HotWaterThe difference between starting and finishing temperature
Temperature rise is the difference between incoming water and the heater’s output. In HotWaterLogic, the planning output is fixed at 120°F. An inlet of 40°F creates an 80°F rise; an inlet of 70°F creates a 50°F rise. Those are calculation scenarios, not measurements of your water supply or instructions to adjust a thermostat.
Why the hot-water fraction changes too
A shower typically mixes hot and cold water. If the desired shower temperature stays the same while the cold side becomes colder, a larger fraction of that shower’s flow must come from the hot side. Our model accounts for this ideal mixture instead of treating every fixture as pure hot-water demand.
Use your water supply, not just the weather forecast
Air temperature is not the same as water temperature at your heater. Your source, distribution system, season and building conditions all matter. A utility’s appropriate winter design figure or a professional measurement is more useful than assuming an outdoor temperature equals the inlet temperature.
The planner’s 40°F, 50°F, 60°F and 70°F chart uses the same fixtures and delivered temperatures in each scenario. It shows how theoretical heat delivered to the water changes; it is not a local forecast, utility feed or predicted energy bill.
Bring the right values to the manufacturer chart
For a tankless comparison, ask the installer to use the hot-side flow requirement and corresponding temperature rise. The theoretical thermal load in our tool excludes equipment efficiency, distribution loss and installation constraints. It must not be used to select a gas line, breaker, electrical service or installation procedure.
Explore your cold-water scenarios
Source: U.S. Department of Energy — tankless water-heater sizing principles.