HOW COLD IS YOUR INCOMING WATER?
Required temperature rise is the difference between your incoming ground water temperature and your desired output temperature.
You know your required flow rate is 2.5 and you live in Atlanta where groundwater temperature is 55°. You would need a water heater that produces a minimum temperature rise of 45°.
The map provides average ground water temperatures. You should use the expected coldest ground water temperature for your area. After determining your ground water temperature, 55° in Atlanta for example, you will subtract this from your desired output temperature. For most uses, you’ll want your water heated to around 100–115°.
Using the previous examples of 2.5 gallons per minute needed and a required minimum temperature rise of 45°, you can review your water heater options.
Each Tankless Water Heater lists the max GPM Flow Rate at different required temperature rises. In this example, Flow Rate at 45° temperature rise would meet / exceed the 2.5 GPM need.
Now that you have determined your peak water flow demand in gallons per minute and you know what your required temperature rise is, you can select the right unit.