HVAC Load Calculation (Manual J Estimator)

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HVAC Load Calculation (Manual J Estimator)

Estimates residential heating and cooling loads based on ACCA Manual J methodology. Enter your home's characteristics to calculate the required HVAC system capacity in BTU/hr and tons.

Conditioned Floor Area (sq ft)

Average Ceiling Height (ft)

Outdoor Summer Design Temp (°F)

Outdoor Winter Design Temp (°F)

Indoor Summer Setpoint (°F)

Indoor Winter Setpoint (°F)

Wall Insulation Level

Poor (R-7, older construction) Standard (R-13, 2x4 walls) Good (R-19, 2x6 walls) Excellent (R-25+, spray foam)

Ceiling/Attic Insulation Level

Poor (R-11) Standard (R-19 to R-25) Good (R-30 to R-38) Excellent (R-49+)

Total Window Area (sq ft)

Window Type

Single pane (U=1.1) Double pane (U=0.5) Double pane Low-E (U=0.35) Triple pane Low-E (U=0.25)

Air Infiltration / Leakage

Leaky (older home, ACH=0.6) Average (ACH=0.4) Tight (ACH=0.25) Very Tight (ACH=0.15, Energy Star)

Number of Occupants

Climate Zone

Hot & Humid (Zone 1-2, FL, TX Gulf) Mixed Humid (Zone 3-4, SE/Mid-Atlantic) Mixed Dry (Zone 3-4, SW interior) Cold (Zone 5-6, Midwest/NE) Very Cold (Zone 7, MN, MT, ND)

Number of Stories

1 Story 2 Stories 3 Stories

Calculate HVAC Load

function hvaCalc() { // --- Gather Inputs --- var floorArea = parseFloat(document.getElementById('hva-floor-area').value); var ceilHeight = parseFloat(document.getElementById('hva-ceiling-height').value); var outdoorSummer = parseFloat(document.getElementById('hva-outdoor-summer').value); var outdoorWinter = parseFloat(document.getElementById('hva-outdoor-winter').value); var indoorSummer = parseFloat(document.getElementById('hva-indoor-summer').value); var indoorWinter = parseFloat(document.getElementById('hva-indoor-winter').value); var wallU = parseFloat(document.getElementById('hva-wall-insulation').value); var ceilU = parseFloat(document.getElementById('hva-ceiling-insulation').value); var windowArea = parseFloat(document.getElementById('hva-window-area').value); var windowU = parseFloat(document.getElementById('hva-window-type').value); var ach = parseFloat(document.getElementById('hva-infiltration').value); var occupants = parseFloat(document.getElementById('hva-occupants').value); var climateZone = document.getElementById('hva-climate-zone').value; var stories = parseFloat(document.getElementById('hva-stories').value);

var errors = []; if (isNaN(floorArea) || floorArea 20000) errors.push("Floor area must be 100–20,000 sq ft."); if (isNaN(ceilHeight) || ceilHeight 20) errors.push("Ceiling height must be 7–20 ft."); if (isNaN(outdoorSummer) || outdoorSummer 120) errors.push("Outdoor summer temp must be 70–120°F."); if (isNaN(outdoorWinter) || outdoorWinter 60) errors.push("Outdoor winter temp must be -30–60°F."); if (isNaN(indoorSummer) || indoorSummer 80) errors.push("Indoor summer setpoint must be 68–80°F."); if (isNaN(indoorWinter) || indoorWinter 78) errors.push("Indoor winter setpoint must be 60–78°F."); if (isNaN(windowArea) || windowArea 5000) errors.push("Window area must be 0–5,000 sq ft."); if (isNaN(occupants) || occupants 20) errors.push("Occupants must be 1–20."); if (outdoorSummer 0) { document.getElementById('hva-result').innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --------------------------------------------------------------- // MANUAL J CORE CALCULATIONS // ---------------------------------------------------------------

// --- Geometry --- var volume = floorArea * ceilHeight; // ft³ var wallArea = (floorArea / stories) * 4 * Math.sqrt(floorArea / stories) / stories * stories; // Perimeter-based wall area: assume square footprint per story var footprint = floorArea / stories; var sideLength = Math.sqrt(footprint); var perimeter = 4 * sideLength; var grossWallArea = perimeter * ceilHeight * stories; // ft² var netWallArea = Math.max(grossWallArea - windowArea, 0); // ft²

// --- Design Temperature Differences --- var deltaT_cool = outdoorSummer - indoorSummer; // °F cooling ΔT var deltaT_heat = indoorWinter - outdoorWinter; // °F heating ΔT

// --- Envelope Conduction Loads (Q = U × A × ΔT) --- // Cooling var wallCool = wallU * netWallArea * deltaT_cool; // BTU/hr var ceilCool = ceilU * floorArea * deltaT_cool; var windowCool = windowU * windowArea * deltaT_cool;

// Heating var wallHeat = wallU * netWallArea * deltaT_heat; var ceilHeat = ceilU * floorArea * deltaT_heat; var windowHeat = windowU * windowArea * deltaT_heat;

// Floor/slab (simplified — assume R-5 effective for slab/crawl) var floorU = 0.10; var floorCool = floorU * floorArea * (deltaT_cool * 0.5); // reduced ΔT for ground contact var floorHeat = floorU * floorArea * (deltaT_heat * 0.5);

// --- Solar Heat Gain (Cooling Only) --- // SHGC varies by climate; assume 0.25 Low-E default, 0.87 single pane var shgcMap = { 1.1: 0.86, 0.5: 0.40, 0.35: 0.25, 0.25: 0.20 }; var shgc = shgcMap[windowU] || 0.35; // Peak solar irradiance by climate zone (BTU/hr·ft²) var solarMap = { hot_humid: 200, mixed_humid: 185, mixed_dry: 220, cold: 170, very_cold: 155 }; var solarIrr = solarMap[climateZone] || 185; // Assume 40% of windows face south/west (worst case) var solarGain = shgc * solarIrr * windowArea * 0.40; // BTU/hr

// --- Infiltration Load --- // Q_inf = 1.1 × CFM × ΔT (sensible) // CFM = (ACH × Volume) / 60 var cfm = (ach * volume) / 60; var infCool = 1.1 * cfm * deltaT_cool; var infHeat = 1.1 * cfm * deltaT_heat;

// --- Latent Cooling Load --- // Q_lat = 0.68 × CFM × ΔW (grain difference) // ΔW by climate zone (grains/lb) var latentMap = { hot_humid: 50, mixed_humid: 38, mixed_dry: 20, cold: 18, very_cold: 12 }; var deltaW = latentMap[climateZone] || 30; var latentInf = 0.68 * cfm * deltaW;

// --- Internal Gains (Cooling Only) --- // People: 250 BTU/hr sensible + 200 BTU/hr latent per person (ASHRAE) var peopleSens = occupants * 250; var peopleLat = occupants * 200; // Lighting & appliances: ~3 BTU/hr per sq ft (residential estimate) var applianceGain = floorArea * 3.0;

// --- Duct Loss/Gain Factor --- // Assume ducts in unconditioned attic/crawl: 15% penalty var ductFactor = 1.15;

// --------------------------------------------------------------- // TOTALS // ---------------------------------------------------------------

// Sensible Cooling Load var sensibleCool = (wallCool + ceilCool + windowCool + floorCool + solarGain + infCool + peopleSens + applianceGain) * ductFactor;

// Latent Cooling Load var latentCool = (latentInf + peopleLat) * ductFactor;

// Total Cooling Load var totalCool = sensibleCool + latentCool; // BTU/hr var coolTons = totalCool / 12000; // 1 ton = 12,000 BTU/hr

// Total Heating Load var totalHeat = (wallHeat + ceilHeat + windowHeat + floorHeat + infHeat) * ductFactor; var heatKBtu = totalHeat / 1000; // kBTU/hr

// Sensible Heat Ratio var shr = sensibleCool / totalCool;

// Recommended equipment sizes (round up to nearest 0.5 ton / 10 kBTU) var recCoolTons = Math.ceil(coolTons * 2) / 2; var recHeatKBtu = Math.ceil(heatKBtu / 10) * 10;

// --------------------------------------------------------------- // OUTPUT // --------------------------------------------------------------- function fmt(n, d) { return n.toFixed(d !== undefined ? d : 0).replace(/\B(?=(\d{3})+(?!\d))/g, ','); }

document.getElementById('hva-result').innerHTML = '### Manual J Load Estimate Results ' +

'' + 'Load ComponentCooling (BTU/hr)Heating (BTU/hr)' + '' + 'Wall Conduction' + fmt(wallCool) + '' + fmt(wallHeat) + '' + 'Ceiling / Roof' + fmt(ceilCool) + '' + fmt(ceilHeat) + '' + 'Windows (Conduction)' + fmt(windowCool) + '' + fmt(windowHeat) + '' + 'Floor / Slab' + fmt(floorCool) + '' + fmt(floorHeat) + '' + 'Solar Heat Gain' + fmt(solarGain) + '—' + 'Infiltration (Sensible)' + fmt(infCool) + '' + fmt(infHeat) + '' + 'Infiltration (Latent)' + fmt(latentInf) + '—' + 'People (Sensible + Latent)' + fmt(peopleSens + peopleLat) + '—' + 'Appliances / Lighting' + fmt(applianceGain) + '—' + 'Duct Loss Factor (15%)Applied to all components' + '' + '' + 'Sensible Cooling' + fmt(sensibleCool) + ' BTU/hr—' + 'Latent Cooling' + fmt(latentCool) + ' BTU/hr—' + 'TOTAL LOAD' + fmt(totalCool) + ' BTU/hr' + fmt(totalHeat) + ' BTU/hr' + '' +

'' + '#### Equipment Sizing Summary ' + 'Cooling Load: ' + fmt(totalCool) + ' BTU/hr  |  ' + fmt(coolTons, 2) + ' tons

' + 'Heating Load: ' + fmt(totalHeat) + ' BTU/hr  |  ' + fmt(heatKBtu, 1) + ' kBTU/hr

' + 'Sensible Heat Ratio (SHR): ' + fmt(shr * 100, 1) + '%

' + 'Infiltration CFM: ' + fmt(cfm, 1) + ' CFM  (ACH = ' + ach + ')

' + '' + 'Recommended Cooling Unit: ' + fmt(recCoolTons, 1) + ' tons (' + fmt(recCoolTons * 12000) + ' BTU/hr)

' + 'Recommended Heating Unit: ' + fmt(recHeatKBtu) + ' kBTU/hr output

' + '⚠ Round up to the next standard equipment size. A licensed HVAC contractor should perform a full Manual J calculation before purchase.

' + ''; }

#### Formulas Used

Conduction Load: Q = U × A × ΔT

Solar Heat Gain: Qsolar = SHGC × Isolar × Awindow × orientation factor

Infiltration (Sensible): Qinf = 1.1 × CFM × ΔT  |  CFM = (ACH × Volume) / 60

Infiltration (Latent): Qlat = 0.68 × CFM × ΔW  (ΔW in grains/lb)

People Load: 250 BTU/hr sensible + 200 BTU/hr latent per occupant (ASHRAE 62.2)

Total Cooling: Qtotal = (Qwall + Qceil + Qwin + Qfloor + Qsolar + Qinf + Qpeople + Qappliance) × duct factor

Tons of Cooling: Tons = Total BTU/hr ÷ 12,000

#### Assumptions & References

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