Furnace Efficiency & AFUE Savings Calculator
ANA›Life Services Authority›National Calculator Authority›Furnace Efficiency & AFUE Savings 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; } }
Furnace Efficiency & AFUE Savings Calculator
Estimate your annual heating cost savings when upgrading from an older furnace to a higher AFUE (Annual Fuel Utilization Efficiency) rated model.
Current Furnace AFUE (%)
Typical older furnaces: 56–80%. Check your furnace nameplate or manual.
New Furnace AFUE (%)
High-efficiency furnaces: 90–98.5% AFUE. ENERGY STAR minimum: 95%.
Fuel Type
Natural Gas (therms) Propane (gallons) Heating Oil (gallons) Electric (kWh)
Annual Fuel Usage (therms)
Check your utility bills for the past 12 months of heating fuel usage.
Fuel Cost per therm ($)
Find this on your utility bill. U.S. avg: natural gas ~$1.20/therm, propane ~$2.50/gal, oil ~$3.80/gal, electric ~$0.13/kWh.
New Furnace Installed Cost ($) (optional)
Include equipment + installation. Used to calculate simple payback period.
Calculate Savings
function furUpdateFuelLabel() { var fuel = document.getElementById('fur-fuel-type').value; var unitMap = { natural_gas: ['therms', 'therm'], propane: ['gallons', 'gallon'], oil: ['gallons', 'gallon'], electric: ['kWh', 'kWh'] }; var units = unitMap[fuel]; document.getElementById('fur-unit-label').textContent = units[0]; document.getElementById('fur-cost-unit-label').textContent = units[1];
// Suggest default cost var defaultCosts = { natural_gas: 1.20, propane: 2.50, oil: 3.80, electric: 0.13 }; document.getElementById('fur-fuel-cost').placeholder = 'e.g. ' + defaultCosts[fuel].toFixed(2); }
function furCalc() { var errors = [];
var currentAFUE = parseFloat(document.getElementById('fur-current-afue').value); var newAFUE = parseFloat(document.getElementById('fur-new-afue').value); var annualUsage = parseFloat(document.getElementById('fur-annual-usage').value); var fuelCost = parseFloat(document.getElementById('fur-fuel-cost').value); var newFurnaceCost = parseFloat(document.getElementById('fur-new-cost').value); var fuelType = document.getElementById('fur-fuel-type').value;
// Validation if (isNaN(currentAFUE) || currentAFUE 99) { errors.push('Current AFUE must be between 50% and 99%.'); } if (isNaN(newAFUE) || newAFUE 99) { errors.push('New AFUE must be between 50% and 99%.'); } if (!isNaN(currentAFUE) && !isNaN(newAFUE) && newAFUE 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// Core formula: // Annual heating cost = Annual Fuel Usage × Fuel Cost per Unit // Efficiency ratio = Current AFUE / New AFUE // Annual savings = Current Annual Cost × (1 - Current AFUE / New AFUE) // // Derivation: // Heat delivered (current) = Usage_current × AFUE_current // To deliver same heat with new furnace: Usage_new = Heat_delivered / AFUE_new // Usage_new = Usage_current × (AFUE_current / AFUE_new) // Savings = (Usage_current - Usage_new) × Cost = Usage_current × Cost × (1 - AFUE_current/AFUE_new)
var currentAFUEDecimal = currentAFUE / 100; var newAFUEDecimal = newAFUE / 100;
var currentAnnualCost = annualUsage * fuelCost; var efficiencyRatio = currentAFUEDecimal / newAFUEDecimal; var newAnnualUsage = annualUsage * efficiencyRatio; var newAnnualCost = newAnnualUsage * fuelCost; var annualSavings = currentAnnualCost - newAnnualCost; var savingsPercent = (1 - efficiencyRatio) * 100;
var savings5yr = annualSavings * 5; var savings10yr = annualSavings * 10; var savings20yr = annualSavings * 20;
var unitMap2 = { natural_gas: 'therms', propane: 'gallons', oil: 'gallons', electric: 'kWh' }; var unitLabel = unitMap2[fuelType];
var paybackHTML = ''; if (!isNaN(newFurnaceCost) && newFurnaceCost > 0) { if (annualSavings > 0) { var payback = newFurnaceCost / annualSavings; var paybackYears = Math.floor(payback); var paybackMonths = Math.round((payback - paybackYears) * 12); paybackHTML = 'Simple Payback Period' + paybackYears + ' yr ' + paybackMonths + ' mo' + 'Net Savings Over 10 Years$' + (savings10yr - newFurnaceCost).toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + 'Net Savings Over 20 Years$' + (savings20yr - newFurnaceCost).toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + ''; } else { paybackHTML = 'Payback PeriodN/A (no savings)'; } }
resultDiv.innerHTML = '### Results ' + '' + 'MetricValue' + 'Current Annual Fuel Usage' + annualUsage.toLocaleString() + ' ' + unitLabel + '' + 'New Annual Fuel Usage' + newAnnualUsage.toLocaleString('en-US', {minimumFractionDigits:1, maximumFractionDigits:1}) + ' ' + unitLabel + '' + 'Fuel Saved Per Year' + (annualUsage - newAnnualUsage).toLocaleString('en-US', {minimumFractionDigits:1, maximumFractionDigits:1}) + ' ' + unitLabel + '' + 'Current Annual Heating Cost$' + currentAnnualCost.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + 'New Annual Heating Cost$' + newAnnualCost.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + 'Annual Savings$' + annualSavings.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + ' (' + savingsPercent.toFixed(1) + '%)' + '5-Year Savings$' + savings5yr.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + '10-Year Savings$' + savings10yr.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + '20-Year Savings$' + savings20yr.toLocaleString('en-US', {minimumFractionDigits:2, maximumFractionDigits:2}) + '' + paybackHTML + ''; }
#### Formula
Step 1 – Current Annual Heating Cost: Costcurrent = Usageannual × Priceper unit
Step 2 – New Annual Fuel Usage (same heat output, higher efficiency): Usagenew = Usagecurrent × (AFUEcurrent / AFUEnew) This works because: Heat delivered = Usage × AFUE. To deliver the same heat at a higher AFUE, less fuel is needed.
Step 3 – Annual Savings: Savings = (Usagecurrent − Usagenew) × Price = Costcurrent × (1 − AFUEcurrent / AFUEnew)
Step 4 – Simple Payback Period: Payback (years) = Installed Cost / Annual Savings
Example: Upgrading from 80% to 96% AFUE with 800 therms/year at $1.20/therm: New usage = 800 × (0.80/0.96) = 666.7 therms/yr Annual savings = (800 − 666.7) × $1.20 = $160/year (16.7%)
#### Assumptions & References
- AFUE (Annual Fuel Utilization Efficiency) measures what percentage of fuel is converted to usable heat over a heating season, per DOE 10 CFR Part 430.
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