Zone 1 (Hot & Humid)
Zone 2 (Hot & Dry / Warm & Humid)
Zone 3 (Temperate)
Zone 4 (Cool)
Zone 5 (Cold)
Zone 6 (Very Cold)
Zone 7 (Extremely Cold)
Standard
Good (e.g., added attic insulation, better windows)
Excellent (e.g., high-performance windows, well-sealed)
—
BTU/hr
Understanding Mini-Split Heat Pump Sizing
Selecting the correct size for a mini-split heat pump is crucial for efficient and effective heating and cooling. An oversized unit can lead to short-cycling, poor dehumidification, and increased wear and tear. An undersized unit will struggle to maintain desired temperatures, especially during extreme weather, leading to discomfort and higher energy bills.
This calculator provides an *estimated* heating capacity (in BTU/hr) required for a specific space. It considers several key factors influencing heat loss and gain:
Living Area Square Footage: The primary driver of heating and cooling load. Larger areas require more capacity.
Climate Zone: Different regions have vastly different temperature extremes. Colder climates require more heating capacity, while hotter climates demand more cooling capacity (though this calculator focuses on the heating BTU estimate). The climate zone is simplified here using numerical values representing general temperature ranges.
Insulation Quality: Better insulation reduces heat transfer, meaning less capacity is needed. The multiplier adjusts the base calculation based on your home's insulation.
Ceiling Height: Higher ceilings mean a larger volume of air to heat or cool.
Window Area & Sun Exposure: Windows are significant sources of heat loss in winter and heat gain in summer. The total window area and the percentage of sun exposure (which can increase heat gain) are factored in.
Occupants: Each person generates body heat, which contributes to the overall load.
The Calculation Logic (Simplified)
The calculation is a simplified heat loss/gain estimation. It starts with a base BTU requirement per square foot, adjusted by climate zone. Then, it modifies this based on other factors:
Base Load: A starting point of BTU per square foot, increasing with colder climate zones.
Ceiling Volume Adjustment: The base load is scaled by the ratio of actual ceiling volume to a standard 8-foot ceiling. (Ceiling Height / 8)
Insulation Adjustment: The result is multiplied by the insulation quality factor. (Insulation Quality Factor)
Window Load Adjustment: An additional load is added based on window area and sun exposure. This is a simplified approach, as detailed window heat transfer calculations are complex.
Occupant Load: The heat generated by occupants is added directly. (Number of Occupants * 400 BTU/hr)
Formula Approximation:Estimated BTU/hr = (Square Footage * Base BTU/sq ft [climate-dependent] * (Ceiling Height / 8) * Insulation Quality Factor) + (Window Area * Sun Exposure % * Window Factor) + (Number of Occupants * 400)
*Note: The "Base BTU/sq ft" and "Window Factor" are internal values used in the JavaScript to approximate typical loads.*
Important Considerations:
This is an Estimate: For precise sizing, consult a qualified HVAC professional. They will perform a Manual J calculation, which is a detailed load calculation that accounts for factors like building materials, air infiltration, ductwork, and specific climate data.
Cooling vs. Heating: This calculator primarily estimates the *heating* BTUs needed. Mini-splits are also rated for cooling BTUs. While there's overlap, dedicated cooling calculations might be necessary in very hot climates.
Multiple Zones: If you are heating/cooling multiple distinct areas, you may need multiple indoor units, each sized appropriately for its zone, or a single larger outdoor unit capable of serving multiple indoor units.
Ductless Mini-Splits: These systems are efficient and offer zoned comfort, making them ideal for additions, older homes without ductwork, or for creating different temperature zones within a larger space.
function calculateBtu() {
var sqft = parseFloat(document.getElementById("squareFootage").value);
var climate = parseInt(document.getElementById("climateZone").value);
var insulation = parseFloat(document.getElementById("insulationQuality").value);
var ceiling = parseFloat(document.getElementById("ceilingHeight").value);
var windowArea = parseFloat(document.getElementById("windowArea").value);
var sunExposure = parseFloat(document.getElementById("sunExposure").value);
var occupants = parseInt(document.getElementById("occupants").value);
var baseBtuPerSqft = 0;
var windowFactor = 50; // Simplified factor for window heat transfer per sq ft per % exposure
// Base BTU per square foot based on climate zone (approximations)
if (climate === 1) { // Zone 1 (Hot & Humid)
baseBtuPerSqft = 20; // More emphasis on cooling, but heating still needed
} else if (climate === 2) { // Zone 2 (Hot & Dry / Warm & Humid)
baseBtuPerSqft = 25;
} else if (climate === 3) { // Zone 3 (Temperate)
baseBtuPerSqft = 30;
} else if (climate === 4) { // Zone 4 (Cool)
baseBtuPerSqft = 35;
} else if (climate === 5) { // Zone 5 (Cold)
baseBtuPerSqft = 40;
} else if (climate === 6) { // Zone 6 (Very Cold)
baseBtuPerSqft = 45;
} else if (climate === 7) { // Zone 7 (Extremely Cold)
baseBtuPerSqft = 50;
}
// Validate inputs
if (isNaN(sqft) || sqft <= 0 ||
isNaN(climate) ||
isNaN(insulation) ||
isNaN(ceiling) || ceiling 15 ||
isNaN(windowArea) || windowArea < 0 ||
isNaN(sunExposure) || sunExposure 100 ||
isNaN(occupants) || occupants < 0) {
document.getElementById("result-value").innerText = "Invalid Input";
document.getElementById("result-unit").innerText = "";
return;
}
// Calculate base load adjusted for ceiling height
var baseLoad = sqft * baseBtuPerSqft * (ceiling / 8);
// Apply insulation quality adjustment
var insulatedLoad = baseLoad * insulation;
// Calculate window load adjustment
var windowLoad = (windowArea * (sunExposure / 100)) * windowFactor;
// Calculate occupant load
var occupantLoad = occupants * 400;
// Total estimated BTU/hr
var totalBtu = insulatedLoad + windowLoad + occupantLoad;
// Round to nearest whole number
totalBtu = Math.round(totalBtu);
// Display the result
document.getElementById("result-value").innerText = totalBtu.toLocaleString();
document.getElementById("result-unit").innerText = "BTU/hr";
}