What is my inlet water temperature?

It is the temperature of the water arriving at your house, which is groundwater temperature and varies by region and by season. Northern regions can see inlets near 40 F in late winter while southern ones stay near 70 F all year. Measure it rather than guessing: run an outside tap for a minute and put a probe thermometer in a glass of the water.

Why does temperature rise matter so much for a tankless heater?

Because a tankless unit has no storage to fall back on, so everything it delivers has to be heated in the moment. The energy needed is flow times rise, so doubling the rise halves the flow the unit can produce. A tank heater is far less sensitive, because a bigger rise only means a slower recovery rather than a thinner stream.

Does temperature rise change through the year?

Yes, and it is the reason a tankless unit that felt fine in the autumn disappoints in late winter. Groundwater temperature lags air temperature by a couple of months and can swing 20 F or more across a year in northern regions. Size for the coldest inlet you will see, not the average, or the unit will be undersized exactly when you want it most.

What outlet temperature should I size for?

Most households size to 120 F, which is the common setpoint guidance for scald safety. If you store hotter for Legionella control and mix down with a thermostatic mixing valve, size the heater to the stored temperature rather than the delivered one, because the heater still has to make that rise. That distinction changes the answer by a meaningful margin.

Sizing calculator

Water Heater Temperature Rise Calculator

Quick answer

Temperature rise is the outlet temperature minus the inlet temperature. A 50 F inlet and a 120 F outlet is a 70 F rise, and because rise sits in the denominator of the flow equation, that same unit delivers roughly half the flow it would at a 35 F rise.

Result

Temperature rise 70 F Published standard

Energy per gallon 583 BTU Published standard

That unit flow at your rise 4.2 GPM Convention

The flow figure is a straight inverse scaling of the rated point and is a planning estimate, not a manufacturer curve. Real units modulate and their curves are not perfectly linear, so read the published curve in the installation manual before you buy.

The formula

Rise = outlet temperature - inlet temperature. Flow at a new rise = flow at the rated rise x (rated rise / new rise).

Inlet water temperature is groundwater temperature at your location and it varies by season as well as by region, which is why a tankless unit that copes in the autumn struggles in late winter. Measure yours at an outside tap with a probe thermometer rather than estimating it.

The figures above are computed with these exact inputs: Inlet water temperature 50 F, Outlet temperature 120 F, Unit rated flow 8.4 GPM, Rise that rating is at 35 F.

What this calculator assumes

Some of these are published test procedures or code tables and some are planning conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Water Weight: 8.33	Published standard	Published density of water at 60 F, 8.33 lb per gallon
Setpoint120: 120	Convention	The 120 F setpoint is a widely repeated scald guidance figure, not a test procedure. Legionella control argues for storing hotter and mixing down

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

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Sediment Buster

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SEDIMENT GONE

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Yacumama

Digital Water Thermometer, Long Probe

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Frequently asked questions

What is my inlet water temperature?: It is the temperature of the water arriving at your house, which is groundwater temperature and varies by region and by season. Northern regions can see inlets near 40 F in late winter while southern ones stay near 70 F all year. Measure it rather than guessing: run an outside tap for a minute and put a probe thermometer in a glass of the water.
Why does temperature rise matter so much for a tankless heater?: Because a tankless unit has no storage to fall back on, so everything it delivers has to be heated in the moment. The energy needed is flow times rise, so doubling the rise halves the flow the unit can produce. A tank heater is far less sensitive, because a bigger rise only means a slower recovery rather than a thinner stream.
Does temperature rise change through the year?: Yes, and it is the reason a tankless unit that felt fine in the autumn disappoints in late winter. Groundwater temperature lags air temperature by a couple of months and can swing 20 F or more across a year in northern regions. Size for the coldest inlet you will see, not the average, or the unit will be undersized exactly when you want it most.
What outlet temperature should I size for?: Most households size to 120 F, which is the common setpoint guidance for scald safety. If you store hotter for Legionella control and mix down with a thermostatic mixing valve, size the heater to the stored temperature rather than the delivered one, because the heater still has to make that rise. That distinction changes the answer by a meaningful margin.

Researched guidance, not professional advice. These results are planning figures from published test procedures, code tables and manufacturer specifications, and they do not replace a sizing done by the person who will sign the permit. A water heater is a pressure vessel: never cap, plug or remove the temperature and pressure relief valve, and replace one that will not open when the test lever is lifted. Gas line, flue and vent work belongs to a licensed plumber or gas fitter, because improper venting produces carbon monoxide. Water at 140 F scalds a child in seconds. Turn off the breaker before touching an element, and never energise an element that is not submerged.