Calculate the estimated heat loss for a room or building to determine appropriate heating system capacity.
U-Factor
R-Value
— BTU/hr —
Understanding BTU Heat Loss
Calculating the heat loss for a space is crucial for properly sizing heating systems like furnaces or boilers.
An undersized system won't adequately heat the space during cold weather, while an oversized system can lead to
inefficiency, short cycling, and uneven temperatures. The British Thermal Unit (BTU) is a standard unit
of energy, often used to measure the heating or cooling capacity of systems. This calculator provides an
estimation of the heat loss in BTUs per hour (BTU/hr) that a space experiences.
The primary factors contributing to heat loss are conduction through walls, windows, and doors, and infiltration
(air leakage) into the space. This calculator uses simplified formulas to estimate these losses.
Key Factors and Formulas:
Volume Heat Loss (Convection & Infiltration): This accounts for the heat required to warm the air
within the space and replace air that leaks out.
BTU/hr (Volume) = Volume (cu ft) * Air Changes per Hour * 0.018 BTU/(cu ft * °F) * Temperature Difference (°F)
Area (sq ft) is the surface area of the component (e.g., Wall Area).
R-Value is the thermal resistance of the material. Higher R-value means better insulation.
For simplicity, this calculator uses a single R-value for all walls, ceiling, and floor, assuming they are similar.
Conduction Heat Loss (Windows & Doors): Windows and doors are typically less insulated.
We use U-Factor or R-Value. If U-Factor is provided, it's converted to R-Value (R = 1/U).
R-Value (Window/Door) is the thermal resistance specific to windows and doors.
Total Estimated Heat Loss = BTU/hr (Volume) + BTU/hr (Walls/Ceiling/Floor) + BTU/hr (Windows/Doors)
Disclaimer: This calculator provides an estimation. Actual heat loss can be influenced by many other factors,
including building orientation, wind exposure, thermal bridging, and the exact construction of walls, windows, and doors.
For precise sizing, consult a qualified HVAC professional.
Example Calculation:
Consider a room that is 15 ft long, 10 ft wide, and 8 ft high. The desired inside temperature is 70°F, and the
outside design temperature is 0°F. The room has 20 sq ft of window area and 15 sq ft of door area.
The walls have an R-value of 13, and the windows have a U-factor of 0.5 (which is an R-value of 2).
Assume an infiltration rate of 0.5 air changes per hour.
Temperature Difference = 70°F – 0°F = 70°F
Room Volume = 15 ft * 10 ft * 8 ft = 1200 cu ft
Total Wall Area (approximate, assuming 4 walls) = 2 * (15*8) + 2 * (10*8) = 240 + 160 = 400 sq ft
Total Door/Window Area = 20 sq ft + 15 sq ft = 35 sq ft
Volume Heat Loss = 1200 cu ft * 0.5 ACH * 0.018 * 70°F ≈ 630 BTU/hr
Wall Heat Loss = (400 sq ft * 70°F) / 13 R-Value ≈ 2154 BTU/hr
Window/Door Heat Loss = (35 sq ft * 70°F) / 2 R-Value ≈ 1225 BTU/hr
Total Estimated Heat Loss = 630 + 2154 + 1225 ≈ 4009 BTU/hr
This means a heating system should be sized to provide at least approximately 4009 BTU/hr to maintain the desired temperature.
function calculateHeatLoss() {
var roomLength = parseFloat(document.getElementById("roomLength").value);
var roomWidth = parseFloat(document.getElementById("roomWidth").value);
var roomHeight = parseFloat(document.getElementById("roomHeight").value);
var outsideTemp = parseFloat(document.getElementById("outsideTemp").value);
var insideTemp = parseFloat(document.getElementById("insideTemp").value);
var windowArea = parseFloat(document.getElementById("windowArea").value);
var doorArea = parseFloat(document.getElementById("doorArea").value);
var wallInsulation = parseFloat(document.getElementById("wallInsulation").value);
var windowInsulationType = document.getElementById("windowInsulationType").value;
var windowInsulationValue = parseFloat(document.getElementById("windowInsulationValue").value);
var infiltrationRate = parseFloat(document.getElementById("infiltrationRate").value);
var resultDiv = document.getElementById("result");
resultDiv.textContent = "– BTU/hr –"; // Reset result
// Input validation
if (isNaN(roomLength) || isNaN(roomWidth) || isNaN(roomHeight) ||
isNaN(outsideTemp) || isNaN(insideTemp) || isNaN(windowArea) ||
isNaN(doorArea) || isNaN(wallInsulation) || isNaN(windowInsulationValue) ||
isNaN(infiltrationRate)) {
resultDiv.textContent = "Error: Please enter valid numbers for all fields.";
resultDiv.style.backgroundColor = "#dc3545"; // Red for error
return;
}
if (wallInsulation <= 0 || windowInsulationValue <= 0 || infiltrationRate < 0) {
resultDiv.textContent = "Error: Insulation R-values and infiltration rate must be positive.";
resultDiv.style.backgroundColor = "#dc3545"; // Red for error
return;
}
if (insideTemp <= outsideTemp) {
resultDiv.textContent = "Error: Inside temperature must be higher than outside temperature.";
resultDiv.style.backgroundColor = "#dc3545"; // Red for error
return;
}
var tempDifference = insideTemp – outsideTemp;
var roomVolume = roomLength * roomWidth * roomHeight;
var totalWallArea = 2 * (roomLength * roomHeight) + 2 * (roomWidth * roomHeight);
var totalWindowDoorArea = windowArea + doorArea;
// Calculate R-Value for windows if U-Factor is provided
var windowDoorRValue;
if (windowInsulationType === "u-factor") {
if (windowInsulationValue === 0) { // Avoid division by zero
resultDiv.textContent = "Error: Window U-Factor cannot be zero.";
resultDiv.style.backgroundColor = "#dc3545";
return;
}
windowDoorRValue = 1 / windowInsulationValue;
} else {
windowDoorRValue = windowInsulationValue;
}
// — Heat Loss Calculations —
// 1. Volume Heat Loss (Infiltration)
var volumeHeatLoss = roomVolume * infiltrationRate * 0.018 * tempDifference;
// 2. Conduction Heat Loss (Walls, Ceiling, Floor)
// Simplified: Using total wall area and wall R-value for all surfaces.
// In a more complex calculator, separate values for ceiling/floor would be used.
var conductionWallLoss = (totalWallArea * tempDifference) / wallInsulation;
// 3. Conduction Heat Loss (Windows & Doors)
var conductionWindowDoorLoss = (totalWindowDoorArea * tempDifference) / windowDoorRValue;
// Total Heat Loss
var totalHeatLoss = volumeHeatLoss + conductionWallLoss + conductionWindowDoorLoss;
// Display result
resultDiv.textContent = totalHeatLoss.toFixed(0) + " BTU/hr";
resultDiv.style.backgroundColor = "#28a745"; // Success green
}