Temperature Rise and Inlet Water
Updated July 2026
Winter inlet water runs 42°F up north to 72°F on the Gulf. Against a 120°F setpoint, that is a 78°F rise vs 48°F, and heater capacity scales with it directly.
Every water heater's job is measured in degrees of lift: from the temperature water arrives at your house to the 120°F on the thermostat. That arrival temperature tracks deep ground temperature, which tracks where you live. In a Minnesota January the inlet runs around 42°F, a 78°F rise. On the Gulf coast the same January delivers 72°F water, a 48°F rise. The Florida heater does 60% of the Minnesota heater's work per gallon, every gallon.
Tankless units feel it directly
A tankless has no storage to hide behind; its flow capacity is heat divided by rise. The biggest residential gas unit, 199,000 BTU input at 82% efficiency, delivers about 6.8 GPM on the Gulf and 4.2 GPM in the far north. Same box, half the heater. Brochure ratings are quoted at a 35°F rise that no US winter provides, which is how a "9.8 GPM" unit ends up throttling two showers in Buffalo. Size on winter rise, always; summer takes care of itself.
Tanks feel it as a longer stopwatch
A tank's stored gallons do not care where you live, but its recovery does: heat-up time scales with the rise. A 50-gallon electric from stone cold takes about 2 h 10 min in the far north against 1 h 20 min on the Gulf. Northern all-electric homes get both ends of the stick, slower recovery and colder refills, which nudges their right answer a size up from the same household in the south.
Finding your number
DOE's default planning assumption is a 50°F inlet. Better: run a bathtub cold tap for two minutes in February and read a cooking thermometer; that is your design inlet. Our tankless calculator carries five regional presets (42, 50, 58, 66, and 72°F) which bracket the continental US; they are planning figures, and your February thermometer outranks them.