function updateLabels() {
var source = document.getElementById("heatSource").value;
var powerLabel = document.getElementById("powerLabel");
var powerHelp = document.getElementById("powerHelp");
var effInput = document.getElementById("efficiency");
var effHelp = document.getElementById("effHelp");
if (source === "electric") {
powerLabel.textContent = "Heating Power (Watts)";
powerHelp.textContent = "Typical electric element: 3500-5500 Watts";
effInput.value = 98;
effHelp.textContent = "Electric heaters are typically ~98% efficient";
} else {
powerLabel.textContent = "Input Rating (BTU/hr)";
powerHelp.textContent = "Typical gas burner: 30,000-50,000 BTU";
effInput.value = 75;
effHelp.textContent = "Standard Gas: 60-65%, High Efficency: 80%+";
}
}
function calculateRecovery() {
var source = document.getElementById("heatSource").value;
var power = parseFloat(document.getElementById("powerInput").value);
var tankSize = parseFloat(document.getElementById("tankSize").value);
var efficiency = parseFloat(document.getElementById("efficiency").value);
var inletTemp = parseFloat(document.getElementById("inletTemp").value);
var targetTemp = parseFloat(document.getElementById("targetTemp").value);
// Validation
if (isNaN(power) || isNaN(tankSize) || isNaN(efficiency) || isNaN(inletTemp) || isNaN(targetTemp)) {
alert("Please fill in all fields with valid numbers.");
return;
}
if (inletTemp >= targetTemp) {
alert("Target temperature must be higher than inlet temperature.");
return;
}
// 1. Calculate Temperature Rise
var tempRise = targetTemp – inletTemp;
// 2. Determine BTU Input based on source
var btuInput = 0;
if (source === "electric") {
// 1 Watt = 3.412 BTU/hr
btuInput = power * 3.412;
} else {
// Gas is already in BTU
btuInput = power;
}
// 3. Apply Efficiency to get Output BTUs
var efficiencyDecimal = efficiency / 100;
var outputBTU = btuInput * efficiencyDecimal;
// 4. Calculate Recovery Rate (GPH)
// Formula: GPH = (Output BTU) / (Temp Rise * 8.33)
// 8.33 is lbs per gallon of water
var gph = outputBTU / (tempRise * 8.33);
// 5. Calculate Recovery Time for Full Tank
// Time (Hours) = Tank Capacity / GPH
var timeHours = tankSize / gph;
var timeMinutes = timeHours * 60;
// Display Results
document.getElementById("resTempRise").textContent = Math.round(tempRise) + " °F";
document.getElementById("resGPH").textContent = gph.toFixed(1) + " Gallons/Hour";
var timeDisplay = "";
if (timeMinutes < 60) {
timeDisplay = Math.round(timeMinutes) + " Minutes";
} else {
var h = Math.floor(timeMinutes / 60);
var m = Math.round(timeMinutes % 60);
timeDisplay = h + " hrs " + m + " min";
}
document.getElementById("resTime").textContent = timeDisplay;
document.getElementById("resBTU").textContent = Math.round(outputBTU).toLocaleString() + " BTU/hr";
document.getElementById("resultBox").style.display = "block";
}
Water Heater Recovery Rate Guide
Whether you have a large family competing for showers in the morning or you run a small business that relies on hot water, understanding your water heater's Recovery Rate is crucial. This metric determines how quickly your unit can generate hot water after the tank has been depleted.
What is Recovery Rate?
The Recovery Rate is defined as the amount of hot water (measured in gallons per hour, or GPH) that a water heater can produce at a specific temperature rise. Unlike the "First Hour Rating," which accounts for the water already stored in the tank plus recovery, the Recovery Rate focuses purely on the heater's ability to heat new incoming cold water.
For example, if you have a recovery rate of 40 GPH, your water heater can heat 40 gallons of incoming cold water to your desired temperature every hour.
How is Recovery Rate Calculated?
The calculation involves thermodynamics physics, specifically the energy required to raise the temperature of water. The standard formula used by manufacturers and plumbers is:
Input BTUs: The energy rating of the burner (Gas) or element (Electric). For electric units, 1 kW ≈ 3,412 BTUs.
Efficiency: The percentage of energy that actually goes into the water rather than being lost as exhaust.
Temp Rise: The difference between your cold groundwater temperature and your hot water set point (e.g., 120°F – 50°F = 70°F rise).
8.33: The weight of a gallon of water in pounds. Since 1 BTU raises 1 lb of water by 1°F, raising 1 gallon (8.33 lbs) by 1°F requires 8.33 BTUs.
Gas vs. Electric Recovery Rates
There is often a significant difference between gas and electric water heater recovery rates:
Electric Water Heaters
Electric units are highly efficient (often 98% to 100% efficient) because the heating elements are immersed directly in the water. However, the total power input is limited by household electrical circuits. A standard 4,500-watt element produces about 15,350 BTUs/hr, resulting in a recovery rate of roughly 20-25 GPH at a 70°F rise.
Gas Water Heaters
Gas heaters often have lower efficiency ratings (60-80% for standard models) because heat escapes up the flue. However, they burn fuel with a much higher energy density. A standard 40,000 BTU gas burner, even at 65% efficiency, delivers about 26,000 effective BTUs/hr. This typically results in a recovery rate of 35-45 GPH or more, making gas faster at replenishing hot water.
Why Temperature Rise Matters
Your geographic location significantly impacts your heater's performance. In colder climates (like Minnesota or Maine), groundwater temperatures can drop to 40°F in winter. In warmer climates (like Florida or Arizona), groundwater might be 70°F.
If you set your thermostat to 120°F:
Cold Climate (40°F inlet): Temperature rise needed is 80°F. The heater works harder, and recovery is slower.
Warm Climate (70°F inlet): Temperature rise needed is only 50°F. The heater recovers much faster.
Use the calculator above to adjust the "Inlet Cold Water Temp" to see how seasonal changes affect your hot water supply.