HVAC System BTU Sizing Calculator

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HVAC System BTU Sizing Calculator

Estimate the heating and cooling BTU capacity required for your space using the Manual J load calculation methodology.

### Room Dimensions

Room Length (ft)

Room Width (ft)

Ceiling Height (ft)

Total Window Area (sq ft)

Number of Exterior Doors

### Climate & Exposure

Climate Zone

Hot & Humid (e.g. Miami, Houston) Hot & Dry (e.g. Phoenix, Las Vegas) Mixed / Moderate (e.g. Atlanta, Dallas) Cold (e.g. Chicago, Denver) Very Cold (e.g. Minneapolis, Anchorage)

Sun Exposure

Mostly Shaded Average Mostly Sunny / South-Facing

Floor Type

Concrete Slab Crawl Space Basement / Conditioned Below Above Conditioned Space

Number of Stories

1 Story 2 Stories 3+ Stories

### Insulation & Construction

Insulation Quality

Poor (pre-1970s, no insulation) Average (1970s–1990s) Good (2000s construction) Excellent (modern / Energy Star)

Window Type

Single Pane Double Pane Triple Pane / Low-E

Number of Occupants

Kitchen Present?

Yes No

Calculate BTU Requirements

### BTU Sizing Results

function hvaCalc() { const errEl = document.getElementById('hva-error'); const resEl = document.getElementById('hva-result'); errEl.style.display = 'none'; resEl.style.display = 'none';

// --- Gather inputs --- const length = parseFloat(document.getElementById('hva-length').value); const width = parseFloat(document.getElementById('hva-width').value); const ceiling = parseFloat(document.getElementById('hva-ceiling').value); const winArea = parseFloat(document.getElementById('hva-windows').value); const doors = parseInt(document.getElementById('hva-doors').value) || 0; const occupants = parseInt(document.getElementById('hva-occupants').value) || 0;

const climate = document.getElementById('hva-climate').value; const sun = document.getElementById('hva-sun').value; const floor = document.getElementById('hva-floor').value; const stories = parseInt(document.getElementById('hva-stories').value); const insulation = document.getElementById('hva-insulation').value; const winType = document.getElementById('hva-windows-type').value; const kitchen = document.getElementById('hva-kitchen').value;

// --- Validation --- const errors = []; if (isNaN(length) || length floorArea * 0.5) errors.push("Window area seems too large (> 50% of floor area). Please verify."); if (errors.length) { errEl.innerHTML = errors.map(e => '⚠ ' + e).join(''); errEl.style.display = 'block'; return; }

// ============================================================ // MANUAL J SIMPLIFIED LOAD CALCULATION // Reference: ACCA Manual J (8th Edition) simplified method // BTU/hr = Area × ΔT × U-value (conduction) + infiltration + internal gains // ============================================================

// --- Design Temperature Differences (°F) --- // Cooling: indoor 75°F vs outdoor design temp // Heating: indoor 70°F vs outdoor design temp const climateData = { hot_humid: { coolDT: 20, heatDT: 25, coolingFactor: 1.15 }, hot_dry: { coolDT: 30, heatDT: 20, coolingFactor: 1.20 }, mixed: { coolDT: 18, heatDT: 35, coolingFactor: 1.00 }, cold: { coolDT: 12, heatDT: 55, coolingFactor: 0.90 }, very_cold: { coolDT: 8, heatDT: 75, coolingFactor: 0.85 } }; const cd = climateData[climate];

// --- Overall U-values (BTU/hr·ft²·°F) by insulation quality --- // Wall U-values (includes framing factor) const wallU = { poor: 0.20, average: 0.10, good: 0.065, excellent: 0.040 }; // Ceiling/roof U-values const ceilU = { poor: 0.15, average: 0.065, good: 0.038, excellent: 0.022 }; // Floor U-values const floorU_map = { slab: { poor: 0.18, average: 0.10, good: 0.06, excellent: 0.04 }, crawl: { poor: 0.15, average: 0.08, good: 0.05, excellent: 0.03 }, basement: { poor: 0.05, average: 0.03, good: 0.02, excellent: 0.01 }, above: { poor: 0.02, average: 0.01, good: 0.005, excellent: 0.003 } };

// --- Window U-values and SHGC --- const windowData = { single: { U: 1.10, SHGC: 0.86 }, double: { U: 0.48, SHGC: 0.57 }, triple: { U: 0.22, SHGC: 0.32 } }; const wd = windowData[winType];

// --- Sun exposure multiplier for solar heat gain --- const sunMult = { shaded: 0.6, average: 1.0, sunny: 1.4 };

// --- Geometry ---

const ceilArea = floorArea; // ft²

const perimeter = 2 * (length + width); // ft

const wallArea = perimeter * ceiling // gross wall area ft²

- winArea // minus windows

- doors * 20; // minus doors (avg 20 ft² each)

const doorArea = doors * 20; // ft²

const doorU = insulation === 'excellent' ? 0.17

insulation === 'good' ? 0.25

insulation === 'average' ? 0.35 : 0.50;

// --- Volume & ACH (Air Changes per Hour) ---

const volume = floorArea * ceiling; // ft³

const ach = insulation === 'excellent' ? 0.25

insulation === 'good' ? 0.40

insulation === 'average' ? 0.60 : 1.00; // Infiltration load factor: 0.018 BTU/ft³·°F (air at standard conditions) const infiltFactor = 0.018;

// ============================================================ // COOLING LOAD (BTU/hr) // ============================================================ const coolDT = cd.coolDT;

// Conduction loads (cooling) const wallCoolCond = wallArea * wallU[insulation] * coolDT; const ceilCoolCond = ceilArea * ceilU[insulation] * coolDT; const floorCoolCond = floorArea * floorU_map[floor][insulation] * coolDT; const winCoolCond = winArea * wd.U * coolDT; const doorCoolCond = doorArea * doorU * coolDT;

// Solar heat gain through windows (cooling only) // SHGC × Solar irradiance (avg 200 BTU/hr·ft² peak, use 150 for average day) const solarIrr = 150; // BTU/hr·ft² average peak const solarGain = winArea * wd.SHGC * solarIrr * sunMult[sun];

// Infiltration (cooling) const infiltCool = volume * ach * infiltFactor * coolDT;

// Internal gains const occupantGain = occupants * 250; // 250 BTU/hr per person (sensible) const kitchenGain = kitchen === 'yes' ? 1200 : 0; // kitchen appliances const lightingGain = floorArea * 3; // 3 BTU/hr per ft² lighting estimate

// Multi-story factor (more exterior surface per floor area) const storyFactor = stories === 1 ? 1.0 : stories === 2 ? 0.95 : 0.90;

const totalCoolingBTU = ( wallCoolCond + ceilCoolCond + floorCoolCond + winCoolCond + doorCoolCond + solarGain + infiltCool + occupantGain + kitchenGain + lightingGain ) * cd.coolingFactor * storyFactor;

// ============================================================ // HEATING LOAD (BTU/hr) // ============================================================ const heatDT = cd.heatDT;

// Conduction loads (heating) — no solar credit in heating calc (conservative) const wallHeatCond = wallArea * wallU[insulation] * heatDT; const ceilHeatCond = ceilArea * ceilU[insulation] * heatDT; const floorHeatCond = floorArea * floorU_map[floor][insulation] * heatDT; const winHeatCond = winArea * wd.U * heatDT; const doorHeatCond = doorArea * doorU * heatDT;

// Infiltration (heating) const infiltHeat = volume * ach * infiltFactor * heatDT;

const totalHeatingBTU = ( wallHeatCond + ceilHeatCond + floorHeatCond + winHeatCond + doorHeatCond + infiltHeat ) * storyFactor;

// ============================================================ // TONNAGE & RECOMMENDATIONS // ============================================================ const coolingTons = totalCoolingBTU / 12000; const heatingTons = totalHeatingBTU / 12000;

// Round up to nearest 0.5 ton for equipment sizing const roundHalfUp = v => Math.ceil(v * 2) / 2; const recCoolTons = roundHalfUp(coolingTons); const recHeatTons = roundHalfUp(heatingTons);

// SEER recommendation based on climate

const seerRec = (climate === 'hot_humid' || climate === 'hot_dry') ? '18–21 SEER2'

climate === 'mixed' ? '16–18 SEER2' : '14–16 SEER2';

const afueRec = (climate === 'cold' || climate === 'very_cold') ? '96–98% AFUE'

'80–96% AFUE';

// ============================================================ // OUTPUT // ============================================================ const fmt = n => n.toLocaleString('en-US', {maximumFractionDigits: 0}); const fmtD = (n, d=1) => n.toLocaleString('en-US', {minimumFractionDigits: d, maximumFractionDigits: d});

document.getElementById('hva-output').innerHTML = `

ParameterValue

Floor Area${fmt(floorArea)} ft² Volume${fmt(volume)} ft³ Net Wall Area${fmt(Math.max(0,wallArea))} ft² Window Area${fmt(winArea)} ft² Total Cooling Load${fmt(totalCoolingBTU)} BTU/hr   — Conduction (walls, ceiling, floor, windows, doors) ${fmt(wallCoolCond+ceilCoolCond+floorCoolCond+winCoolCond+doorCoolCond)} BTU/hr   — Solar Heat Gain${fmt(solarGain)} BTU/hr   — Infiltration${fmt(infiltCool)} BTU/hr   — Internal Gains (people, kitchen, lighting) ${fmt(occupantGain+kitchenGain+lightingGain)} BTU/hr Cooling Capacity (Tons)${fmtD(coolingTons)} tons → recommend ${fmtD(recCoolTons)} tons Total Heating Load${fmt(totalHeatingBTU)} BTU/hr   — Conduction ${fmt(wallHeatCond+ceilHeatCond+floorHeatCond+winHeatCond+doorHeatCond)} BTU/hr   — Infiltration${fmt(infiltHeat)} BTU/hr Heating Capacity (Tons equiv.)${fmtD(heatingTons)} tons → recommend ${fmtD(recHeatTons)} tons Recommended Cooling Efficiency${seerRec} Recommended Heating Efficiency${afueRec} Design Cooling ΔT${cd.coolDT}°F Design Heating ΔT${cd.heatDT}°F Infiltration ACH${ach} ACH

⚠️ This is a simplified Manual J estimate. A certified HVAC professional should perform a full Manual J load calculation before equipment purchase.

`; resEl.style.display = 'block'; }

#### Formulas Used

Cooling / Heating Conduction Load (BTU/hr):

Q_cond = A × U × ΔT Where A = surface area (ft²), U = overall heat transfer coefficient (BTU/hr·ft²·°F), ΔT = design temperature difference (°F).

Solar Heat Gain (cooling only):

Q_solar = A_window × SHGC × I_solar × SunMultiplier Where SHGC = Solar Heat Gain Coefficient, I_solar = 150 BTU/hr·ft² (average peak irradiance).

Infiltration Load (BTU/hr):

Q_inf = Volume × ACH × 0.018 × ΔT Where 0.018 BTU/ft³·°F is the volumetric heat capacity of air at standard conditions, ACH = air changes per hour.

Internal Gains (cooling):

Q_internal = (Occupants × 250) + Kitchen + (Floor Area × 3) 250 BTU/hr per person (sensible), 1,200 BTU/hr for kitchen, 3 BTU/hr·ft² for lighting.

Total Cooling Load:

Q_cool = (Q_cond + Q_solar + Q_inf + Q_internal) × ClimateFactor × StoryFactor

Tonnage:

Tons = BTU/hr ÷ 12,000 1 ton of refrigeration = 12,000 BTU/hr. Equipment is rounded up to the nearest 0.5 ton.

#### Assumptions & References

• Based on ACCA Manual J (8th Edition) simplified residential load calculation methodology.

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