Estimate the heating requirements for a residential space.
Input Parameters
Estimated Heat Load
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Understanding Heat Load Calculation
The heat load of a building or a specific room is the amount of heat energy that must be supplied to maintain a comfortable indoor temperature during colder periods. This calculation is crucial for properly sizing heating systems (like furnaces, boilers, or heat pumps) to ensure they are neither undersized (leading to insufficient heating) nor oversized (leading to inefficiency, short-cycling, and discomfort).
This calculator provides an estimate of the sensible heat load, which is the heat required to raise the temperature of air and building materials. It considers several key factors:
Building Envelope Losses: Heat escaping through walls, windows, and the roof.
Infiltration Losses: Heat lost due to cold outside air entering the building through cracks and openings.
Ventilation Losses: Heat lost when fresh air is intentionally brought into the building for air quality. (Simplified in this calculator by the infiltration rate).
The Math Behind the Calculation
The primary components of heat loss are conduction through the building envelope and air infiltration.
1. Conduction Losses (Through Walls and Windows)
The rate of heat loss through a surface (like a wall or window) due to conduction is calculated using the formula:
$Q_{conduction} = U \times A \times \Delta T$
Where:
$Q_{conduction}$ is the heat loss rate in BTU per hour (BTU/hr).
$U$ is the U-value of the material (BTU/hr/sq ft/°F). This represents how well the material insulates; lower U-values mean better insulation.
$A$ is the surface area of the material in square feet (sq ft).
$\Delta T$ is the temperature difference between the inside and outside ($T_{inside} – T_{outside}$) in degrees Fahrenheit (°F).
2. Infiltration Losses
This accounts for heat lost from air exchange. The formula often used is:
$Q_{infiltration} = 0.018 \times CFM \times \Delta T$
Where:
$Q_{infiltration}$ is the heat loss rate in BTU/hr.
$0.018$ is a constant that approximates the specific heat and density of air.
$CFM$ is the cubic feet per minute of air infiltration. This is often estimated from the Air Changes per Hour (ACH) and the building volume.
$\Delta T$ is the temperature difference ($T_{inside} – T_{outside}$) in °F.
To estimate CFM from ACH:
$CFM = (ACH \times Room Volume (cubic ft)) / 60$
The Room Volume is calculated as:
$Room Volume = Room Surface Area \times Room Height$
Total Heat Load
The total estimated heat load is the sum of conduction losses through all exterior surfaces (walls and windows) and the infiltration losses.
$Total Heat Load = (Q_{walls} + Q_{windows}) + Q_{infiltration}$
How to Use This Calculator
Gather Information: Measure the surface area of the room, its height, the number and average size of windows, and identify how many walls are exterior.
Determine U-Values: U-values are a measure of thermal transmittance. For windows, a typical double-pane window might have a U-value around 0.5 BTU/hr/sq ft/°F. For walls, it varies greatly based on insulation; well-insulated walls might be 0.1 or lower, while uninsulated walls could be 0.5 or higher. You may need to consult building specifications or online resources for typical values if exact data isn't available.
Set Temperatures: Input your desired indoor temperature and the "design outdoor temperature." The design outdoor temperature is the expected lowest temperature for your region during winter, used for sizing heating equipment. Local building codes or ASHRAE data can provide this.
Estimate Infiltration: Air Changes per Hour (ACH) represents how many times the entire volume of air in the room is replaced by outside air each hour. A typical older home might have 1.0 ACH, while a tightly sealed new home might be 0.3 ACH or less. 0.5 ACH is a reasonable average for many homes.
Calculate: Click the "Calculate Heat Load" button.
The result will be an estimated heat load in BTU/hr. This value is essential for selecting a heating unit that can adequately compensate for heat loss and keep your space warm and comfortable during the coldest days. Always consult with a qualified HVAC professional for precise system sizing and installation.
function calculateHeatLoad() {
var roomArea = parseFloat(document.getElementById(" pomieszczeniePowierzchnia").value);
var roomHeight = parseFloat(document.getElementById(" pomieszczenieWysokosc").value);
var numWindows = parseFloat(document.getElementById(" pomieszczenieLiczbaOkien").value);
var avgWindowArea = parseFloat(document.getElementById("oknoPowierzchnia").value);
var windowUValue = parseFloat(document.getElementById("oknoWspolczynnikU").value);
var numExteriorWalls = parseFloat(document.getElementById("pomieszczenieLiczbaScianZewnetrznych").value);
var wallUValue = parseFloat(document.getElementById("scianaWspolczynnikU").value);
var desiredTemp = parseFloat(document.getElementById("temperaturaWewnetrzna").value);
var outdoorTemp = parseFloat(document.getElementById("temperaturaZewnetrzna").value);
var infiltrationRate = parseFloat(document.getElementById("infiltracja").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "–"; // Clear previous result
// Input validation
if (isNaN(roomArea) || roomArea <= 0 ||
isNaN(roomHeight) || roomHeight <= 0 ||
isNaN(numWindows) || numWindows < 0 ||
isNaN(avgWindowArea) || avgWindowArea < 0 ||
isNaN(windowUValue) || windowUValue <= 0 ||
isNaN(numExteriorWalls) || numExteriorWalls < 0 ||
isNaN(wallUValue) || wallUValue <= 0 ||
isNaN(desiredTemp) ||
isNaN(outdoorTemp) ||
isNaN(infiltrationRate) || infiltrationRate < 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
if (outdoorTemp >= desiredTemp) {
resultDiv.innerHTML = "Outdoor temperature should be lower than desired indoor temperature for heating calculation.";
return;
}
// Calculations
var temperatureDifference = desiredTemp – outdoorTemp;
// Conduction Loss through Windows
var totalWindowArea = numWindows * avgWindowArea;
var windowConductionLoss = windowUValue * totalWindowArea * temperatureDifference;
// Conduction Loss through Exterior Walls
// Estimate wall area assuming standard room dimensions (e.g., ~10ft length per wall for simplicity in estimation)
// A more accurate calculation would require room length and width.
// For simplicity here, we'll use perimeter if numExteriorWalls > 0.
// If numExteriorWalls is 1, 2, 3 or 4, we assume a basic rectangular room.
// A more robust calculation would require room length and width.
// For a simplified approach: assume each exterior wall contributes to the perimeter.
// Let's assume a typical room depth/width for estimation if only one wall is exterior.
var estimatedWallLengthPerWall = 10; // Default assumption for wall length if only one wall is exterior.
var totalExteriorWallArea = 0;
if (numExteriorWalls > 0) {
// If multiple exterior walls, assume they form a perimeter.
// We approximate perimeter for simplicity, or use a standard length if only one wall.
var perimeter = 0;
if (numExteriorWalls === 1) {
perimeter = estimatedWallLengthPerWall; // Simple case
} else if (numExteriorWalls === 2) {
perimeter = 2 * estimatedWallLengthPerWall; // Simple case
} else if (numExteriorWalls === 3) {
perimeter = 3 * estimatedWallLengthPerWall;
} else if (numExteriorWalls >= 4) { // Assume square-ish for 4+
perimeter = 4 * estimatedWallLengthPerWall;
}
// Clamp perimeter to avoid excessive area with many exterior walls in a small room context.
// This is a simplification. A real calculation needs room L/W.
perimeter = Math.min(perimeter, roomArea * 1.5); // Heuristic to avoid unrealistic wall area
totalExteriorWallArea = perimeter * roomHeight;
// Ensure wall area doesn't exceed total surface area minus window area
totalExteriorWallArea = Math.min(totalExteriorWallArea, roomArea – totalWindowArea);
totalExteriorWallArea = Math.max(0, totalExteriorWallArea); // Ensure it's not negative
}
var wallConductionLoss = wallUValue * totalExteriorWallArea * temperatureDifference;
// Infiltration Loss
var roomVolume = roomArea * roomHeight;
var airChangesPerHour = infiltrationRate;
var cfm = (airChangesPerHour * roomVolume) / 60;
var infiltrationLoss = 0.018 * cfm * temperatureDifference;
// Total Heat Load
var totalHeatLoad = windowConductionLoss + wallConductionLoss + infiltrationLoss;
// Display Result
resultDiv.innerHTML = "" + totalHeatLoad.toFixed(0) + " BTU/hr";
}