Natural Ventilation Rate Calculator
Estimate Airflow and Air Changes per Hour (ACH)
Understanding Natural Ventilation Calculations
Natural ventilation is the process of supplying and removing air through an indoor space by natural means, without using mechanical systems. Calculating the natural ventilation rate is critical for building design, ensuring thermal comfort, and maintaining healthy indoor air quality.
1. Wind-Driven Ventilation
This occurs when wind creates a pressure difference across a building. The formula used in this calculator is:
Qw = K × A × V
- Qw: Airflow rate (m³/s)
- K: Effectiveness of opening (typically 0.5 – 0.6 for perpendicular wind)
- A: Free area of inlet opening (m²)
- V: Wind speed (m/s)
2. The Stack Effect (Buoyancy-Driven)
The stack effect occurs due to the difference in air density caused by temperature variations between the inside and outside of a building. Hot air rises and escapes through upper openings, while cooler air is drawn in through lower openings.
Qs = Cd × A × √(2 × g × H × ΔT / Ti)
Our calculator simplifies this using the standard coefficient for architectural approximations: Qs = 0.65 × A × √(h × ΔT).
3. Air Changes per Hour (ACH)
ACH is a measure of how many times the entire volume of air in a room is replaced within one hour. For residential living rooms, an ACH of 0.35 to 2.0 is often targeted, while kitchens or high-occupancy areas may require much higher rates.
Practical Example
Imagine a room with a volume of 120 m³. You have a window open by 1.2 m². If the wind speed outside is 2.5 m/s (diagonal wind) and the temperature inside is 4°C warmer than outside:
- Wind Ventilation: 0.3 × 1.2 × 2.5 = 0.9 m³/s (3240 m³/h)
- Stack Ventilation: 0.65 × 1.2 × √(2m × 4°C) ≈ 2.2 m³/s
- Combined: The calculation uses the square root of the sum of squares to account for simultaneous effects.