Use this calculator to get a rough estimate of the heating and cooling loads for a residential or light commercial space. This tool considers key factors like area, insulation, windows, climate, and internal gains to provide an approximate BTU/hr requirement for your HVAC system.
Typical coldest temperature for your region.
Typical hottest temperature for your region.
Poor (Old construction, minimal insulation)
Average (Standard modern construction)
Good (Above average insulation)
Excellent (High-performance, well-insulated)
Single Pane
Double Pane
Triple Pane
Low (Minimal electronics, efficient lighting)
Medium (Typical household)
High (Many electronics, high-wattage lighting)
Drafty (Old, leaky building)
Average (Typical existing home)
Tight (Newer, well-sealed construction)
Very Tight (Energy-efficient, passive house standards)
Estimated Loads:
Heating Load: 0 BTU/hr
Sensible Cooling Load: 0 BTU/hr
Cooling Capacity: 0 Tons
function calculateLoads() {
// Constants
var indoorWinterTemp = 70; // °F
var indoorSummerTemp = 75; // °F
var heatPerPerson = 300; // BTU/hr sensible per person
var airDensityFactor = 0.018; // For infiltration Q = 0.018 * Volume * ACH * DeltaT
// U-Factors (simplified, representative values)
var uWallPoor = 0.15; var uRoofPoor = 0.25;
var uWallAverage = 0.08; var uRoofAverage = 0.15;
var uWallGood = 0.05; var uRoofGood = 0.08;
var uWallExcellent = 0.03; var uRoofExcellent = 0.05;
var uWindowSingle = 1.1;
var uWindowDouble = 0.5;
var uWindowTriple = 0.3;
// ACH Factors (Air Changes per Hour)
var achDrafty = 0.75;
var achAverage = 0.5;
var achTight = 0.35;
var achVeryTight = 0.25;
// Internal Gain Factors (BTU/hr/sqft)
var sensibleApplianceFactorLow = 3;
var sensibleApplianceFactorMedium = 5;
var sensibleApplianceFactorHigh = 8;
// Get Input Values
var floorArea = parseFloat(document.getElementById("floorArea").value);
var ceilingHeight = parseFloat(document.getElementById("ceilingHeight").value);
var outdoorWinterTemp = parseFloat(document.getElementById("outdoorWinterTemp").value);
var outdoorSummerTemp = parseFloat(document.getElementById("outdoorSummerTemp").value);
var insulationQuality = document.getElementById("insulationQuality").value;
var windowArea = parseFloat(document.getElementById("windowArea").value);
var windowType = document.getElementById("windowType").value;
var numOccupants = parseFloat(document.getElementById("numOccupants").value);
var internalGainFactor = document.getElementById("internalGainFactor").value;
var constructionTightness = document.getElementById("constructionTightness").value;
// Validate Inputs
if (isNaN(floorArea) || floorArea <= 0 ||
isNaN(ceilingHeight) || ceilingHeight <= 0 ||
isNaN(outdoorWinterTemp) ||
isNaN(outdoorSummerTemp) ||
isNaN(windowArea) || windowArea < 0 ||
isNaN(numOccupants) || numOccupants < 0) {
document.getElementById("heatingLoadResult").innerText = "Invalid Input";
document.getElementById("coolingLoadResult").innerText = "Invalid Input";
document.getElementById("coolingTonsResult").innerText = "Invalid Input";
return;
}
// Derived Values
var volume = floorArea * ceilingHeight;
// Rough estimate for wall area: assuming a typical single-story building where wall area is roughly equal to floor area for 8ft ceilings.
// This is a simplification for a general calculator.
var wallArea = floorArea * 1.0;
var netWallArea = Math.max(0, wallArea – windowArea); // Ensure net wall area is not negative
var roofArea = floorArea; // Roof area is typically equal to floor area
// Determine U-factors based on selections
var uWall, uRoof;
switch (insulationQuality) {
case "poor": uWall = uWallPoor; uRoof = uRoofPoor; break;
case "average": uWall = uWallAverage; uRoof = uRoofAverage; break;
case "good": uWall = uWallGood; uRoof = uRoofGood; break;
case "excellent": uWall = uWallExcellent; uRoof = uRoofExcellent; break;
}
var uWindow;
switch (windowType) {
case "single": uWindow = uWindowSingle; break;
case "double": uWindow = uWindowDouble; break;
case "triple": uWindow = uWindowTriple; break;
}
var ach;
switch (constructionTightness) {
case "drafty": ach = achDrafty; break;
case "average": ach = achAverage; break;
case "tight": ach = achTight; break;
case "veryTight": ach = achVeryTight; break;
}
var applianceFactor;
switch (internalGainFactor) {
case "low": applianceFactor = sensibleApplianceFactorLow; break;
case "medium": applianceFactor = sensibleApplianceFactorMedium; break;
case "high": applianceFactor = sensibleApplianceFactorHigh; break;
}
// — Heating Load Calculation (BTU/hr) —
var deltaT_heating = indoorWinterTemp – outdoorWinterTemp;
if (deltaT_heating < 0) deltaT_heating = 0; // No heating load if outdoor temp is higher than indoor setpoint
var q_wall_heating = uWall * netWallArea * deltaT_heating;
var q_roof_heating = uRoof * roofArea * deltaT_heating;
var q_window_heating = uWindow * windowArea * deltaT_heating;
var q_infiltration_heating = airDensityFactor * volume * ach * deltaT_heating;
var totalHeatingLoad = q_wall_heating + q_roof_heating + q_window_heating + q_infiltration_heating;
// — Cooling Load Calculation (BTU/hr – Sensible) —
var deltaT_cooling = outdoorSummerTemp – indoorSummerTemp;
if (deltaT_cooling < 0) deltaT_cooling = 0; // No cooling load if outdoor temp is lower than indoor setpoint
var q_wall_cooling = uWall * netWallArea * deltaT_cooling;
var q_roof_cooling = uRoof * roofArea * deltaT_cooling;
// For windows, solar gain is a major factor, but for simplicity, we'll use U*A*DeltaT as a baseline transmission gain.
// This is a significant simplification as solar heat gain through windows is often the largest cooling load component.
var q_window_cooling = uWindow * windowArea * deltaT_cooling;
var q_infiltration_cooling = airDensityFactor * volume * ach * deltaT_cooling;
var q_occupant_cooling = numOccupants * heatPerPerson;
var q_internal_cooling = applianceFactor * floorArea;
var totalCoolingLoadSensible = q_wall_cooling + q_roof_cooling + q_window_cooling + q_infiltration_cooling + q_occupant_cooling + q_internal_cooling;
var coolingTons = totalCoolingLoadSensible / 12000; // 1 Ton = 12,000 BTU/hr
// Display Results
document.getElementById("heatingLoadResult").innerText = totalHeatingLoad.toFixed(0);
document.getElementById("coolingLoadResult").innerText = totalCoolingLoadSensible.toFixed(0);
document.getElementById("coolingTonsResult").innerText = coolingTons.toFixed(2);
}
Understanding Heating and Cooling Loads
Heating and cooling loads represent the amount of heat energy that needs to be added to or removed from a space to maintain a comfortable indoor temperature. Calculating these loads is a critical first step in designing and sizing an efficient HVAC (Heating, Ventilation, and Air Conditioning) system for any building.
Why Calculate Loads?
Proper Sizing: An undersized HVAC system won't be able to maintain comfort on extreme days, while an oversized system will cycle on and off too frequently (short-cycling), leading to reduced efficiency, premature wear, poor dehumidification (for cooling), and uncomfortable temperature swings.
Energy Efficiency: Understanding where heat is gained or lost helps identify areas for improvement, such as adding insulation, upgrading windows, or sealing air leaks, ultimately reducing energy consumption.
Cost Savings: An efficiently sized and operating system uses less energy, leading to lower utility bills over the long term.
Key Factors Influencing Loads:
Building Envelope (Walls, Roof, Floor): The materials and insulation levels of these components determine how much heat transfers through them. Better insulation (lower U-factor, higher R-value) reduces transmission loads.
Windows and Doors: These are often the weakest points in the building envelope. Their size, type (single, double, triple pane), and orientation significantly impact heat gain (especially solar radiation in summer) and heat loss.
Air Leakage (Infiltration/Exfiltration): Uncontrolled air movement through cracks and gaps in the building envelope brings in unconditioned outdoor air, adding to both heating and cooling loads.
Climate and Outdoor Temperatures: The difference between indoor and outdoor design temperatures directly drives transmission and infiltration loads. Colder winters mean higher heating loads; hotter, more humid summers mean higher cooling loads.
Internal Heat Gains:
Occupants: People generate body heat (sensible) and moisture (latent).
Appliances and Lighting: Electronic devices, lights, and cooking appliances all contribute heat to the space.
Building Orientation and Shading: (Not directly in this simplified calculator, but important) The direction a building faces and the presence of shading (overhangs, trees) can significantly impact solar heat gain through windows.
How to Use This Calculator:
Conditioned Floor Area: Enter the total square footage of the space you want to heat or cool.
Average Ceiling Height: Provide the average height of your ceilings. This helps determine the building's volume.
Outdoor Design Temperatures: Input the typical extreme winter low and summer high temperatures for your location. These are often available from local weather data or HVAC design guides.
Wall & Roof Insulation Quality: Select the option that best describes your building's insulation levels. This impacts the U-factor (heat transfer coefficient) of your envelope.
Total Window Area: Estimate the combined square footage of all windows in the space.
Window Type: Choose the type of glazing (single, double, or triple pane) to reflect its insulating properties.
Number of Occupants: Enter the typical number of people present in the space.
Internal Heat Gains: Select a factor for heat generated by appliances, electronics, and lighting.
Building Air Tightness: Choose an option that reflects how well-sealed your building is against air leaks.
Understanding the Results:
Heating Load (BTU/hr): This is the estimated amount of heat energy (in British Thermal Units per hour) your heating system needs to provide on the coldest design day to maintain comfort.
Sensible Cooling Load (BTU/hr): This is the estimated amount of sensible heat energy your cooling system needs to remove on the hottest design day. Sensible heat relates to temperature change, not humidity.
Cooling Capacity (Tons): Cooling loads are often expressed in "tons" of refrigeration. One ton of cooling capacity is equivalent to removing 12,000 BTU/hr of heat. This value helps in sizing air conditioners.
Important Considerations and Disclaimer:
This calculator provides a simplified estimate for educational and preliminary planning purposes only. It uses generalized factors and does not account for many complex variables that a professional HVAC load calculation (like ACCA Manual J) would include, such as:
Specific wall, roof, and floor construction details (R-values of individual layers)
Detailed window properties (SHGC – Solar Heat Gain Coefficient, U-factor, orientation, shading)
Duct losses and gains
Latent heat loads (moisture removal during cooling)
Specific internal heat gains from individual appliances
Ventilation requirements
Thermal mass of the building
Local climate data specifics (humidity, solar radiation)
Always consult with a qualified HVAC professional for an accurate load calculation and system design tailored to your specific building and local climate. An expert can perform a detailed analysis to ensure your HVAC system is perfectly sized for optimal comfort, efficiency, and longevity.