MVHR: Displacement vs. Mixing Ventilation – Why "High Vents" Isn't Always the Best Choice

In the world of Mechanical Ventilation with Heat Recovery (MVHR), one of the most common setups involves installing both supply and extract vents at ceiling level. It's a simple solution from an installation perspective, but from a physics and Indoor Air Quality (IAQ) standpoint, it doesn't always represent the optimal choice.

In fact, a more efficient approach exists, based on precise thermodynamic principles: displacement ventilation. Understanding the difference between the two strategies is crucial for anyone seeking the best performance from their system.

Ventilation by Dilution (or Mixing)

This is the most common strategy. Fresh air is supplied from above (at the ceiling or high on the wall) at a certain velocity. The air jet mixes with the existing air in the room, diluting the concentration of pollutants (CO₂, VOCs, humidity). The air is then also extracted from above, typically in service rooms. It works, but it's not the most efficient way to "clean" the air we breathe.

Displacement Ventilation

This qualitatively superior approach is based on a different logic: not mixing, but displacing.

• Low-Level Supply: Fresh, slightly cooler air is supplied at low velocity from vents located at a low level (in the floor or low on the wall). Being denser, it spreads across the floor, creating a "lake" of fresh, clean air in the occupied zone.
• Convective Plumes: Heat sources (people, computers, appliances) create "thermal plumes": the air in contact with them heats up, becomes lighter, and rises, carrying pollutants produced at the source along with it.
• High-Level Extract: The stale, warm, and pollutant-laden air stratifies at the top of the room and is extracted through vents positioned at the ceiling.

The result is a targeted removal of pollutants, which are drawn away from the breathing zone before they can spread. The air quality in the occupied zone is significantly higher compared to a mixing system.

A Design Choice, Not an Afterthought

Choosing between displacement and mixing is not an installation detail but an engineering decision that directly impacts comfort and health. Displacement ventilation represents the state of the art for those who desire a genuinely healthy home environment.

This attention to the physics of airflow is the same we apply in all our energy analyses. For a consultation that integrates efficiency and well-being, contact us.