Home Heating Load Calculator
ANA›Life Services Authority›National Calculator Authority›Home Heating Load Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Home Heating Load Calculator
Estimate the heating load (BTU/hr) required to keep your home comfortable during cold weather using the Manual J simplified method.
Total Floor Area (sq ft)
Average Ceiling Height (ft)
Desired Indoor Temperature (°F)
Outdoor Design Temperature (°F)
Wall Insulation Level
Poor (no insulation, R-0) Below Average (R-7) Average (R-13) Good (R-19) Excellent (R-30+)
Window Type
Single Pane Double Pane Double Pane Low-E Triple Pane
Total Window Area (sq ft)
Number of Stories
1 Story 2 Stories 3 Stories
Basement / Foundation Type
Slab on Grade (uninsulated) Slab on Grade (insulated) Unfinished Basement Finished / Insulated Basement Crawl Space
Air Infiltration / Leakage
Tight (new construction, 0.5 ACH) Average (1.0 ACH) Leaky (older home, 1.5 ACH) Very Leaky (drafty, 2.5 ACH)
Calculate Heating Load
function homCalc() { // --- Gather inputs --- var floorArea = parseFloat(document.getElementById('hom-floor-area').value); var ceilHeight = parseFloat(document.getElementById('hom-ceiling-height').value); var indoorTemp = parseFloat(document.getElementById('hom-indoor-temp').value); var outdoorTemp = parseFloat(document.getElementById('hom-outdoor-temp').value); var wallU = parseFloat(document.getElementById('hom-insulation').value); var windowU = parseFloat(document.getElementById('hom-windows').value); var windowArea = parseFloat(document.getElementById('hom-window-area').value); var stories = parseInt(document.getElementById('hom-num-stories').value); var floorU = parseFloat(document.getElementById('hom-basement').value); var ach = parseFloat(document.getElementById('hom-infiltration').value);
var resultDiv = document.getElementById('hom-result');
// --- Validation --- var errors = []; if (isNaN(floorArea) || floorArea 20000) errors.push("Floor area must be between 100 and 20,000 sq ft."); if (isNaN(ceilHeight) || ceilHeight 20) errors.push("Ceiling height must be between 6 and 20 ft."); if (isNaN(indoorTemp) || indoorTemp 80) errors.push("Indoor temperature must be between 60°F and 80°F."); if (isNaN(outdoorTemp) || outdoorTemp 60) errors.push("Outdoor design temperature must be between -40°F and 60°F."); if (isNaN(windowArea) || windowArea 5000) errors.push("Window area must be between 0 and 5,000 sq ft."); if (!isNaN(indoorTemp) && !isNaN(outdoorTemp) && outdoorTemp >= indoorTemp) errors.push("Outdoor design temperature must be lower than indoor temperature for a heating load calculation.");
if (errors.length > 0) { resultDiv.style.display = "block"; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --------------------------------------------------------------- // FORMULA — Manual J Simplified Method // // 1. Temperature Difference (ΔT) // ΔT = T_indoor − T_outdoor // // 2. Wall Area // Perimeter ≈ 4 × √(floorArea / stories) [assumes square footprint per story] // wallArea = Perimeter × ceilHeight × stories − windowArea // // 3. Ceiling Area // ceilingArea = floorArea / stories [only top-story ceiling loses heat] // Ceiling U-value fixed at 0.026 (≈ R-38 attic insulation, typical) // // 4. Conduction Heat Loss (BTU/hr) // Q_walls = U_wall × wallArea × ΔT // Q_windows = U_window × windowArea × ΔT // Q_ceiling = U_ceil × ceilingArea × ΔT // Q_floor = U_floor × floorArea × ΔT // // 5. Infiltration Heat Loss (BTU/hr) // Volume = floorArea × ceilHeight // Q_inf = 0.018 × ACH × Volume × ΔT // (0.018 BTU/ft³·°F = specific heat of air × density) // // 6. Total Heating Load // Q_total = Q_walls + Q_windows + Q_ceiling + Q_floor + Q_inf // // 7. Safety Factor: multiply by 1.25 (25% design margin per ACCA Manual J) // ---------------------------------------------------------------
var deltaT = indoorTemp - outdoorTemp;
// Wall area (net of windows) var footprintPerStory = floorArea / stories; var sideLength = Math.sqrt(footprintPerStory); var perimeter = 4 * sideLength; var grossWallArea = perimeter * ceilHeight * stories; var netWallArea = Math.max(grossWallArea - windowArea, 0);
// Ceiling (top story only) var ceilingArea = footprintPerStory; var uCeiling = 0.026; // ≈ R-38
// Conduction losses var qWalls = wallU * netWallArea * deltaT; var qWindows = windowU * windowArea * deltaT; var qCeiling = uCeiling * ceilingArea * deltaT; var qFloor = floorU * floorArea * deltaT;
// Infiltration loss var volume = floorArea * ceilHeight; var qInf = 0.018 * ach * volume * deltaT;
// Total with 25% safety factor var qTotal = qWalls + qWindows + qCeiling + qFloor + qInf; var qDesign = qTotal * 1.25;
// Convert to useful units var qKBtu = qDesign / 1000; var qTons = qDesign / 12000; var qKW = qDesign * 0.000293071; var sqFtBtu = qDesign / floorArea; // BTU/hr per sq ft (rule-of-thumb check)
// Efficiency sizing suggestion var furnaceSize = Math.ceil(qDesign / 10000) * 10000; // round up to nearest 10k BTU
// ---- Format output ---- resultDiv.style.display = "block"; resultDiv.innerHTML = '### Heating Load Results ' + '' + 'ComponentHeat Loss (BTU/hr)' + 'Walls (net ' + netWallArea.toFixed(0) + ' sq ft)' + qWalls.toFixed(0) + '' + 'Windows (' + windowArea.toFixed(0) + ' sq ft)' + qWindows.toFixed(0) + '' + 'Ceiling (' + ceilingArea.toFixed(0) + ' sq ft)' + qCeiling.toFixed(0) + '' + 'Floor / Foundation' + qFloor.toFixed(0) + '' + 'Infiltration (' + ach + ' ACH)' + qInf.toFixed(0) + '' + 'Subtotal' + qTotal.toFixed(0) + '' + 'Design Load (×1.25 safety factor)' + qDesign.toFixed(0) + '' + '' + '' + 'Design Heating Load: ' + qDesign.toFixed(0) + ' BTU/hr
' + '' + qKBtu.toFixed(1) + ' kBTU/hr | ' + qTons.toFixed(2) + ' tons | ' + qKW.toFixed(2) + ' kW
' + 'Load intensity: ' + sqFtBtu.toFixed(1) + ' BTU/hr per sq ft
' + 'Suggested minimum furnace/boiler size: ' + furnaceSize.toLocaleString() + ' BTU/hr
' + ''; }
#### Formulas Used
1. Temperature Difference: ΔT = Tindoor − Toutdoor
2. Conduction Heat Loss (each surface): Q = U × A × ΔT where U = thermal transmittance (BTU/hr·ft²·°F), A = surface area (ft²)
3. Infiltration Heat Loss: Qinf = 0.018 × ACH × Volume × ΔT where 0.018 BTU/ft³·°F is the volumetric heat capacity of air, ACH = air changes per hour
4. Total Design Load: Qdesign = (Qwalls + Qwindows + Qceiling + Qfloor + Qinf) × 1.25
5. Wall Area Estimation: Perimeter = 4 × √(Floor Area / Stories) (assumes square footprint) Net Wall Area = Perimeter × Ceiling Height × Stories − Window Area
Unit Conversions: 1 ton = 12,000 BTU/hr | 1 BTU/hr = 0.000293071 kW
#### Assumptions & References
- Based on ACCA Manual J (Residential Load Calculation, 8th Edition) simplified method.
- Ceiling U-value fixed at 0.026 BTU/hr·ft²·°F (≈ R-38 attic insulation), a common code-minimum for cold climates.
- Home footprint is assumed square for perimeter estimation; irregular shapes may increase wall area and load.
- A 25% safety factor (×1.25) is applied per ACCA Manual J to account for unusually cold days, duct losses, and equipment cycling.
- Infiltration constant 0.018 BTU/ft³·°F = density of air (0.075 lb/ft³) × specific heat (0.24 BTU/lb·°F).
- Outdoor design temperature should be the 99% heating design temperature for your location (available from ASHRAE or local weather data).
- Internal heat gains (occupants, appliances, solar) are not subtracted in this simplified model, providing a conservative (safe) estimate.
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