When There's Too Much Fresh Air – Why Demand-Controlled Ventilation Matters

Because modern residential buildings are so airtight, mechanical ventilation is now essential to prevent mold growth and maintain healthy indoor CO₂ levels. The problem starts when a system is run on the “more fresh air is always better” principle, operating continuously at a high, unregulated airflow rate throughout the winter months. Good ventilation isn’t about maximum airflow — it’s about delivering exactly the amount that’s needed.

The physical cause of winter air dryness

In winter, the absolute moisture content of cold outdoor air — the actual mass of water vapor it carries — is extremely low. When that air is brought indoors and heated to 22 °C, its relative humidity drops drastically.

If the volume of dry air introduced (we often see unjustifiably high, continuous rates of 120–200 m³/h) significantly exceeds what the occupants’ own moisture production (cooking, laundry, simply being present) can replenish, indoor humidity is bound to collapse.

The risks of persistently low relative humidity (below 20–25%) include reduced comfort (dry throat, burning and red eyes, dry skin, static buildup) and a genuine health risk: while dry air itself carries no infection, dried-out mucous membranes lose their natural defenses, which significantly raises the risk of respiratory irritation and viral infection — especially in children.

The proper engineering solution

Too little fresh air is harmful too, causing fatigue, elevated CO₂ levels and headaches. The solution is demand-controlled ventilation: automatic control based on CO₂ and humidity levels, which only ramps up the ventilation unit’s speed when deteriorating air quality actually calls for it — combined with an enthalpy heat exchanger, a special type of heat exchanger that recovers not just heat but a significant share of the moisture from outgoing air, returning it to the living space.