important to reset the timer, otherwise the system will keep showing the “Replace filters” message even though new filters are installed.
- ⚙️ The timer is based on runtime, not on actual filter contamination.
- ⏳ If not reset, the controller may continue to display maintenance warnings unnecessarily.
How to reset the filter timer:
- In some units, simply turn the system off and on again to restart the timer.
- Others have a dedicated RESET TIMER button on the controller menu.
- Modern MVHR systems often allow you to reset it via the mobile app.
Once the timer has been reset, your unit will start a new filter cycle and notify you at the right time for the next replacement.
🧰 How to change your filters – short step-by-step guide
You can find a short step-by-step guide and video tutorial here 👉 How to change your ventilation unit filters
This guide shows you how to safely remove old filters, correctly install new F7 and G4 filters, and check the airflow direction to keep your MVHR system running efficiently.
Need help? If you’re unsure whether your F7 filter is fitted correctly, the airflow direction is right, or which side the G4 filter belongs on, feel free to contact us:
- 📩 Email: [email protected]
- 💬 Live chat on our website: cleanfilter.co.uk
The CleanFilter team is always ready to help you keep your heat recovery system clean, efficient and easy to maintain.