Continuous Mechanical Extract Ventilation (cMEV), (formerly known as System 3), is a mechanical ventilation system designed to continuously remove stale air from the interior of a building. It operates at a low level to extract indoor air, thereby reducing humidity and pollutants, particularly in wet rooms such as bathrooms and kitchens.
cMEV systems are integral to maintaining indoor air quality and preventing issues such as condensation, mould growth, and the accumulation of indoor pollutants. They are particularly effective in residential settings where airtightness is relatively poor, as they rely on passive air infiltration to replace the extracted air.
Key Features:
- Continuous Operation: Unlike intermittent extract fans, cMEV systems run continuously, ensuring consistent air quality.
- Low Energy Consumption: Designed to operate efficiently with minimal energy use.
- Wet Room Focus: Commonly installed in areas with high moisture generation, such as bathrooms and kitchens.
Building Regulations:
cMEV systems are governed by Part F of the Building Regulations (Ventilation), which outlines the requirements for mechanical ventilation in residential buildings. The latest revision of Approved Document F (2021) specifies that cMEV systems must provide a minimum extract rate to ensure adequate ventilation.
Practical Example:
In a UK residential retrofit project, a cMEV system might be installed in a bathroom to address persistent dampness. The system continuously extracts humid air, reducing the risk of mould growth and improving overall air quality.
Related Terms:
- Mechanical Ventilation with Heat Recovery (MVHR): A system that extracts stale air while recovering heat to warm incoming fresh air.
- Passive Ventilation: Natural airflow through windows, vents, or other openings without mechanical assistance.
- Intermittent Extract Fans: Fans that operate only when needed, typically triggered by humidity or user activation.
- Air Tightness: The measure of how well a building prevents uncontrolled air leakage.
- Indoor Air Quality (IAQ): The quality of air within a building, particularly in relation to occupant health and comfort.
- Condensation Risk: The likelihood of moisture condensing on surfaces due to high humidity levels.
- Building Regulations Part L: Focuses on the conservation of fuel and power, often intersecting with ventilation requirements.
Last Updated on 21 August 2025 by Bradley Pallister