Continuous Mechanical Extract Ventilation (cMEV), also known as a System 3 ventilation system, is a type of mechanical ventilation that operates constantly to remove stale, moisture-laden air from a building. This system improves indoor air quality by continuously extracting air from "wet rooms"—areas like kitchens, bathrooms, utility rooms, and toilets where moisture and pollutants are generated.

The system comprises a central fan unit or individual extractor fans that are wired to operate at a low, continuous trickle speed. The fan unit is typically located in a loft or cupboard space, with ducting running to discreet extract grilles in the wet rooms. In an extension or renovation context, a cMEV system is an excellent choice for a new bathroom or kitchen. A single fan unit can be used to ventilate multiple wet rooms, with the benefit of a single penetration through the building's envelope to the outside. This is more efficient and aesthetically pleasing than multiple individual fans.

When higher ventilation rates are needed, for instance, during a shower or while cooking, the fan speed can be boosted manually via a switch (often integrated with the light switch) or automatically via humidity or motion sensors. This ensures that the system can handle peak moisture and pollutant generation effectively.

According to Approved Document F (ADF) of the Building Regulations, which covers the ventilation of buildings in England, a cMEV system must be designed and installed to meet specific minimum extract rates. For example, in a new build or a major renovation, the system must be able to achieve the higher of either the room-specific extract rate or a whole-building ventilation rate based on the number of bedrooms. The continuous operation of a cMEV system helps to prevent the build-up of moisture, which in turn reduces the risk of condensation, damp, and mould growth. This makes it a highly effective solution for maintaining a healthy and comfortable indoor environment.