How to Design Ventilation Systems for Dwellings

1. Start Here: Determine Your Dwelling’s Airtightness

Every ventilation system design begins with one critical question: Is your dwelling designed to be highly airtight?

• Highly airtight = Design air permeability <5 m³/(h·m²) at 50Pa or as-built air permeability <3 m³/(h·m²).
• Less airtight = All other cases.

This decision determines whether you’ll use natural ventilation or mechanical systems. Let’s explore both paths.

2. Path A: Natural Ventilation (For Less Airtight Dwellings)

Step 2.1 – Install Intermittent Extract Fans in Wet Rooms

• Kitchens:
  • With cooker hood extracting externally: Minimum 30 l/s extract rate.
  • Without external cooker hood: 60 l/s extract rate (Diagram 1.2).
• Bathrooms/Utility Rooms: 15 l/s (bathrooms) / 30 l/s (utility rooms).
• Installation Rules:
  • Position fans ≤400mm below ceilings (Diagram 1.1).
  • Avoid placing fans and background ventilators <500mm apart.

Step 2.2 – Background Ventilators

• Habitable Rooms/Kitchens:
  • Multi-storey: 8,000mm² equivalent area per room.
  • Single-storey: 10,000mm² per room.
• Bathrooms: 4,000mm² minimum.
• Total Units: ≥4 ventilators for 1-bed; ≥5 for >1-bed.

Example: In a 3-bed house, install 5+ ventilators distributed across ≥2 façades for cross-ventilation.

Step 3 – Purge Ventilation (Rapid Air Exchange)

• Window Requirements:
  • Hinged windows (15–30° opening): Total area ≥1/10 of floor area.
  • Hinged windows (≥30°): ≥1/20 floor area.
• No External Windows? Use adjacent rooms:
  • Permanent opening ≥1/20 combined floor area.
  • Ensure background ventilators (≥10,000mm²) in the adjacent room.

Step 4 – Final Checks

• Doors: 10mm undercut (or 20mm if no floor finish).
• Noise Areas: Install noise-attenuating ventilators near roads.
• Access: Ensure all components are reachable for cleaning/filter changes.

3. Path B: Mechanical Ventilation (For Highly Airtight Dwellings)

Option 1: Continuous Mechanical Extract Ventilation (CMEV)

• Extract Rates (Table 1.2):
  • Kitchen: 13 l/s (high rate), 8 l/s (continuous).
  • Bathroom: 8 l/s (high), 8 l/s (continuous).
• Ductwork:
  • Use rigid ducts; limit flexible ducts to ≤1.5m (BSRIA BG 43).
  • Seal joints to prevent leaks.
• Background Ventilators:
  • 4,000mm² in habitable rooms (no ventilators in wet rooms).

Option 2: Mechanical Ventilation with Heat Recovery (MVHR)

• Balanced System:
  • Match supply/extract airflow (±10% tolerance).
  • Heat recovery efficiency ≥70% (BS EN 13141-7).
• Frost Protection: Required for outdoor intakes in temperatures <0°C.
• Condensate Drainage: Connect to soil stacks or external soakaways.

Controls & Noise

• Noise Limits: ≤30 dB(A) in bedrooms; ≤45 dB(A) in kitchens.
• Automatic Controls: Humidity sensors in wet rooms; CO₂ sensors in living areas.
• Manual Overrides: Local switches for boost mode.

4. Commissioning & Compliance

Airflow Testing

• Use UKAS-calibrated equipment to verify rates (Appendix C).
• Example: A 4-bed home needs whole-dwelling ventilation ≥37 l/s (Table 1.3).

Homeowner Documentation

• Provide:
  • Filter replacement schedules (MVHR: every 6–12 months).
  • Fan cleaning instructions (CMEV: quarterly).
  • Signed commissioning sheet (Appendix C).

5. Special Cases

Existing Dwellings & Retrofits

• Window Replacements: Match original ventilator sizes or install 8,000mm² in habitable rooms.
• Energy Efficiency Upgrades: Follow Table 3.1 to assess ventilation impact (e.g., loft insulation = “minor” measure).

Basements & Internal Rooms

• Treat basements as separate single-storey dwellings.
• Use permanent openings (≥1/20 floor area) for rooms without external windows.

Final Note

Proper ventilation isn’t just about compliance – it prevents mould, controls humidity, and ensures healthy indoor air. Always cross-reference with Approved Document F (2021) and consult specialists for non-standard designs.

For full details: Download Approved Document F Volume 1.