In many German and Austrian homes, balanced ventilation with heat recovery is described as a KWL system (Kontrollierte Wohnraumlüftung) or Wärmerückgewinnungsanlage. In English, we often use the term HRU – Heat Recovery Unit. Whatever the name, one element is always the same: good filters are essential to protect both air quality and the heat exchanger.
When users search online, they often type phrases such as “KWL Filter”, “Filter Wärmerückgewinnungsanlage” or “HRU filter ePM1”. But what really matters when choosing a replacement filter for your system?
What HRU / KWL filters do
In a typical residential HRU/KWL system there are usually two main filtration stages:
- Extract side: a coarse or medium filter that helps protect the heat exchanger from dust, fibres and grease coming from the rooms.
- Supply side: a more efficient filter (e.g. ePM1 or ePM2,5 according to ISO 16890) that helps protect occupants from fine outdoor particles such as pollen, soot and traffic dust.
Clean, correctly selected filters contribute to:
- maintaining a stable airflow and avoiding unnecessary fan energy use,
- protecting the efficiency and lifespan of the heat recovery core,
- improving comfort for people who are sensitive to dust, pollen and outdoor pollution.
What to check before ordering a KWL / HRU filter
Before you order, it is worth checking a few simple but important details:
- Unit brand and model: often written on the data plate inside or on the casing of the unit.
- Filter size: width, height and thickness in mm. If in doubt, you can measure your current filter.
- Filter type: flat filter mat, bag filter, cassette filter or compact V-filter.
- Filter class: indicated according to ISO 16890 (ePM1, ePM2,5, ePM10, Coarse, etc.). Higher ePM1 values usually mean better fine dust protection.
Many online catalogues allow you to search by unit model or directly by dimensions, which is particularly useful when you are looking for a compatible replacement for your KWL system.
Keeping HRU / KWL filters in good condition
The lifetime of an HRU / KWL filter depends on several factors: outdoor air quality, the location of the building (for example, near a busy road or in a rural area), indoor dust levels and how many hours per day the unit runs.
Rather than following a fixed rule, it is generally recommended to:
- follow the maintenance instructions provided by the unit manufacturer,
- inspect filters regularly for visible dust build-up and discoloration,
- pay attention to possible warning signals such as reduced airflow, higher noise levels or system alarms.
If filters appear heavily loaded with dust or if the unit indicates that airflow is restricted, it is usually a sign that replacement should be planned. In some cases, monitoring pressure drop across the filter can provide an additional, more precise indication of when a filter has reached the end of its useful life.
Choosing the right HRU / KWL filter – with the correct dimensions and a suitable ePM class – and combining it with regular, documented maintenance is a simple but effective way to protect your heat recovery unit, improve indoor air quality and keep your ventilation system working efficiently over time.