Natural ventilation is a sustainable method of supplying fresh air to a space without the use of mechanical systems like fans or air conditioners. Understanding how to calculate the natural ventilation rate is crucial for architects, HVAC engineers, and building owners aiming to improve Indoor Air Quality (IAQ) and reduce energy consumption.
Understanding the Physics of Airflow
Natural ventilation is primarily driven by two natural forces:
Wind Pressure (Cross Ventilation): Wind blowing against a building creates high pressure on the windward side and low pressure on the leeward side, driving air through openings.
Stack Effect (Buoyancy): Differences in temperature (and thus density) between indoor and outdoor air cause warm air to rise and escape through high openings, drawing cool air in from lower openings.
This calculator utilizes the Wind-Driven ventilation model, which is the most common method for estimating airflow in standard rooms with windows.
The Calculation Formula
To calculate the volumetric flow rate due to wind, we use the following equation:
Q = Cv × A × V
Where:
Q: Airflow rate (m³/s)
Cv: Effectiveness coefficient (based on wind angle)
A: Free area of inlet openings (m²)
V: Wind velocity (m/s)
About the Coefficient (Cv)
The efficiency of the wind entering a window changes based on the angle. If the wind hits the window head-on (perpendicular), the coefficient is typically around 0.5 to 0.6. If the wind hits at a sharp angle (oblique), the efficiency drops to roughly 0.25 to 0.35.
Calculating Air Changes Per Hour (ACH)
While the flow rate (Q) tells you how much air is moving, the ACH tells you how effective that ventilation is for the specific size of your room. It represents how many times the entire volume of air in the room is replaced in one hour.
ACH = (Q × 3600) / Room Volume
Note: We multiply Q (m³/s) by 3600 to convert seconds to hours.
Recommended Ventilation Rates
Different spaces require different ventilation rates to ensure occupant health and comfort:
Residential Living Areas: 4 – 6 ACH
Bathrooms/Kitchens: 10 – 15 ACH (to remove moisture/odors)
Classrooms/Offices: 4 – 8 ACH
Warehouses: 2 – 4 ACH
Tips for Improving Natural Ventilation
If your calculation shows a low ACH, consider these modifications:
Increase Opening Area: Open windows fully or install larger vents.
Cross Ventilation: Ensure openings are on opposite walls to create a clear path for airflow.
Maximize Height Diff: For stack effect, place outlets as high as possible and inlets as low as possible.
Orient to Wind: Design building openings to face prevailing wind directions.
function calculateVentilation() {
// 1. Get Input Elements by ID
var areaInput = document.getElementById("openingArea");
var windInput = document.getElementById("windSpeed");
var volumeInput = document.getElementById("roomVolume");
var angleSelect = document.getElementById("windAngle");
// 2. Parse Values
var area = parseFloat(areaInput.value);
var wind = parseFloat(windInput.value);
var volume = parseFloat(volumeInput.value);
var coeff = parseFloat(angleSelect.value);
// 3. Validation
if (isNaN(area) || area <= 0) {
alert("Please enter a valid Opening Area greater than 0.");
return;
}
if (isNaN(wind) || wind < 0) {
alert("Please enter a valid Wind Speed (cannot be negative).");
return;
}
if (isNaN(volume) || volume <= 0) {
alert("Please enter a valid Room Volume greater than 0.");
return;
}
// 4. Calculate Flow Rate (Q) in m³/s
// Formula: Q = Cv * A * V
var flowRateSeconds = coeff * area * wind;
// 5. Calculate Flow Rate (Q) in m³/h
// Conversion: m³/s * 3600 seconds/hour
var flowRateHours = flowRateSeconds * 3600;
// 6. Calculate ACH (Air Changes Per Hour)
// Formula: ACH = Flow Rate (m³/h) / Volume (m³)
var ach = flowRateHours / volume;
// 7. Determine Status
var statusText = "";
var statusColor = "#333";
if (ach < 2) {
statusText = "Poor (Stagnant Air)";
statusColor = "#e74c3c";
} else if (ach < 4) {
statusText = "Fair (Low Ventilation)";
statusColor = "#f39c12";
} else if (ach < 15) {
statusText = "Good (Standard Ventilation)";
statusColor = "#2ecc71";
} else {
statusText = "High (Breezy/Drafty)";
statusColor = "#3498db";
}
// 8. Update Result Elements
document.getElementById("resultQSec").innerText = flowRateSeconds.toFixed(3) + " m³/s";
document.getElementById("resultQHour").innerText = flowRateHours.toFixed(1) + " m³/h";
document.getElementById("resultACH").innerText = ach.toFixed(1);
var statusEl = document.getElementById("resultStatus");
statusEl.innerText = statusText;
statusEl.style.color = statusColor;
statusEl.style.fontWeight = "bold";
// 9. Show Results
document.getElementById("resultsBox").classList.add("visible");
}