HotWaterTwo views of the same household
Tankless load estimates simultaneous hot-side flow. Tank demand estimates heater-temperature water used during the busiest hour. The “Together” field affects simultaneous flow only. “Minutes per use” and “Uses per hour” affect hourly volume only. Set them separately to describe a realistic routine.
Ideal mixing calculation
For each fixture, the hot-water fraction equals (delivered temperature − inlet temperature) ÷ (heater output temperature − inlet temperature). Hot-side GPM equals fixture GPM × the number running together × that fraction. Hourly hot-water gallons equal fixture GPM × minutes per use × uses per hour × that fraction.
The planning heater output is 120°F. Default delivered temperatures are 105°F for showers and bath fills, 110°F for faucets and 120°F for appliance hot fill. These are editable-model assumptions in the source, not measured household settings or thermostat recommendations.
Starting fixture assumptions
| Fixture | Flow | Duration | Uses / busy hour | Together |
|---|---|---|---|---|
| Shower | 2 GPM | 8 minutes | 2 | 1 |
| Faucet | 1 GPM | 3 minutes | 2 | 1 |
| Bath fill | 4 GPM | 8 minutes | 0 | 0 |
| Appliance hot fill | 1 GPM | 5 minutes | 0 | 0 |
Replace these starting values with observations where possible. They are not industry averages. Appliance hot fill represents only water entering from the hot supply; internal heating and cycle behavior vary.
Theoretical heat delivered to the water
The estimate in Btu/hour is hot-side GPM × temperature rise in °F × 8.34 pounds per gallon × 60 minutes per hour. The conversion to kW divides by 3,412.142 Btu/hour per kW. Water properties are approximated. This is thermal output to the water, not fuel input, element rating or electrical-service sizing.
What this model leaves out
The model assumes steady fixture flow and ideal mixing. It does not model pipe heat loss, distribution delay, recirculation, storage stratification, equipment recovery over time, minimum activation flow, control behavior or fuel/electrical constraints. The hourly estimate is not a certified first-hour rating or a tank-size recommendation. A plumber must compare matched conditions with the manufacturer’s data.
The winter chart repeats the simultaneous calculation at four assumed inlet temperatures. It is not a utility measurement or a forecast. If you set all simultaneous counts to zero, the chart shows zero load; it does not infer another household scenario.
Sources behind the approach
- U.S. DOE: tankless sizing depends on flow and temperature rise.
- U.S. DOE: heat-pump capacity and installation decision guidance.
- Greater Salt Lake MSD: local permit example used in the Utah guide.
- USU Extension: drinking-water information.
HotWaterLogic model version 1.0.0. Content reviewed October 26, 2026. This is an independent planning resource; no government agency or equipment manufacturer endorses the tool.
Footage: Diana / Pexels, used and edited under the Pexels license. The technical system graphic is an original schematic representation, not a specified equipment model.