Groundwater Temperature Impact on Unit Size
In addition to the number and type of fixtures you want simultaneously served by the tankless water heater, you will also have to consider the temperature of your groundwater. And that is determined by where in the country you live.
In the chart above I have delineated what is generally accepted as the cool / warm dividing line of about 55°F groundwater. The colder the groundwater gets the less hot water can be produced by a unit for a given GPM flow rate. This means a tankless water heater in Florida would have to be rated 33% to 50% larger in Michigan to serve the same number and types of fixtures.
Why? Well because the tankless unit heater has to warm the incoming cold water more in Michigan than Florida since the groundwater coming into the unit can be 30°F colder in Michigan (42°F) than Florida (72°F).
Let's see how this really affects determining unit size and temperature rise in the next section.