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Humidity, Foggy Windows and Mold – Causes and Solutions for Homes with a Ventilation Unit

date_range 03 November 2025
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🍂 During autumn and winter, many residents notice that windows fog up 💧, mold spots appear on walls 🦠, and humidity lingers indoors even though the AHU/MVHR is running. This is common, especially in November, when outdoor air is humid and we spend more time at home.

⚙️ There can be several causes, but the most common is insufficient airflow or a sub-optimal ventilation unit setup.

💨 1. Insufficient Airflow Indoors

If ventilation filters or AHU/MVHR filters are clogged, the airflow decreases, preventing proper removal of indoor humidity. As a result, windows start fogging up 💧, condensation builds up, and conditions become ideal for mold growth 🦠.

🔍 Most Common Causes:

🧩 Clogged filters in the ventilation unit

This accounts for about 80% of cases. When filters get dirty, the ventilation unit can no longer maintain proper airflow, and humidity remains indoors. ✅ Solution: increase the airflow (fan speed) and replace the filters. Even in new homes, filters may already be contaminated if ducts were left open during installation or renovation.

⚙️ Other components may be dirty

If the filters have already been replaced, it’s worth checking:

  • 🌬️ outdoor air grilles
  • 🔁 heat exchanger – inspection and cleaning if needed (once per year)
  • 📦 air ducts
  • 🎯 diffusers
  • 🧱 additional pre-filters or exhaust filters

⚖️ Filters that are too efficient or an undersized ventilation unit

In some cases, installers choose a ventilation unit with too low capacity, and when higher-class filters (e.g., F7 or F9) are installed, airflow drops even further. ✅ Solution: temporarily use a lower-class filter (e.g., M5) or increase the fan speed if the ventilation unit allows it.

A clogged filter – the main cause of reduced airflow
The heat exchanger can also become clogged over time

📚 Read more about ventilation unit (AHU/MVHR) maintenance here: https://cleanfilter.co.uk/blog/post/is-your-ventilation-unit-ready-for-the-heating-season

🔗 Read the article

🏠 2. New House and Enthalpy Heat Exchanger

If you’ve recently moved into a new home, increased humidity 💧 can be a natural process — during construction, the building materials absorb a large amount of moisture that evaporates over several months. Enthalpy or sorption heat exchangers return part of this moisture back into the rooms, so the relative humidity can reach as high as 80–85%.

✅ Solutions:

  • 💨 Increase ventilation temporarily — raise the ventilation unit (AHU/MVHR) speed and enable the “by-pass” function if available.
  • 🪟 Open windows slightly from time to time for natural air exchange.
  • ⏳ After 3–6 months, the situation usually stabilizes naturally.

💎 Why Choose CleanFilter Ventilation Unit Filters?

CleanFilter offers high-quality filters for AHU/MVHR systems, compatible with most ventilation units used across Europe. They help maintain optimal airflow 💨 and clean indoor air 🌿.

✅ Why Choose CleanFilter:

  • 🇩🇪 Certified German materials compliant with the VDI 6022 hygiene standard, free from PFAS compounds.
  • 📊 Proven efficiency based on testing, not just theoretical data.
  • 🧱 High sealing quality – the filter prevents leakage of contaminants.
Flexit Nordic S2 S3 filter F7

FLEXIT Nordic S2/S3 F7+F7 Filter set (Efficient)

Price £12.96

Ex VAT: £10.80

Manufacturer of MVHRFlexit
ManufacturerCleanFilter
Air handling unit
  • FLEXIT Nordic S2
  • FLEXIT Nordic S3
Filter dimensions166x272x48
Class EN779F7+F7
Class ISO16890ePM1 70%
Protection levelHigh: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters2
Advantages
  • Top filtering efficiency: ePM1 70%. Higher protection from smog particles, than using ePM1 50% or ePM1 55% filters.
  • Made in the EU.
  • Premium class certified synthetic filtering material (SANDLER AG, Germany)
  • Tested according to ISO16890 at the accredited laboratory in Germany.
Manufacturer product number
  • 116597
  • 116597
Zehnder ComfoAir Q 350/450/600 G4+G4 Filter set
-5%
  • -5%

Zehnder ComfoAir Q 350/450/600 G4+G4 Filter set (Standard)

Regular price £12.76 Price £12.12

Ex VAT: £10.10

Manufacturer of MVHRZehnder
ManufacturerCleanFilter
Air handling unit
  • ComfoAir Q 450
  • ComfoAir Q 350
  • ComfoAir Q 600
Filter dimensions500x159x22
Class EN779G4+G4
Class ISO16890Coarse 75%
Protection levelBasic: larger solid particles, dust, fluff.
Quantity of filters2
Advantages
  • Low airflow resistance: longer servicing term, lower energy consumption.
  • Made in the EU – quality and safety.
  • German filtration material – hygienic (VDI 6022) and PFAS-free for cleaner, safer air.
  • ISO 16890 tested – proven efficiency in accredited German lab.
  • Premium analog filters: tested efficiency, certified quality, and German filtering materials.
Manufacturer product number
  • 400502014
  • 400502012
Brink Flair 325/400 1xF7...

Brink Flair 325/400 1xF7 Filter (Smog&Pollen)

Price £20.91

Ex VAT: £17.43

Manufacturer of MVHRBrink
ManufacturerCleanFilter
Air handling unit
  • Brink Flair 325
  • Brink Flair 400
Filter dimensions245x510x22
Class EN779F7
Class ISO16890ePM1 70%
Protection levelHigh: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters1
Advantages
  • Top filtering efficiency: ePM1 70%. Higher protection from smog particles, than using ePM1 50% or ePM1 55% filters.
  • Made in the EU.
  • Premium class certified synthetic filtering material (SANDLER AG, Germany)
  • Tested according to ISO16890 at the accredited laboratory in Germany.
Manufacturer product number
  • 532703
  • 532702
  • 580616
RemarksSupply air filter. Extract air filtration requires an additional G4/M5 filter, available in our shop.
Domekt R 450 V 1xF7/AC Aktyvintos anglies filtras (kvapų)
-8%
  • -8%

Domekt R 450 V 1xF7/AC Activated carbon filter (Odor)

Regular price £27.32 Price £25.14

Ex VAT: £20.95

Manufacturer of MVHRKomfovent
ManufacturerCleanFilter
Air handling unitDomekt R 450 V (Rego 450 VE)
Filter dimensions470x240x46
Class EN779F7/AC
Class ISO16890ePM1 70%
Protection levelParticles and odors: fine particles, dust, pollen, mold spores, combustion particles, volatile particles, NOx, O3 gases.
Quantity of filters1
Advantages
  • Made in the EU – quality and safety.
  • German filtration material – hygienic (VDI 6022) and PFAS-free for cleaner, safer air.
  • ISO 16890 tested – proven efficiency in accredited German lab.
  • Absorbs volatile particles (odors) and harmful gases as NOx, O3, etc.
Remarks
  • ⚠️ Not suitable for C6M models (from 10/2020). Please check model or filter dimensions before ordering.
  • 🌟 High-quality replacement filter: made from certified materials (Germany, SANDLER AG) with tested performance. Provides reliable air filtration and optimal service life.
NIBE ERS 10-400 / ERS 10-500 F7+G4 Filter set (Efficient)

NIBE ERS 10-400 / ERS 10-500 F7+G4 Filter set (Efficient)

Price £16.40

Ex VAT: £13.67

Manufacturer of MVHRNIBE
ManufacturerCleanFilter
Air handling unit
  • NIBE ERS 10-400
  • NIBE ERS 10-500
Filter dimensions248x226x48
Class EN779F7+G4
Class ISO16890ePM1 70%
Protection levelHigh: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters2
Advantages
  • Top filtering efficiency: ePM1 70%. Higher protection from smog particles, than using ePM1 50% or ePM1 55% filters.
  • Made in the EU.
  • Premium class certified synthetic filtering material (SANDLER AG, Germany)
  • Tested according to ISO16890 at the accredited laboratory in Germany.
Paul Novus 300/450 F7+G4...

Paul Novus 300/450 F7+G4 Filter set (Efficient)

Price £20.19

Ex VAT: £16.82

Manufacturer of MVHRPaul
ManufacturerCleanFilter
Air handling unit
  • Paul Novus 300
  • Paul Novus 450
Filter dimensions180x475x50
Class EN779F7+G4
Class ISO16890ePM1 70%
Protection levelHigh: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters2
Advantages
  • Top filtering efficiency: ePM1 70%. Higher protection from smog particles, than using ePM1 50% or ePM1 55% filters.
  • Made in the EU.
  • Premium class certified synthetic filtering material (SANDLER AG, Germany)
  • Tested according to ISO16890 at the accredited laboratory in Germany.
Manufacturer product number527003440
SAVE VSR 300 (PF) F7+M5 filtrų komplektas (Efektyvus)

SAVE VSR 300 (PF) F7+M5 Filter set (Efficient)

Price £21.90

Ex VAT: £18.25

Manufacturer of MVHRSystemair
ManufacturerCleanFilter
Air handling unitSAVE VSR 300 (PF)
Filter dimensions
  • 397x277x24
  • 398x210x24
Class EN779F7+M5
Class ISO16890ePM1 70%
Protection levelHigh: fine particles, dust, pollen, mold spores, combustion particles, smog particles, insecticide dust.
Quantity of filters2
Advantages
  • Top filtering efficiency: ePM1 70%. Higher protection from smog particles, than using ePM1 50% or ePM1 55% filters.
  • Made in the EU.
  • Premium class certified synthetic filtering material (SANDLER AG, Germany)
  • Tested according to ISO16890 at the accredited laboratory in Germany.
Manufacturer product number25211
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🧾 Summary

If your ventilation unit (AHU/MVHR) is running but windows are fogging up 💧 and you feel humidity indoors, start by checking the filters. In most cases, a simple filter replacement and airflow adjustment ⚙️ solve the problem within a few days.

🌿 Properly selected and maintained ventilation filters ensure not only clean air, but also reliable protection against mold 🦠 and excess moisture in your home.

Tags: ahu filters foggy windows home humidity indoor air quality mold indoors mvhr filters reduce humidity at home ventilation system maintenance ventilation unit filters |
Categories: Indoor air quality