![\"Marko](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/08\/MarkoKannisto-150x150.webp\")
\n\t\tWriter: Marko Kannisto, Product Manager<\/p>\n\t\t\t\t\t\t
Summary<\/h2>\n\n\n\n
Note to the reader:<\/em><\/strong> The figures used in this article are illustrative examples. In real-world applications, results vary depending on the building, its usage profile, the climate zone and the ventilation system. Assessing the annual efficiency of a ventilation unit often requires the use of default values, as conditions are not constant throughout the year. Nevertheless, these examples clearly illustrate the scale of the phenomenon and its mechanisms of action at a practical level.<\/em><\/p>\n\n\n\n An imbalance in air flows is one of the most common reasons for a decline in the actual annual efficiency of a ventilation unit \u2013 and a clogged fine filter is a good, concrete example of this. This article uses an example to illustrate how a dirty fine filter can, in practice, reduce heat recovery and increase heating energy consumption, and how MyVallox CFi units maintain balanced airflows throughout the entire operating cycle.<\/p>\n\n\n\n When discussing the energy efficiency of a ventilation unit, attention is often focused on the efficiency of the heat recovery unit. In actual use, however, what matters most is the annual efficiency of the entire system \u2013 and, in particular, whether the supply and extract air flows remain in balance throughout the year.<\/p>\n\n\n\n Balanced airflows are not just a technical detail. They have a direct impact on how much thermal energy is actually recovered in a building \u2013 and how much energy escapes unnoticed.<\/p>\n\n\n\n Let us first consider a situation in which the ventilation unit is operating as intended. Assumptions:<\/p>\n\n\n\n When the airflows are balanced, heat recovery operates at its best. The thermal energy recovered from the extract air is efficiently transferred to the supply air, and the high efficiency of the heat exchanger is also reflected in a good average annual efficiency figure throughout the year.<\/p>\n\n\n\n In actual use, the fine filter for the supply air will inevitably become dirty over time. This increases the pressure drop on the supply air side and leads to a reduction in the supply air flow, whilst the extract air flow often remains virtually unchanged. As a result, the airflows in the ventilation unit are no longer balanced.<\/p>\n\n\n\n The pressure drop caused by a new fine filter is typically around 20 Pa. When the filter becomes dirty, it is reasonable to estimate that the pressure drop caused by the filter will increase by around 25\u201330 per cent. In ventilation technology, the pressure drop is proportional to the square of the air flow rate; therefore, the air flow rate does not decrease in the same proportion as the pressure drop increases, but rather in proportion to the square root of the pressure drop.<\/p>\n\n\n\n When a fine filter becomes clogged, causing its pressure drop to increase by around 25\u201330 per cent, the air flow is calculated to decrease by around 10\u201312 per cent. In this example, the supply air flow rate therefore decreases from 65 l\/s to 58 l\/s. In other words: when the fine filter becomes clogged, the airflows are no longer balanced.<\/p>\n\n\n\n An example of a situation where the fine filter has become dirty as described above:<\/p>\n\n\n\n Even if the efficiency of the heat recovery cell itself were still 89 per cent, the system as a whole would no longer be able to make full use of it.<\/p>\n\n\n\n In heat recovery, the heat transfer can be described, in simple terms, by the following formula:<\/p>\n\n\n where:<\/p>\n\n\n The amount of heat energy transferred is always limited by the lower of the two air flow rates. When the supply air flow decreases, some of the thermal energy in the extract air is lost from the building without being utilised. In other words, even if the heat exchanger\u2019s efficiency were still 89 per cent, it would not be possible to recover the same amount of energy from the exhaust air as before.<\/p>\n\n\n\n In a balanced situation:<\/p>\n\n\n In the event of an imbalance (the fine filter is dirty):<\/p>\n\n\n The proportion of the thermal energy in the exhaust air flow that can be utilised is therefore:<\/p>\n\n\n In other words, only around 89 per cent of the thermal energy in the extract air flow can be transferred to the supply air.<\/p>\n\n\n\n A simplified expression for the change in annual efficiency can be written as:<\/p>\n\n\n In practice, the machine\u2019s actual annual efficiency therefore falls from 83 per cent to 74 per cent when the supply air flow decreases as a result of the fine filter becoming clogged.<\/p>\n\n\n\n Please note the following regarding the energy consumption of the fans:<\/em><\/strong> In practice, compensating for the clogging of the supply air fine filter requires increasing the speed of the supply fan, which increases the fan\u2019s electricity consumption to some extent. However, modern EC fans are extremely energy-efficient, and their additional energy consumption is typically very low compared with the loss of heating energy caused by imbalances in air flow. For this reason, the increased energy consumption of the fans has not been specifically taken into account in these calculations, and it has no significant impact on the overall result.<\/em><\/p>\n\n\n\n Using the example air flow rates, the heating energy requirement for ventilation can be calculated as 9,301 kWh per year. This has been calculated in accordance with the guidelines issued by the Ministry of the Environment, for climate zone I (Helsinki-Vantaa), test year TRY 2020.<\/p>\n\n\n\n Next, let\u2019s work out how much energy can be recovered from this using heat recovery.<\/p>\n\n\n\n Balanced airflows (annual efficiency 83 per cent):<\/p>\n\n\n\n 9 301 kWh \u00d7 0,83 = 7 720 kWh<\/p>\n\n\n\n Unbalanced airflows (annual efficiency 74 per cent):<\/p>\n\n\n\n 9 301 kWh \u00d7 0,74 = 6 883 kWh<\/p>\n\n\n\n The difference is 837 kWh per year.<\/p>\n\n\n\n As the filters do not remain dirty for the whole year, let us consider an average scenario in which the effect lasts, for example, for half the year:<\/p>\n\n\n Assuming the price of electricity is 0.20 \u20ac\/kWh, the cost is:<\/p>\n\n\n\n 420 kWh \u00d7 0,20 \u20ac = 84 \u20ac \/ year<\/p>\n\n\n\n This may not be a particularly significant additional cost over the course of a single year, but it is nonetheless a hidden cost that is worth bearing in mind. When considering the entire life cycle of the equipment, this already adds up to a significant cost. It is also worth noting that, in this case, the user has done nothing wrong. However, given the current high cost of electricity, it is worth being aware of this hidden cost as well.<\/p>\n\n\n\n Well, is there anything we could do to sort this out? Does the resident have to adjust the airflows at set intervals, and do so based on a \u2018gut feeling\u2019?<\/p>\n\n\n\n That is, of course, one possibility, but there is also a slightly more elegant alternative. Indeed, Vallox\u2019s new MyVallox CFi ventilation units are designed to operate energy-efficiently even under real-world operating conditions.<\/p>\n\n\n\n MyVallox CFi units:<\/p>\n\n\n\n In this case, the high annual efficiency is maintained in practice as well \u2013 not just under laboratory conditions.<\/p>\n\n\n\n Balanced airflows are a key prerequisite for good annual efficiency. Without intelligent control, a clogged fine filter can reduce energy efficiency by a surprisingly large amount and increase energy costs.<\/p>\n\n\n\n MyVallox CFi units offer a solution to this problem. They help to save energy, reduce heating costs and ensure that the ventilation system operates efficiently all year round.<\/p>\n\n\n\n The result is better indoor air quality, less wasted energy and greater savings for the user.<\/p>\n\n\n\n1. Framing the problem<\/h2>\n\n\n\n
Initial situation: airflows in balance<\/h3>\n\n\n\n
\n
A clogged fine filter \u2013 an unnoticed factor reducing energy efficiency<\/h3>\n\n\n\n
A reduction in airflow as the filter becomes dirty<\/h3>\n\n\n\n
\n
2. Method \/ principle<\/h2>\n\n\n\n
![\"Q](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/vuosihyotysuhde-1-300x14.png\"\n)
\n <\/div>\n\n\n\n![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/selitteet1-ENG-1-300x44.png\"\n)
\n <\/div>\n\n\n\nEstimation based on air flows<\/h3>\n\n\n\n
![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/Arviolaskelma1-ENG-300x32.png\"\n)
\n <\/div>\n\n\n\n![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/Arviolaskelma2-ENG-300x35.png\"\n)
\n <\/div>\n\n\n\n![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/arviolaskelma3-300x20.png\"\n)
\n <\/div>\n\n\n\nEffect on the machine\u2019s annual efficiency ratio<\/h3>\n\n\n\n
![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/vaikutus1-ENG-300x30.png\"\n)
\n <\/div>\n\n\n![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/vaikutus2-ENG-300x82.png\"\n)
\n <\/div>\n\n\n\n3. Application example<\/h2>\n\n\n\n
![\"\"\n](\"https:\/\/www.vallox.com\/app\/uploads\/2026\/06\/Keskiarvo_ENG-300x35.png\"\n)
\n <\/div>\n\n\n\n4. Design recommendation<\/h2>\n\n\n\n
\n
In conclusion: energy efficiency is no accident<\/h3>\n\n\n\n