Heat Load Calculator (Manual J Estimate)
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Heat Load Calculator (Manual J Estimate)
Estimates residential heating and cooling loads using the Manual J simplified methodology. Enter your home's characteristics to calculate BTU/hr requirements for properly sizing HVAC equipment.
### Climate & Design Conditions
Outdoor Winter Design Temp (°F)
99% design temperature for your location
Outdoor Summer Design Temp (°F)
1% design temperature for your location
Indoor Winter Setpoint (°F)
Indoor Summer Setpoint (°F)
### Building Envelope
Conditioned Floor Area (sq ft)
Average Ceiling Height (ft)
Gross Exterior Wall Area (sq ft)
Total Window Area (sq ft)
Ceiling / Roof Area (sq ft)
Floor / Foundation Type
Slab-on-Grade Vented Crawlspace Conditioned Basement Floor Over Garage/Unconditioned
### Insulation & Construction
Wall Assembly R-Value (hr·ft²·°F/BTU)
Typical: R-13 (2×4), R-20 (2×6)
Ceiling/Attic R-Value
Typical: R-30 to R-60
Window U-Factor (BTU/hr·ft²·°F)
Double-pane ≈ 0.30; Single-pane ≈ 0.80
Window SHGC (Solar Heat Gain Coefficient)
Low-e ≈ 0.25; Clear glass ≈ 0.60
Floor/Foundation R-Value
Slab edge insulation or floor insulation
Infiltration Rate (ACH)
Tight=0.25, Average=0.35, Leaky=0.60+
### Internal & Solar Gains (Cooling)
Number of Occupants
Each person ≈ 250 BTU/hr sensible
Lighting & Appliance Density (W/sq ft)
Typical residential: 1.0–2.0 W/sq ft
Primary Window Orientation
North (low solar gain) East South West (high solar gain) Mixed / All Directions
Exterior Shading
None Partial (overhangs/trees) Full shading
Calculate Heat Load
Heating Load — BTU/hr —
Cooling Load — BTU/hr —
#### Load Breakdown
Component Heating (BTU/hr) Cooling (BTU/hr)
function heaCalc() { const errEl = document.getElementById('hea-error'); const resEl = document.getElementById('hea-result'); errEl.style.display = 'none'; resEl.style.display = 'none';
// --- Read inputs --- const outdoorWinter = parseFloat(document.getElementById('hea-outdoor-winter').value); const outdoorSummer = parseFloat(document.getElementById('hea-outdoor-summer').value); const indoorWinter = parseFloat(document.getElementById('hea-indoor-winter').value); const indoorSummer = parseFloat(document.getElementById('hea-indoor-summer').value); const floorArea = parseFloat(document.getElementById('hea-floor-area').value); const ceilHeight = parseFloat(document.getElementById('hea-ceiling-height').value); const wallArea = parseFloat(document.getElementById('hea-wall-area').value); const windowArea = parseFloat(document.getElementById('hea-window-area').value); const ceilingArea = parseFloat(document.getElementById('hea-ceiling-area').value); const floorType = document.getElementById('hea-floor-type').value; const wallR = parseFloat(document.getElementById('hea-wall-rvalue').value); const ceilR = parseFloat(document.getElementById('hea-ceiling-rvalue').value); const windowU = parseFloat(document.getElementById('hea-window-ufactor').value); const windowSHGC = parseFloat(document.getElementById('hea-window-shgc').value); const floorR = parseFloat(document.getElementById('hea-floor-rvalue').value); const ach = parseFloat(document.getElementById('hea-infiltration').value); const occupants = parseFloat(document.getElementById('hea-occupants').value); const lightingDens = parseFloat(document.getElementById('hea-lighting-density').value); const orientation = document.getElementById('hea-orientation').value; const shading = document.getElementById('hea-shading').value;
// --- Validation --- const errors = []; if (isNaN(outdoorWinter) || isNaN(outdoorSummer)) errors.push("Enter valid outdoor design temperatures."); if (isNaN(indoorWinter) || isNaN(indoorSummer)) errors.push("Enter valid indoor setpoint temperatures."); if (isNaN(floorArea) || floorArea = wallArea) errors.push("Window area must be less than wall area."); if (isNaN(ceilingArea) || ceilingArea 1.20) errors.push("Window U-factor must be between 0.10 and 1.20."); if (isNaN(windowSHGC) || windowSHGC 0.87) errors.push("SHGC must be between 0.10 and 0.87."); if (isNaN(floorR) || floorR 2.0) errors.push("Infiltration ACH must be between 0.10 and 2.0."); if (isNaN(occupants) || occupants 0) { errEl.innerHTML = errors.map(e => '⚠ ' + e).join(''); errEl.style.display = 'block'; return; }
// ============================================================ // HEATING LOAD CALCULATIONS // ============================================================ const deltaT_heat = indoorWinter - outdoorWinter; // °F temperature difference
// Net opaque wall area (subtract windows) const netWallArea = wallArea - windowArea;
// Wall conduction: Q = A × U × ΔT where U = 1/R const wallU_heat = 1 / wallR; const Q_wall_heat = netWallArea * wallU_heat * deltaT_heat;
// Ceiling/roof conduction const ceilU_heat = 1 / ceilR; const Q_ceil_heat = ceilingArea * ceilU_heat * deltaT_heat;
// Window conduction: Q = A × U × ΔT const Q_win_heat = windowArea * windowU * deltaT_heat;
// Floor/foundation heat loss // Slab: perimeter × F2 factor; others: area × U × ΔT let Q_floor_heat = 0; const floorU_heat = floorR > 0 ? 1 / floorR : 1 / 2; // default R-2 if uninsulated if (floorType === 'slab') { // Slab perimeter estimate: assume square footprint const perimeterFt = 4 * Math.sqrt(floorArea); // F2 factor (BTU/hr·ft·°F): insulated slab ≈ 0.73, uninsulated ≈ 1.10 const F2 = floorR >= 5 ? 0.73 : 1.10; Q_floor_heat = perimeterFt * F2 * deltaT_heat; } else if (floorType === 'crawl') { // Vented crawlspace: full ΔT applies Q_floor_heat = floorArea * floorU_heat * deltaT_heat; } else if (floorType === 'basement') { // Conditioned basement: reduced ΔT (ground temp ≈ 55°F) const groundTemp = 55; const deltaT_basement = indoorWinter - groundTemp; Q_floor_heat = floorArea * floorU_heat * deltaT_basement; } else if (floorType === 'over-garage') { // Floor over unconditioned space: full ΔT Q_floor_heat = floorArea * floorU_heat * deltaT_heat; }
// Infiltration heat loss: // Q_inf = 0.018 × CFM × ΔT (0.018 = ρ·Cp for air in BTU/ft³·°F) // CFM = (ACH × Volume) / 60 const volume = floorArea * ceilHeight; // ft³ const cfm = (ach * volume) / 60; const Q_inf_heat = 0.018 * cfm * deltaT_heat;
// Total heating load const Q_heat_total = Q_wall_heat + Q_ceil_heat + Q_win_heat + Q_floor_heat + Q_inf_heat;
// ============================================================ // COOLING LOAD CALCULATIONS // ============================================================ const deltaT_cool = outdoorSummer - indoorSummer; // °F temperature difference
// Wall conduction (cooling) // Apply CLTD (Cooling Load Temperature Difference) correction ≈ 1.2× for solar absorption const CLTD_wall = deltaT_cool * 1.2; const Q_wall_cool = netWallArea * wallU_heat * CLTD_wall;
// Ceiling conduction (cooling) — attic amplifies solar gain const CLTD_ceil = deltaT_cool * 1.5; // attic can be 30–50°F above outdoor const Q_ceil_cool = ceilingArea * ceilU_heat * CLTD_ceil;
// Window conduction (cooling) const Q_win_cond_cool = windowArea * windowU * deltaT_cool;
// Solar heat gain through windows: // Q_solar = A × SHGC × SC_factor × shading_factor // Peak solar intensity by orientation (BTU/hr·ft²) — summer peak values const solarIntensity = { north: 30, east: 150, south: 100, west: 175, mixed: 120 }; const shadingFactor = { none: 1.0, partial: 0.75, full: 0.45 }; const peakSolar = solarIntensity[orientation] || 120; const shadeFactor = shadingFactor[shading] || 0.75; const Q_solar_cool = windowArea * windowSHGC * peakSolar * shadeFactor;
- // Floor conduction (cooling) — minimal for most floor types
- const Q_floor_cool = floorType === 'over-garage'
- ? floorArea * floorU_heat * deltaT_cool * 0.5
- 0;
// Infiltration cooling load (sensible only): const Q_inf_cool = 0.018 * cfm * deltaT_cool;
// Internal gains (sensible): // Occupants: 250 BTU/hr sensible per person (ASHRAE) const Q_occupants = occupants * 250;
// Lighting & appliances: 1 W = 3.412 BTU/hr const Q_lighting = lightingDens * floorArea * 3.412;
// Total sensible cooling load const Q_cool_sensible = Q_wall_cool + Q_ceil_cool + Q_win_cond_cool + Q_solar_cool + Q_floor_cool + Q_inf_cool + Q_occupants + Q_lighting;
// Latent cooling load (moisture removal): // Latent ≈ 20–30% of sensible for typical residential const latentFraction = 0.25; const Q_cool_latent = Q_cool_sensible * latentFraction; const Q_cool_total = Q_cool_sensible + Q_cool_latent;
// ============================================================ // DISPLAY RESULTS // ============================================================ const fmt = n => Math.round(n).toLocaleString(); const toTons = btu => (btu / 12000).toFixed(1);
document.getElementById('hea-heating-result').textContent = fmt(Q_heat_total) + ' BTU/hr'; document.getElementById('hea-heating-tons').textContent = toTons(Q_heat_total) + ' tons equivalent'; document.getElementById('hea-cooling-result').textContent = fmt(Q_cool_total) + ' BTU/hr'; document.getElementById('hea-cooling-tons').textContent = toTons(Q_cool_total) + ' tons (' + toTons(Q_cool_sensible) + ' sensible + ' + toTons(Q_cool_latent) + ' latent)';
// Breakdown table const rows = [ ['Opaque Walls', fmt(Q_wall_heat), fmt(Q_wall_cool)], ['Ceiling / Roof', fmt(Q_ceil_heat), fmt(Q_ceil_cool)], ['Windows (Conduction)', fmt(Q_win_heat), fmt(Q_win_cond_cool)], ['Windows (Solar Gain)', '—', fmt(Q_solar_cool)], ['Floor / Foundation', fmt(Q_floor_heat), fmt(Q_floor_cool)], ['Infiltration', fmt(Q_inf_heat), fmt(Q_inf_cool)], ['Occupants', '—', fmt(Q_occupants)], ['Lighting & Appliances', '—', fmt(Q_lighting)], ['Latent (Moisture)', '—', fmt(Q_cool_latent)], ['TOTAL','' + fmt(Q_heat_total) + '', '' + fmt(Q_cool_total) + ''], ];
const tbody = document.getElementById('hea-breakdown-body');
tbody.innerHTML = rows.map((r, i) => {
const bg = i === rows.length - 1 ? 'background:#dee2e6;font-weight:700;' : (i % 2 === 0 ? '' : 'background:#f1f3f5;');
return ${r[0]}
${r[1]}
${r[2]};
}).join('');
// Equipment sizing recommendation const heatTons = Q_heat_total / 12000; const coolTons = Q_cool_total / 12000; const designTons = Math.max(heatTons, coolTons); const stdSizes = [1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 5.0]; const recommended = stdSizes.find(s => s >= designTons) || (Math.ceil(designTons * 2) / 2);
document.getElementById('hea-recommendation').innerHTML =
**🏠 Equipment Sizing Recommendation**
Heating: **${toTons(Q_heat_total)} tons** (${fmt(Q_heat_total)} BTU/hr) |
Cooling: **${toTons(Q_cool_total)} tons** (${fmt(Q_cool_total)} BTU/hr)
**Recommended unit size: ${recommended} tons** — next standard size above your peak load of ${designTons.toFixed(2)} tons.
Note: Oversizing by more than 25% reduces efficiency and comfort. Consult a licensed HVAC contractor for a full Manual J calculation.;
resEl.style.display = 'block'; }
#### Formulas Used
Conduction Heat Loss/Gain: Q = A × U × ΔT where U = 1/R, ΔT = design temperature difference (°F)
Infiltration Load: Qinf = 0.018 × CFM × ΔT where CFM = (ACH × Volume) / 60 0.018 BTU/ft³·°F = density × specific heat of air
Solar Heat Gain (Cooling): Qsolar = Awindow × SHGC × Ipeak × SF where Ipeak = peak solar intensity by orientation (BTU/hr·ft²), SF = shading factor
Cooling Load Temperature Difference (CLTD): Walls: CLTD = ΔT × 1.2 (solar absorption correction) Ceiling: CLTD = ΔT × 1.5 (attic amplification)
Latent Cooling Load: Qlatent = Qsensible × 0.25 (25% latent fraction, typical residential)
Slab-on-Grade Heat Loss: Qslab = P × F2 × ΔT where P = perimeter (ft), F2 = edge loss factor (BTU/hr·ft·°F)
Tons of Refrigeration: 1 ton = 12,000 BTU/hr
#### Assumptions & References
- Based on ACCA Manual J (8th Edition) simplified residential load calculation methodology.
- Outdoor design temperatures should be obtained from ASHRAE Fundamentals Handbook or local weather data (99% heating / 1% cooling design conditions).
- Occupant sensible heat gain: 250 BTU/hr per person (ASHRAE 62.2 residential assumption).
- Latent load estimated at 25% of sensible — actual value depends on local humidity; humid climates may reach 30–40%.
- Ground temperature for conditioned basement assumed 55°F (average U.S. deep ground temperature).
- This tool provides a simplified estimate only. A full Manual J calculation by a licensed HVAC engineer is required for equipment selection and permit purposes.
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