Back to Knowledge Hub

cMEV Systems: Continuous Mechanical Extract Ventilation

Approx. 6 min read

Introduction

Continuous Mechanical Extract Ventilation (cMEV) systems provide a reliable, centralised method for removing stale air and moisture from wet rooms throughout a property. Unlike decentralized MEV (dMEV), cMEV uses a single fan unit connected to a duct network, delivering constant low-level extraction that ensures balanced ventilation and improved indoor air quality.

cMEV is widely used in both new builds and retrofit projects where discreet, centralised ventilation is preferred.

How cMEV Works

A cMEV system consists of a central fan unit, usually located in a loft, cupboard, or utility room. Extract ducts connect the fan to wet rooms — typically kitchens, bathrooms, en-suites, and utility rooms — drawing out moisture and pollutants. The system operates continuously at a low extraction rate, with a boost function for peak demand.

Key operational principles:

  • Continuous background extraction maintains consistent airflow.
  • Wet rooms are ventilated directly via dedicated ducts.
  • Boost functionality can be manual or humidity-controlled.

System Components

  • Central fan unit – high-efficiency, low-noise, with built-in controls.
  • Ductwork – rigid or semi-rigid ducts connecting wet rooms to the fan.
  • Boost controls – switched live or humidity-triggered boost operation.
  • External terminal – exhausts air to the outside safely.
  • Filters – protect the fan unit and maintain air quality.

Benefits of cMEV Systems

  • Balanced ventilation – constant extraction prevents condensation and mould.
  • Energy-efficient – modern fans with low power consumption.
  • Low maintenance – single fan reduces servicing complexity compared to multiple units.
  • Discreet installation – ducting hidden within ceilings or loft spaces.
  • Noise control – central fan location minimises audible disturbance in living spaces.

Design and Installation Considerations

When specifying cMEV, consider:

  • Extraction rates – calculated per wet room, typically 15–30 L/s for bathrooms, 30–60 L/s for kitchens.
  • Duct layout – minimise bends and pressure loss; smooth, rigid ducts recommended.
  • Noise attenuation – correct fan sizing and acoustic ducting ensure compliance with Part F.
  • Access for maintenance – central fan should be easily serviceable.
  • Control strategy – consider humidity sensors or timers for efficient boost operation.

Compliance and Standards

cMEV systems must comply with:

  • Building Regulations Part F – continuous extraction rates, wet room ventilation, and boost operation.
  • Approved Document F (2021) – ensures indoor air quality and ventilation effectiveness.
  • BS 5925:1991 & BS EN 13141-6 – guidance on mechanical ventilation performance and testing.

Applications

cMEV is suitable for:

  • New build homes where centralised ventilation is preferred.
  • Retrofit projects requiring discreet, efficient wet room extraction.
  • Homes with multiple wet rooms needing coordinated airflow.

Flexivent Expertise

Flexivent designs, supplies, and installs cMEV systems that meet current Building Regulations and Part F compliance. Our engineers ensure optimised extraction, low-noise performance, and long-term reliability. Contact Flexivent to discuss centralised mechanical extract solutions for your property.