Natural vs Mechanical Ventilation — UK

Approved Document F defines four systems: System 1 — intermittent extract fans with background ventilators (trickle vents); System 2 — passive stack ventilation; System 3 — continuous mechanical extract (MEV); System 4 — continuous supply and extract with heat recovery (MVHR). Systems 1 and 2 are natural/hybrid, while 3 and 4 are fully mechanical.

For most existing homes with reasonable air permeability (above 5 m³/h/m²), natural ventilation with trickle vents and extract fans (System 1) is adequate and Part F compliant. However, modern airtight new builds (below 5 m³/h/m²) increasingly need mechanical ventilation (System 3 or 4) to maintain adequate air quality without excessive heat loss.

A continuous MEV system (System 3) costs approximately £20-40/year in electricity. An MVHR system (System 4) costs £30-60/year to run but recovers £200-400/year in heating energy. Natural ventilation has no direct running cost but increases heating bills by £100-300/year due to uncontrolled heat loss through vents and extract fans.

Mechanical ventilation provides more consistent air quality because it operates continuously regardless of wind conditions or occupant behaviour. MVHR with filters also removes pollen, dust and particulates. Natural ventilation depends on wind pressure and occupants remembering to open windows, which often results in under-ventilation in winter and over-ventilation in summer.

Modern MEV and MVHR systems are designed to be very quiet. A well-installed MVHR unit produces 25-35 dB(A) at the unit and is inaudible in rooms when ductwork is properly attenuated. By contrast, intermittent extract fans can be 35-45 dB(A) and are often perceived as more disruptive because they switch on and off. Trickle vents allow external noise ingress, which can be an issue near busy roads.