Estimate the required flow rate and temperature rise for your needs.
Your Sizing Requirements:
Required Temperature Rise:– °F
Required Flow Rate (GPM):– GPM
Estimated Minimum BTU/hr:– BTU/hr
Understanding Your Tankless Water Heater Sizing Needs
Choosing the right tankless water heater is crucial for ensuring you have enough hot water when you need it, without wasting energy. Unlike traditional tank water heaters, tankless systems heat water on demand. This calculator helps you determine the key specifications you'll need for a tankless unit based on your household's peak usage and local climate conditions.
Key Factors Calculated:
Temperature Rise: This is the difference between your incoming cold water temperature and your desired hot water temperature. Colder climates will require a higher temperature rise.
Flow Rate (GPM – Gallons Per Minute): This represents the maximum amount of hot water your household might use simultaneously. Think about all the fixtures that could be running at the same time (e.g., two showers, a dishwasher, and a sink).
Minimum BTU/hr (British Thermal Units per Hour): This is the measure of heating power required to achieve your desired flow rate and temperature rise. Higher BTU/hr ratings mean the unit can heat water faster and supply more hot water at peak times.
How the Calculator Works:
The calculator uses fundamental principles of water heating:
Temperature Rise Calculation:
Temperature Rise = Desired Hot Water Temperature - Cold Water Inlet Temperature
This is a straightforward subtraction. For example, if your cold water is 50°F and you want 120°F hot water, the temperature rise needed is 70°F.
Flow Rate (GPM):
This input is based on estimating your household's peak simultaneous demand. Here are some typical fixture flow rates to help you estimate:
Showerhead: 2.0 – 2.5 GPM
Bathroom Faucet: 0.5 – 1.5 GPM
Kitchen Faucet: 1.0 – 2.0 GPM
Dishwasher: 1.0 – 2.0 GPM (during fill cycles)
Washing Machine: 1.5 – 3.0 GPM (during fill cycles)
Sum the GPM of all fixtures you realistically expect to run at the same time to get your peak flow rate requirement. For instance, two showers (2.5 GPM each) and a sink (1.5 GPM) running simultaneously would require 6.5 GPM.
BTU/hr Calculation:
The core formula to estimate the required heating power is:
BTU/hr = Flow Rate (GPM) × 8.34 (lbs/gallon) × 60 (min/hr) × Specific Heat of Water (1 BTU/lb°F) × Temperature Rise (°F) × Altitude Factor
The constant 500 is a convenient approximation derived from the density of water (8.34 lbs/gallon), minutes in an hour (60), and the specific heat of water (1 BTU/lb°F). The Altitude Factor accounts for the fact that water boils at lower temperatures at higher altitudes, requiring slightly more energy to reach the desired temperature.
Choosing a Tankless Water Heater:
Once you have your calculated required temperature rise, peak flow rate, and estimated minimum BTU/hr, you can start looking at tankless water heater specifications. Always choose a unit that meets or exceeds these requirements, especially the flow rate and temperature rise for your coldest expected inlet temperature. It's often recommended to select a unit with a slightly higher capacity than your calculated minimum to ensure consistent performance during peak demand.
Consider both the GPM rating at your required temperature rise and the total BTU/hr output. Some heaters perform differently at various temperature rises.
Important Considerations:
Natural Gas vs. Electric: Gas tankless heaters generally have higher BTU outputs and flow rates available than electric models.
Venting Requirements: Gas tankless units require proper venting, which can impact installation costs.
Energy Efficiency: Tankless heaters are more energy-efficient than traditional tank heaters because they don't continuously heat a large volume of water.
Professional Installation: It is highly recommended to have a qualified professional install your tankless water heater to ensure safety and proper function.
function updateSliderValue(inputId, sliderId) {
var input = document.getElementById(inputId);
var slider = document.getElementById(sliderId);
input.value = slider.value;
// Optionally trigger calculation when slider changes
// calculateTanklessSizing();
}
function calculateTanklessSizing() {
var peakFlowRate = parseFloat(document.getElementById("peakFlowRate").value);
var coldWaterTemp = parseFloat(document.getElementById("coldWaterTemp").value);
var desiredHotWaterTemp = parseFloat(document.getElementById("desiredHotWaterTemp").value);
var altitudeFactor = parseFloat(document.getElementById("altitude").value);
var tempRiseResultElement = document.getElementById("tempRiseResult");
var flowRateResultElement = document.getElementById("flowRateResult");
var btuResultElement = document.getElementById("btuResult");
// Clear previous results
tempRiseResultElement.textContent = "-";
flowRateResultElement.textContent = "-";
btuResultElement.textContent = "-";
// Validate inputs
if (isNaN(peakFlowRate) || peakFlowRate <= 0) {
alert("Please enter a valid peak flow rate (must be greater than 0).");
return;
}
if (isNaN(coldWaterTemp) || coldWaterTemp <= 0) {
alert("Please enter a valid cold water inlet temperature.");
return;
}
if (isNaN(desiredHotWaterTemp) || desiredHotWaterTemp <= 0) {
alert("Please enter a valid desired hot water temperature.");
return;
}
if (isNaN(altitudeFactor) || altitudeFactor <= 0) {
alert("Please enter a valid altitude factor.");
return;
}
if (desiredHotWaterTemp <= coldWaterTemp) {
alert("Desired hot water temperature must be higher than the cold water inlet temperature.");
return;
}
// Calculations
var temperatureRise = desiredHotWaterTemp – coldWaterTemp;
var requiredBTU = peakFlowRate * 500 * temperatureRise * altitudeFactor;
// Update results
tempRiseResultElement.textContent = temperatureRise.toFixed(1);
flowRateResultElement.textContent = peakFlowRate.toFixed(1);
btuResultElement.textContent = Math.round(requiredBTU);
}