Mechanical Extract Ventilation

Mechanical Extract Ventilation Explained

Mechanical Extract Ventilation (Part F), historically known as a System 2 Ventilation Strategy,  is a building ventilation system commonly used in the UK to remove stale air and control indoor air quality. It relies on mechanical fans to extract air from specific areas, like kitchens and bathrooms, and is an essential component for maintaining a healthy and comfortable indoor environment.

Mechanical extract ventilation is a ventilation system designed to remove stale or polluted air from specific areas within a building and is often governed by Part F of the UK building regulations. It can be categorized into two main types: centralized and decentralized systems.

Centralised Mechanical Extract Ventilation (CMEV)

Centralised System: In a centralized mechanical extract ventilation system, a single central fan or ventilation unit is responsible for extracting air from multiple rooms or areas in a building. The extracted air is then typically exhausted outside through ductwork or a common ventilation system. This type of system is often used in larger buildings and is more complex to design and install but can be more energy-efficient and cost-effective in the long run.

Decentralised Mechanical Extract Ventilation (DMEV)

Decentralised System: Decentralized mechanical extract ventilation systems consist of individual fans or units in each room or area that require ventilation. These units are responsible for extracting air from their specific location and expelling it directly to the outside, without the need for extensive ductwork or a central fan. Decentralized systems are often simpler to install and offer more room-level control, but they may require more maintenance and energy usage as each unit operates independently.

The choice of ventilation strategy, whether centralized or decentralized mechanical extract ventilation, should be based on the specific requirements of the building and its intended use. Factors such as the building’s size, layout, and occupancy should guide this decision, ensuring that the chosen strategy is appropriate for providing effective air quality and humidity control.

Air-Flow Testing and Reporting

Testing and commissioning are crucial for both centralized and decentralized systems. Regular testing and commissioning procedures are necessary to verify that the ventilation system is operating correctly and efficiently. This includes checking the performance of fans, airflow rates, and the control of air quality, ensuring that the system complies with regulations and standards.

Building control systems play a significant role in managing mechanical extract ventilation. They enable automation and remote control of the ventilation system, allowing it to adapt to changing occupancy and environmental conditions. Integrating the ventilation system with the building control system can enhance energy efficiency and occupant comfort, as it can respond to real-time data and adjust ventilation rates accordingly

Mechanical Extract Demand Controls – Purge Ventilation

Incorporating humidity sensors into mechanical extract ventilation systems can be highly beneficial. These sensors help monitor and control indoor humidity levels, which is especially important in areas prone to high moisture levels, such as kitchens and bathrooms. By maintaining optimal humidity, these sensors contribute to occupant comfort and reduce the risk of mould and condensation issues.

MEV Ongoing Maintenance

Maintenance is a critical aspect of both types of mechanical extract ventilation systems. Regular maintenance, including cleaning and servicing of fans, ducts, and filters, is essential to ensure that the system operates efficiently and maintains indoor air quality. Neglecting maintenance can lead to decreased performance and increased energy consumption.


In summary, the appropriateness of the ventilation strategy, testing and commissioning, humidity sensors, maintenance, and building control are all essential considerations when implementing mechanical extract ventilation systems. Proper planning, installation, and ongoing management of these systems are crucial for ensuring indoor air quality, occupant comfort, and compliance with building regulations.