Natural Ventilation with Background Ventilators & Intermittent Extract Fans (System 1)

A hybrid ventilation strategy combining passive background ventilators (e.g., trickle vents in windows or walls) with intermittent mechanical extract fans in high-moisture areas (e.g., kitchens, bathrooms). This system provides continuous low-level airflow (background ventilation) and targeted extraction during pollutant-generating activities (e.g., cooking, showering).

Key Components

1. Background Ventilators
   • Small, adjustable vents installed in windows, walls, or eaves to allow passive airflow.
   • Typically sized to meet minimum airflow requirements per room (e.g., 2500–5000 mm² equivalent area).
   • Example: Trickle vents in retrofit double-glazed windows to comply with Part F without compromising thermal performance.
2. Intermittent Extract Fans
   • Mechanically driven fans activated manually or via humidity sensors in wet rooms.
   • Minimum extract rates: 15 l/s for bathrooms, 30 l/s for kitchens.
   • Example: A humidistat-controlled fan in a social housing retrofit to reduce condensation and mould risk.

Real-World Applications

• New Builds: In a two-bedroom house, habitable rooms (e.g., bedrooms, living rooms) require background ventilators, while extract fans are installed in kitchens and bathrooms.
• Retrofits: Upgrading 1970s terraced housing by adding trickle vents to windows and replacing outdated fans with low-noise, energy-efficient models to meet modern standards.

Regulatory Compliance (UK)

• Approved Document F: System 1 is the baseline solution for dwellings, ensuring minimum airflow rates and moisture control.
• Energy Efficiency: Background ventilators must be closable to prevent heat loss, aligning with Part L (conservation of fuel and power)

Advantages & Challenges