In refurbished buildings, new windows and airtightness improvements can reduce unintended infiltration. External insulation alone does not automatically make a home airtight. When natural air renewal decreases, it is necessary to establish how to provide what people and the building need.
Does that black filter really prove that mechanical ventilation is needed?
Striking images circulate on social media: a new filter beside one blackened after months of use. The message suggests that without that filter we would have breathed in everything it collected. The photograph, however, does not measure people's exposure.
A dirty filter shows that it has trapped particles as air passed through. The supply filter, which treats outdoor air, must be distinguished from the extract filter, which also collects dust and fibres generated indoors. Colour alone does not identify the nature or danger of the material.
The photograph rightly highlights the importance of maintenance; on its own, it does not prove how poor the indoor air was or what system was needed. Ordinary particle filters also do not remove CO₂ or all gaseous pollutants.
What mechanical ventilation does
The main function of mechanical ventilation is to provide controlled air renewal. Filtration is an additional function whose effectiveness depends on the filters and the system. Systems can be centralised or decentralised; not all solutions include a heat exchanger.
In a dual-flow mechanical ventilation system with heat recovery, outgoing air transfers some of its energy to incoming air through a heat exchanger. The diagram below illustrates the winter operating principle of a heat exchanger with separate air streams; it is not a detailed installation design.
Opening the windows is possible
A home with mechanical ventilation is still a home where windows can be opened. With a correctly designed and adjusted system, routine air renewal can also be provided without relying entirely on occupants' habits.
Opening windows can renew the air quickly, but the effect depends on wind, temperatures, openings and duration. Heat is lost in winter; in summer, opening windows can help during cooler hours or bring in heat and humidity. Outdoor air quality also matters.
Recovering heat does not automatically mean saving money
The heat exchanger's rated efficiency describes performance under defined conditions. The annual balance must include fan electricity, pressure losses, any frost protection or preheating, maintenance and filter replacement. Initial investment, climate and heating system must also be considered.
A fair comparison requires the same air renewal rate and guaranteed indoor conditions. If ventilation was previously insufficient, a system may improve the air without reducing the bill: comparing the new service with inadequate ventilation would distort the assessment of efficiency.
In a cold climate, the amount of recoverable heat can be significant. Financial viability must nevertheless be calculated for the specific case, including actual airflow rates, controls and building use.
Analysis, design and performance verification
Temperature, relative humidity and CO₂ help describe how spaces are used. CO₂ is useful for assessing ventilation relative to occupancy when considered alongside outdoor concentration, number of people, activities and length of stay. It is neither a complete indicator of air quality nor a universal health traffic light. [1]
Monitoring can be extended to PM2.5 and PM10 and, for specific purposes, to volatile organic compounds. Low-cost sensors have limitations: an aggregate VOC reading does not identify individual substances. Issues such as radon require dedicated instruments and protocols.
We collect data during representative periods: nights with occupied bedrooms, showers, cooking and window opening. The positioning and reliability of instruments matter. A few days may reveal anomalies, but do not necessarily represent all seasons. Monitoring supports design; it does not replace the requirements applicable to the specific work.
What problem do we need to solve?
Insufficient ventilation, indoor pollution sources, outdoor noise, humidity and consumption call for different assessments. Mould and condensation also need investigation: water ingress, thermal bridges and low surface temperatures may all contribute. Ventilation can help, but it does not repair a leak.
Strategies include reducing sources, local extraction and general air renewal, with filtration where useful. Natural, hybrid and mechanical solutions can be compared on the basis of achievable performance. A draughty building does not automatically guarantee adequate ventilation in every room and in every season. [2]
When assessing mechanical ventilation, air distribution, measured airflow rates, bedroom noise, maintenance access and controls matter. A good system requires verified installation and proper management over time.
The NOAH principle
In existing buildings, measurements and site visits help identify problems. In new buildings and refurbishments, air renewal should be planned from the design stage, considering occupancy, building characteristics and applicable requirements. There is no need to wait for monitoring to reveal poor air quality before designing adequate ventilation.
Mechanical ventilation is a useful tool when it meets a real need and is well designed and managed. Energy efficiency, comfort and air quality must be assessed together.
In both cases, checking performance during use helps establish whether the solution really works.
Understand the building. Design air renewal. Verify performance.