Water Heater Recovery Time Calculator

ANA›Life Services Authority›National Calculator Authority›Water Heater Recovery Time 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; } }

Water Heater Recovery Time Calculator

Estimate how long it takes your water heater to reheat a full tank of cold water based on tank size, BTU rating or wattage, and temperature rise.

Heater Type

Gas (BTU/hr) Electric (Watts)

Tank Size (gallons)

Burner Rating (BTU/hr)

Element Rating (Watts)

Heater Efficiency (%)

Typical gas: 60–80 %; electric: 90–98 %

Incoming Water Temperature (°F)

Target Water Temperature (°F)

Calculate

function watToggleFuel() { var fuel = document.getElementById("wat-fuel").value; document.getElementById("wat-btu-field").style.display = (fuel === "gas") ? "" : "none"; document.getElementById("wat-watt-field").style.display = (fuel === "electric") ? "" : "none"; }

function watCalc() { var resultDiv = document.getElementById("wat-result"); resultDiv.style.display = "block"; resultDiv.className = "calc-result";

// --- read inputs --- var fuel = document.getElementById("wat-fuel").value; var tank = parseFloat(document.getElementById("wat-tank").value); var btu = parseFloat(document.getElementById("wat-btu").value); var watt = parseFloat(document.getElementById("wat-watt").value); var efficiency = parseFloat(document.getElementById("wat-efficiency").value); var inlet = parseFloat(document.getElementById("wat-inlet").value); var target = parseFloat(document.getElementById("wat-target").value);

// --- validation --- var errors = []; if (isNaN(tank) || tank 100) errors.push("Efficiency must be between 1 and 100 %."); if (isNaN(inlet) || inlet 212) errors.push("Target temperature must be ≤ 212 °F."); if (!isNaN(inlet) && !isNaN(target) && target 0) { resultDiv.classList.add("calc-result-error"); resultDiv.innerHTML = "Please fix the following:" + errors.map(function(e){ return ""; }).join("") + ""; return; }

// --- core formula --- // BTU needed = gallons × 8.3454 lb/gal × 1 BTU/(lb·°F) × ΔT var deltaT = target - inlet; // °F var btuNeeded = tank * 8.3454 * deltaT; // BTU

var effectiveBTU; if (fuel === "gas") { effectiveBTU = btu * (efficiency / 100); // BTU/hr delivered } else { // 1 Watt = 3.41214 BTU/hr effectiveBTU = watt * 3.41214 * (efficiency / 100); // BTU/hr delivered }

var timeHours = btuNeeded / effectiveBTU; // hours var timeMinutes = timeHours * 60; // minutes

var hrs = Math.floor(timeHours); var mins = Math.round((timeHours - hrs) * 60); if (mins === 60) { hrs += 1; mins = 0; }

var timeStr = ""; if (hrs > 0 && mins > 0) timeStr = hrs + " hr " + mins + " min"; else if (hrs > 0) timeStr = hrs + " hr"; else timeStr = mins + " min";

// first-hour delivery (FHD) — usable hot water in first hour // FHD ≈ 0.7 × tank + (effectiveBTU / (8.3454 × deltaT)) var fhd = 0.7 * tank + (effectiveBTU / (8.3454 * deltaT));

resultDiv.innerHTML = "### Results " + "" + "Temperature Rise" + deltaT.toFixed(1) + " °F" + "BTU Required" + btuNeeded.toLocaleString(undefined,{maximumFractionDigits:0}) + " BTU" + "Effective Heat Output" + effectiveBTU.toLocaleString(undefined,{maximumFractionDigits:0}) + " BTU/hr" + "Recovery Time" + timeStr + " (" + timeMinutes.toFixed(1) + " min)" + "Est. First-Hour Delivery" + fhd.toFixed(1) + " gallons" + ""; }

#### Formulas Used

BTU Required:

BTU = Gallons × 8.3454 (lb/gal) × ΔT (°F)

Where ΔT = Target Temp − Inlet Temp

Effective Heat Output (Gas):

Effective BTU/hr = Burner BTU/hr × (Efficiency / 100)

Effective Heat Output (Electric):

Effective BTU/hr = Watts × 3.41214 × (Efficiency / 100)

Recovery Time:

Time (hours) = BTU Required / Effective BTU/hr

First-Hour Delivery (FHD) Estimate:

FHD ≈ 0.70 × Tank Size + (Effective BTU/hr / (8.3454 × ΔT))

The 0.70 factor accounts for the ~70 % of stored hot water usable before cold dilution drops temperature below acceptable levels.

#### Assumptions & References

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References

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