Furnace Efficiency Calculator — AFUE to Annual Fuel Cost
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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; } }
Furnace Efficiency Calculator — AFUE to Annual Fuel Cost
Enter your furnace's AFUE rating, annual heat requirement, and local fuel price to estimate your yearly heating fuel cost.
AFUE Rating (%)
Annual Fuel Utilization Efficiency — found on your furnace label or manual. Typical range: 56%–98.5%.
Fuel Type
Natural Gas Propane Heating Oil Electricity
Fuel Price (per therm)
U.S. average natural gas: ~$1.10–$1.50/therm.
Method for Annual Heat Requirement
Enter Annual Heat Load Directly (MMBtu/yr) Calculate from Heating Degree Days
Annual Heat Load (MMBtu/year)
Typical U.S. home: 40–150 MMBtu/year depending on climate and size.
Annual Heating Degree Days (HDD, base 65°F)
Find your city's HDD at NOAA or DegreeDays.net. Examples: Miami ~200, Chicago ~6500, Minneapolis ~8000.
Conditioned Floor Area (sq ft)
Overall Heat Loss Coefficient — UA (BTU/hr·°F)
UA = Home heat loss rate. Rule of thumb: ~0.25 × floor area for average insulation (e.g., 2000 sq ft → ~500 BTU/hr·°F). Better insulation = lower UA.
Calculate Annual Fuel Cost
const furFuelData = { natural_gas: { unit: "therm", btuPerUnit: 100000, priceHint: "U.S. average: ~$1.10–$1.50/therm.", label: "per therm" }, propane: { unit: "gallon", btuPerUnit: 91452, priceHint: "U.S. average: ~$2.50–$3.50/gallon.", label: "per gallon" }, oil: { unit: "gallon", btuPerUnit: 138500, priceHint: "U.S. average: ~$3.50–$4.50/gallon.", label: "per gallon" }, electricity: { unit: "kWh", btuPerUnit: 3412, priceHint: "U.S. average: ~$0.12–$0.18/kWh.", label: "per kWh" } };
function furUpdateFuelUnits() { const fuel = document.getElementById("fur-fuel-type").value; const data = furFuelData[fuel]; document.getElementById("fur-price-unit").textContent = data.label; document.getElementById("fur-price-hint").textContent = data.priceHint; // Electricity note: AFUE doesn't apply to heat pumps; this covers electric resistance furnaces }
function furToggleHeatMethod() { const method = document.getElementById("fur-heat-input").value; document.getElementById("fur-direct-section").style.display = (method === "direct") ? "" : "none"; document.getElementById("fur-dd-section").style.display = (method === "degree_days") ? "" : "none"; }
function furCalc() { const resultDiv = document.getElementById("fur-result"); resultDiv.style.display = "none";
// --- Collect & validate AFUE --- const afueRaw = parseFloat(document.getElementById("fur-afue").value); if (isNaN(afueRaw) || afueRaw 100) { resultDiv.style.display = "block"; resultDiv.innerHTML = '⚠️ Please enter a valid AFUE between 50% and 100%.
'; return; } const afue = afueRaw / 100; // decimal
// --- Fuel --- const fuelType = document.getElementById("fur-fuel-type").value; const fuelData = furFuelData[fuelType]; const fuelPrice = parseFloat(document.getElementById("fur-fuel-price").value); if (isNaN(fuelPrice) || fuelPrice n.toLocaleString(undefined, {minimumFractionDigits: dec, maximumFractionDigits: dec}); const fmtInt = (n) => n.toLocaleString(undefined, {maximumFractionDigits: 0});
let savingsRow = "";
if (afueRaw Savings vs. 95% AFUE furnaceYou'd save $${fmt(savings95)}/yr by upgrading;
} else {
savingsRow =Savings vs. 95% AFUE furnaceAlready at or above 95% AFUE — excellent efficiency!`;
}
let vs80Row = "";
if (afueRaw > 80) {
vs80Row = Savings vs. 80% AFUE furnaceSaving **$${fmt(savingsVs80)}/yr** vs. an 80% AFUE unit;
} else if (afueRaw Savings vs. 80% AFUE furnaceAn 80% AFUE unit would save $${fmt(-savingsVs80)}/yr`;
}
resultDiv.style.display = "block"; resultDiv.innerHTML = ` ### Results
ParameterValue AFUE Rating${afueRaw}% Annual Heat Requirement${fmt(heatLoadMMBtu, 2)} MMBtu/year Annual Fuel Consumed${fmt(annualFuelUnits, 1)} ${fuelData.unit}s/year Fuel Price$${fmt(fuelPrice)} / ${fuelData.unit} 💰 Estimated Annual Fuel Cost$${fmt(annualCost)} / year Monthly Average$${fmt(annualCost / 12)} / month ${savingsRow} ${vs80Row}
${heatLoadNote}
`; }
#### Formula
Annual Fuel Cost = (Annual Heat Load ÷ AFUE) ÷ BTU_per_unit × Fuel_Price
- Annual Heat Load — total useful heat energy your home needs each year (BTU/yr or MMBtu/yr).
- AFUE (Annual Fuel Utilization Efficiency) — fraction of fuel energy converted to useful heat (e.g., 0.80 for 80%).
- Annual Fuel Input = Annual Heat Load ÷ AFUE — accounts for combustion losses up the flue.
- Annual Fuel Units = Annual Fuel Input (BTU) ÷ BTU content per unit of fuel.
- Annual Cost = Annual Fuel Units × Price per unit.
Degree-Day Heat Load Method:
Annual Heat Load (BTU/yr) = UA × HDD × 24
- UA — overall building heat loss coefficient (BTU/hr·°F); sum of all envelope conductances.
- HDD — annual Heating Degree Days (base 65°F); measures heating season severity.
- 24 — converts degree-days (°F·day) to degree-hours (°F·hr) to match BTU/hr·°F units.
Fuel BTU Content (HHV): Natural Gas ≈ 100,000 BTU/therm | Propane ≈ 91,452 BTU/gallon | Heating Oil ≈ 138,500 BTU/gallon | Electricity ≈ 3,412 BTU/kWh.
#### Assumptions & References
- AFUE is defined by ANSI/ASHRAE Standard 103 and regulated by the U.S. Department of Energy (DOE). Minimum federal standard for gas furnaces is 80% AFUE (78% in some regions); high-efficiency units reach 95%–98.5%.
- Fuel BTU values are higher heating values (HHV) per EIA (U.S. Energy Information Administration).
- Heating Degree Day data sourced from NOAA Climate Data Online or DegreeDays.net (base 65°F standard).
- The UA × HDD × 24 method is a standard simplified steady-state heat loss model per ASHRAE Fundamentals Handbook.
- Fuel prices are national averages and vary significantly by region and season. Check current prices at EIA.gov.
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