HVAC Load Calculator

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.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; } }

HVAC Load Calculator

Estimate the heating and cooling load (BTU/hr) for a room or building zone based on area, insulation, climate, occupancy, and internal gains.

Floor Area (sq ft)

Ceiling Height (ft)

Outdoor Summer Design Temp (°F)

Outdoor Winter Design Temp (°F)

Indoor Design Temp (°F)

Wall/Ceiling Insulation Level

Poor (old/uninsulated, U=0.20) Average (standard, U=0.08) Good (well-insulated, U=0.05) Excellent (high-performance, U=0.03)

Wall U-Value (BTU/hr·ft²·°F) — overrides above

Window Area (sq ft)

Window U-Value (BTU/hr·ft²·°F)

Single pane (U=1.10) Double pane (U=0.50) Double pane Low-E (U=0.30) Triple pane (U=0.20)

Number of Occupants

Lighting Load (Watts)

Equipment/Appliance Load (Watts)

Air Changes per Hour (ACH)

Calculate HVAC Load

function hvaCalc() { const err = (msg) => { document.getElementById("hva-result").innerHTML = '⚠ ' + msg + ''; };

const area = parseFloat(document.getElementById("hva-area").value); const ceiling = parseFloat(document.getElementById("hva-ceiling").value); const tOutSummer = parseFloat(document.getElementById("hva-outdoor-summer").value); const tOutWinter = parseFloat(document.getElementById("hva-outdoor-winter").value); const tIndoor = parseFloat(document.getElementById("hva-indoor").value); const winArea = parseFloat(document.getElementById("hva-windows").value) || 0; const winU = parseFloat(document.getElementById("hva-window-u").value); const occupants = parseFloat(document.getElementById("hva-occupants").value) || 0; const lighting = parseFloat(document.getElementById("hva-lighting").value) || 0; const equipment = parseFloat(document.getElementById("hva-equipment").value) || 0; const ach = parseFloat(document.getElementById("hva-ach").value);

// U-value: use manual override if provided, else dropdown let wallU = parseFloat(document.getElementById("hva-insulation-u").value); if (isNaN(wallU) || wallU = tIndoor) return err("Winter outdoor temp must be below indoor temp for heating load."); if (tOutSummer 1.0) return err("Wall U-value must be between 0.01 and 1.0."); if (winU n.toLocaleString(undefined, {minimumFractionDigits:d, maximumFractionDigits:d});

document.getElementById("hva-result").innerHTML = ` ### Results

Parameter Cooling Heating

Envelope Conduction ${fmt(condCool)} BTU/hr ${fmt(condHeat)} BTU/hr

Infiltration / Ventilation ${fmt(infCool)} BTU/hr ${fmt(infHeat)} BTU/hr

Internal Gains (people + lights + equip) ${fmt(internalCool)} BTU/hr —

Total Design Load ${fmt(totalCool)} BTU/hr${fmt(coolTons,2)} tons ${fmt(totalHeat)} BTU/hr${fmt(heatKW,2)} kW

Load per sq ft ${fmt(coolPerSqFt,1)} BTU/hr·ft² ${fmt(heatPerSqFt,1)} BTU/hr·ft²

Recommended Equipment Size (×1.15 / ×1.25) ${fmt(recCoolTons,2)} tons ${fmt(recHeatKW,2)} kW

Volume: ${fmt(volume)} ft³ | Net wall area: ${fmt(netWall)} ft² | Infiltration CFM: ${fmt(cfm,1)} | ΔT cool: ${fmt(dtCool)}°F | ΔT heat: ${fmt(dtHeat)}°F

`; }

#### Formulas Used

Envelope Conduction (sensible): Qcond = U × A × ΔT [BTU/hr] Applied separately to net walls, ceiling, and windows. ΔT = |Toutdoor design − Tindoor|

Infiltration / Ventilation (sensible): CFM = (ACH × Volume) / 60 Qinf = 0.018 × CFM × ΔT [BTU/hr] (0.018 = ρ·cp for air ≈ 0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/hr)

Internal Gains (cooling only): Qpeople = N × 250 BTU/hr (sensible, seated/light activity — ASHRAE 2017 HOF) Qlighting = Watts × 3.412 BTU/W·hr Qequipment = Watts × 3.412 × 0.70 (70% heat fraction to space)

Total Cooling Load: Qcool = Qcond,cool + Qinf,cool + Qinternal Total Heating Load: Qheat = Qcond,heat + Qinf,heat (internal gains omitted — conservative)

Unit Conversions: Tons of cooling = BTU/hr ÷ 12,000 kW of heating = BTU/hr ÷ 3,412.14

Equipment Sizing (ACCA Manual S): Cooling equipment ≤ 1.15× calculated load | Heating equipment ≤ 1.25× calculated load

#### Assumptions & References

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