Tankless Water Heater Flow Rate Calculator
ANA›Life Services Authority›National Calculator Authority›Tankless Water Heater Flow Rate Calculator
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Tankless Water Heater Flow Rate Calculator
Determine the minimum flow rate (GPM) your tankless water heater must deliver to meet your household hot water demand.
Incoming Cold Water Temperature (°F)
Typical groundwater temperature varies by region (40–70 °F).
Desired Hot Water Temperature (°F)
Recommended safe delivery temperature is 120 °F (OSHA / ASHRAE).
Heater Power / BTU Input (BTU/hr)
Check your unit's spec sheet. Common residential units: 120,000–199,000 BTU/hr.
Heater Thermal Efficiency (%)
Most condensing tankless heaters: 90–98 %. Non-condensing: 80–85 %.
Calculate
function tanCalc() { const resultDiv = document.getElementById('tan-result'); resultDiv.innerHTML = '';
const Tin = parseFloat(document.getElementById('tan-inlet-temp').value); const Tout = parseFloat(document.getElementById('tan-outlet-temp').value); const Q = parseFloat(document.getElementById('tan-heater-power').value); const eff = parseFloat(document.getElementById('tan-efficiency').value);
// --- Validation --- const errors = []; if (isNaN(Tin) || Tin 80) errors.push('Inlet temperature must be between 32 °F and 80 °F.'); if (isNaN(Tout) || Tout 160) errors.push('Outlet temperature must be between 90 °F and 160 °F.'); if (!isNaN(Tin) && !isNaN(Tout) && Tout 1000000) errors.push('Heater power must be between 10,000 and 1,000,000 BTU/hr.'); if (isNaN(eff) || eff 100) errors.push('Efficiency must be between 50 % and 100 %.');
if (errors.length > 0) { resultDiv.innerHTML = '' + errors.map(e => '').join('') + ''; return; }
// --- Core Formula --- // Flow Rate (GPM) = (Q_input × η) / (500.4 × ΔT) // // Where: // Q_input = heater BTU/hr input // η = thermal efficiency (decimal) // 500.4 = unit conversion constant = 8.34 lb/gal × 60 min/hr × 1 BTU/(lb·°F) // ΔT = Tout − Tin (°F) // // Derived from: Q = m_dot × Cp × ΔT // m_dot (lb/min) = GPM × 8.34 lb/gal // Q (BTU/min) = BTU/hr ÷ 60
const deltaT = Tout - Tin; const etaDec = eff / 100; const CONSTANT = 500.4; // 8.34 × 60
const flowGPM = (Q * etaDec) / (CONSTANT * deltaT);
// Useful heat delivered const usefulBTU = Q * etaDec;
// Cross-check: temperature rise achievable at common fixture flow rates const showerGPM = 2.0; // low-flow showerhead const faucetGPM = 1.5; const dishwasherGPM = 1.2;
function tempRise(gpm) { return (usefulBTU / (CONSTANT * gpm)).toFixed(1); }
// Simultaneous use examples const simultaneous = [ { label: '1 shower (2.0 GPM)', gpm: 2.0 }, { label: '1 shower + 1 faucet (3.5 GPM)', gpm: 3.5 }, { label: '2 showers (4.0 GPM)', gpm: 4.0 }, { label: '2 showers + 1 faucet (5.5 GPM)', gpm: 5.5 }, ];
let simRows = simultaneous.map(s => { const rise = (usefulBTU / (CONSTANT * s.gpm)); const achievedTemp = Tin + rise; const ok = achievedTemp >= Tout; return '' + '' + s.label + '' + '' + s.gpm.toFixed(1) + ' GPM' + '' + rise.toFixed(1) + ' °F' + '' + achievedTemp.toFixed(1) + ' °F' + '' + (ok ? '✔ Sufficient' : '✗ Insufficient') + '' + ''; }).join('');
resultDiv.innerHTML = '### Results ' + '' + 'ParameterValue' + 'Temperature Rise Required (ΔT)' + deltaT.toFixed(1) + ' °F' + 'Useful Heat Delivered' + usefulBTU.toLocaleString(undefined,{maximumFractionDigits:0}) + ' BTU/hr' + 'Maximum Flow Rate at ' + deltaT.toFixed(0) + ' °F Rise' + flowGPM.toFixed(2) + ' GPM' + '' +
'#### Simultaneous Use Analysis ' + '' + 'ScenarioTotal FlowTemp RiseDelivered TempStatus' + simRows + '' +
'' + 'Recommendation: Select a unit rated at least ' + Math.ceil(flowGPM * 10) / 10 + ' GPM to satisfy your peak demand of ' + deltaT.toFixed(0) + ' °F temperature rise.' + '
'; }
#### Formula
Flow Rate (GPM) = (Qinput × η) / (500.4 × ΔT)
- Qinput — Heater BTU/hr input rating
- η — Thermal efficiency (decimal, e.g. 0.95)
- 500.4 — Unit conversion constant = 8.34 lb/gal × 60 min/hr × 1 BTU/(lb·°F)
- ΔT — Temperature rise = Tout − Tin (°F)
Derived from the steady-state heat transfer equation: Q = ṁ × Cp × ΔT, where water's specific heat Cp ≈ 1 BTU/(lb·°F) and density ≈ 8.34 lb/gal.
#### Assumptions & References
- Recommended hot water delivery temperature: 120 °F per OSHA 3261 and ASHRAE 188 to prevent scalding and Legionella growth.
- Formula source: ASHRAE Fundamentals Handbook, Chapter on Service Water Heating.
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