Question 1

What are the Part F ventilation systems?

Accepted Answer

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.

Question 2

Is natural ventilation enough for modern homes?

Accepted Answer

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.

Question 3

How much does mechanical ventilation cost to run?

Accepted Answer

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.

Question 4

Which system provides better indoor air quality?

Accepted Answer

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.

Question 5

Is mechanical ventilation noisy?

Accepted Answer

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.

Feature	Natural (System 1/2)	Mechanical (System 3/4)
Install cost	£200-500	£1,000-5,000
Running cost	None (but higher heating)	£20-60/year electric
Heat recovery	None	Up to 95% (MVHR)
Air quality consistency	Variable — weather dependent	Consistent year-round
Filtration	None	Pollen and dust filters
Maintenance	Minimal	Filter changes, annual service
Noise ingress	Through trickle vents	Minimal — sealed building
Ideal airtightness	Above 5 m³/h/m²	Below 5 m³/h/m²

Natural vs Mechanical Ventilation — UK

Side-by-Side Comparison

When Natural Ventilation Works

When Mechanical Ventilation Is Better

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