Mechanical ventilation with heat recovery (MVHR) incorporate components that are subject to natural wear and should be replaced at regular intervals. Otherwise, not only is the heat exchange insufficiently effective, but the system fails to perform its function. Filters belong to the group of such components. They work in two sections of the heat recovery units, HRUs: the supply fan and the exhaust fan.
The air filters in the HRUs have two functions. Firstly, the filters clean the air supplied to the room from contamination before it is fed into the house. This way, clean air without dust enters buildings. With finer filters installed, bacteria and viruses that are harmful to our health are not transported with the air into the room. Secondly, filters’ removing of dirt protects other components of the HRUs like the heat exchanger.
Therefore the filters in HRUs both extend the life and efficiency of the entire system by protecting the components from dirt, and provide people indoors fresh and clean air to breathe by purifying the incoming air of bacteria, viruses, dust, mites, spores, pollen and other contaminants.
Filters play a major role in the efficiency of HRUs’ operation and the efficiency and effectiveness of heat recovery. It is also their quality that impacts power consumption during fan's operation. Dirty filters limit the airflow, which increases the intensity of the air handling unit operation, resulting also in an uneven airflow.
Extra consideration needs to be given to an increase in the Specific Power Input factor [SPI]. The more contaminated the filter, the less effective the air purification, but also the more intensive the system operation, and consequently higher power intake and energy consumption, which translates into higher operating costs and higher electricity bills.
This depends on several matters, including the season and weather conditions. The HRU manufacturers’ recommendations as to the frequency of filter replacement usually vary within the range of at least once every 3-4 months. In fact, it vastly depends on the air pollution level in a place of residence. If you live in a place where the air quality is very low, it is possible that you should replace the filters even every month.
All marketed HRUs must be supplied with a “visual warning device for filter replacement”, i.e. a kind of visual alarm, usually in the form of a LED on the HRU or on the controller, which informs the user about the need to check the level of contamination of filters and a possible need to replace them with new ones. Such alarms in HRUs are usually timed, which means that the manufacturer presets the time after which the information “dirty filters” is to be displayed for the user. The best way to ensure your filters are always properly maintained is just to check them regularly and replace them with a new set of filters if necessary.
Typically, dust filters for general ventilation are used for heat recovery units. Two types of filters used to be installed in the past. A filter of G4 grade, as a pre-filter, should retain dust sizes of 1-10 µm > 90% of its mass (average arrestance Am of the test dust), and its average efficiency for finer particles is defined as Em < 40%. The F7 fine filter, on the other hand, should have an average efficiency for particles of 0,4 µm within 80 ≤ Em < 90%. In HRUs, G4 class filters are usually used. They can retain most of the larger contaminants. However, if you need increased protection of your health or if you live in an area particularly exposed to smog pollution, replacing them with EU7 (F7) class filters is recommended.
We wrote “in the past” because the new ISO 16890 standard has been in force since 2016, but since G4 and F7 grades were in use for many years, this outdated nomenclature can still be found in the manufacturers' data sheets.
According to the new ISO 16890 standard “Air filters for general ventilation”, filters with a filtration efficiency of about 60-70%, called coarse dust filters, are nowadays a “market standard”. The new standard defines the way of filtration by specifying size of dust particles filtered and the filtration efficiency expressed as a percentage. For example, to build the HRU-PremAIR unit we used a coarse filter classified as ISO coarse 70%, which means, a 70% efficiency filter. ISO ePM1 55% can be optionally used as a finer filter.
It should be noted that the measurement methods used in the two standards are different and therefore cannot be directly compared. The 779 standard used 0.4 µm particles, while ISO 16890 used 3 fractions: 1 µm, 2.5 µm and 10 µm. In the new standard the filters also undergo an “artificial aging” process to better determine the effectiveness for the filter throughout its life cycle, and not just for a new, clean filter. Moreover, the ISO standard allows for a more precise selection of filters, depending on the outside air quality and the designed indoor air quality.
Why aren't higher efficiency filters such as HEPA used? First of all, to ensure that the filter upkeep, namely replacing them about four times a year, is not too expensive for the user. And secondly, you don’t want filters to generate too much air resistance in the HRU, as a change in filters in such a small device as a HRU significantly affects the characteristics of the device. A filter with a higher filtration class can create very high resistance for the HRU, so its designer would have to design stronger fans, which in turn means a larger HRU, and thus a more expensive product for the customer.
One of the main, if not the primary, tasks of mechanical ventilation is to provide fresh, filtered air. Not only is the heat recovery system responsible for the ventilation of the building, but also it is an effective way to protect you against smog. You are exposed to smog when you are both outdoors and indoors. The undesirable effects of frequent airing of flats are mentioned more often especially in winter. The heat recovery system ensures that there is an adequate level of humidity in the apartment and that the air you breathe is both fresh and clean. However, if the intake air is very polluted and there is a lot of smog in the area you live in, basic G4 filters lose the fight against smog.
To protect your home from smog you will need filters with a higher filtration class. So apart from air filters installed in the device as standard, you will have to have an additional anti-smog filter installed on the intake duct of the MVHR system. It is essential that the decision to install an additional filter box is made at the design stage of the HRU system. Why? Because as with any filter, this high efficiency filtering box will generate high resistance for the unit. So when designing the system and selecting your HRU, you have to calculate the compression ratio drops in the whole system and ensure that your HRU will “handle” the resistances created by the additional anti-smog filter.
Pressure drop charts for the MOCArz-CA series filters
Our company also has such filters – the MOCarz series– and, admittedly, their resistance values are high. But we can take those technical data into account at our design stage and select the right product. If our HRU cannot cope with such a fabric filter, we can turn to more advanced solutions like installing an electrostatic filter which generates the lowest resistance, namely ca 10 Pa. 
For more details about electrostatic filters click here.
An electrostatic filter is, in a nutshell, a filter with a filtration efficiency of as high as 95% of ePM1 and ePM2.5 particles, the main components of the smog. The performance of such a filter is also much better due to it being reusable, as you don't replace it every few months, but just wash it with water instead. What about its downside? Compared to fabric filters, its price is much higher. But as for the operation costs, the payback period of an electrostatic filter when compared to a standard one, depending on the air pollution levels, is 3-5 years.
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Electrostatic precipitator e-MOCarz |
Filter box, duct-mounted MOCarz |
Filter box, with active carbon MOCarz-CA |
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