What is the Minimum Purge Rate (Airflow) that I need for a Habitable Room?

The minimum airflow rate for purge ventilation is 4 air changes per hour (ACH), as specified in Approved Document F, Volume 1. This ensures rapid removal of pollutants and moisture from habitable rooms in UK dwellings.

Understanding Purge Ventilation

Purge ventilation is essential for quickly removing pollutants, odours, and excess moisture from a room. It’s particularly useful in areas like kitchens and bathrooms, where activities such as cooking or showering generate high levels of humidity and airborne contaminants. Approved Document F sets clear guidelines to ensure buildings have effective systems in place for this purpose.

The 4 ACH Requirement

The 4 ACH standard means that the entire volume of air in a room must be replaced four times every hour during purge ventilation. This rate ensures that pollutants are rapidly diluted and expelled, maintaining a healthy indoor environment.

To calculate the required airflow rate in litres per second (l/s), use the formula:
Purge airflow (l/s) = (Room volume * ACH) / 3600 * 100

For example, a room with a volume of 50m³ would require:
(50 * 4) / 3600 * 100 = 5.56 l/s

Free Area of Openings

Approved Document F also specifies the minimum free area for openings used in purge ventilation. The free area must be at least 1/20th of the room’s floor area. For instance, a room with a floor area of 20m² would need a free area of 1m² for effective purge ventilation.

Why Purge Ventilation Matters

  1. Health Benefits: Rapid removal of pollutants reduces the risk of respiratory issues and allergies.
  2. Moisture Control: Prevents condensation and mould growth, which can damage property and harm health.
  3. Comfort: Improves indoor air quality, making spaces more pleasant to live in.

Practical Tips for Effective Purge Ventilation

  1. Use Openable Windows: Ensure windows can open wide enough to meet the free area requirement.
  2. Install Mechanical Systems: In rooms without windows, consider mechanical extract fans to achieve the necessary airflow.
  3. Regular Maintenance: Keep ventilation systems clean and functional to maintain performance.

Ensure your home meets the 4 ACH standard for purge ventilation to maintain a healthy and comfortable living environment.