Temperature rise
Temperature rise is the difference between incoming groundwater temperature and your hot-water setpoint. It determines how much flow a tankless water heater can actually sustain: the colder the groundwater, the fewer GPM you get.
Groundwater ranges from about 40 °F in the northern US to 75 °F in the far south. Heating to a 110–120 °F delivery temperature therefore means a rise of anywhere from 35 °F to 80 °F depending on where you live and the season.
The physics is fixed: GPM = BTU/h × efficiency ÷ (rise × 500). A unit that delivers 8 GPM at a 35 °F rise manages only about 4 GPM at 70 °F — winter in a cold state cuts tankless capacity roughly in half versus the brochure number.
| Formula | Temperature rise = delivery setpoint − incoming groundwater temperature |
|---|---|
| Florida, Gulf Coast | Groundwater 70–75 °F → 45–50 °F rise to 120 °F |
| Southern California, Texas | Groundwater 60–70 °F → 50–60 °F rise |
| Mid-Atlantic, Pacific Northwest | Groundwater 50–60 °F → 60–70 °F rise |
| Midwest, New England | Groundwater 40–50 °F → 70–80 °F rise |
| Flow impact | A 199 kBTU/h tankless does roughly 8.5 GPM at a 45 °F rise and 5.5 GPM at 70 °F |
What sets your groundwater temperature
Groundwater tracks the mean annual air temperature of the place it comes from, which is why the US bands run from about 40 °F along the northern border to the mid-70s on the Gulf Coast. Depth is the second variable: water from a deep well arrives within a few degrees of the local annual average year-round, while a municipal main buried a few feet down follows the seasons with a lag of a month or two.
That lag is why the design case is February, not January. Ground heat storage means the coldest supply water usually arrives several weeks after the coldest air, and a shallow main can swing 10–20 °F between August and late winter. If you size a tankless from a September measurement, you have sized it for the wrong month.
Measure rather than guess when you can. Run a cold tap for two minutes and hold a thermometer in the stream; do it in winter, and again in summer if you want to see the spread. A five-degree error in groundwater temperature is a five-degree error in rise, which moves a tankless unit’s output by roughly 7%.
The arithmetic it drives
Heating one gallon per minute by one degree Fahrenheit requires about 500 BTU/h. Rearranged for a tankless heater: sustainable flow = input BTU/h × efficiency ÷ (rise × 500). The same 199,000 BTU/h condensing unit that delivers 8.5 GPM against a 45 °F rise manages about 5.5 GPM against 70 °F. Nothing has changed except the water it was handed.
Storage tanks feel it differently but no less. Recovery rate in gallons per hour is input × efficiency ÷ (8.33 × rise), so a gas tank that recovers 45 GPH in Houston recovers about 30 GPH in Minneapolis. Stored capacity shifts too: colder inlet water means each drawn gallon is replaced by colder water, so the usable fraction of the tank shrinks slightly and the first-hour rating drops from its rated figure in a cold-climate winter.
This is the single largest reason tankless installations disappoint. A unit sized on a brochure figure computed at a 35–45 °F rise loses roughly a third to a half of its stated flow in a northern winter, and the shortfall arrives precisely when everyone is taking longer showers.
Choosing a setpoint, and the safety trade
The rise is only half yours to control, and the half you control is the setpoint. DOE recommends 120 °F, which is enough for a comfortable shower — most people mix to around 105 °F — while limiting both standby losses and scald risk. Every 10 °F you add raises the rise, cuts a tankless unit’s flow by roughly 8–12%, and increases heat lost through the tank wall.
The counter-argument is Legionella, which multiplies in stagnant water roughly between 77 °F and 113 °F and is killed off quickly above about 140 °F. The standard resolution is to store at 140 °F and fit a thermostatic mixing valve that blends the outlet down to 120 °F. That protects the tank, delivers safe water at the tap, and as a side effect increases usable capacity by roughly 30%, because each stored gallon dilutes into about 1.3 gallons of 120 °F water.
What you must not do is raise the setpoint without a mixing valve. Water at 140 °F causes a third-degree burn on adult skin in about five seconds, at 130 °F in about thirty, and at 120 °F in around five minutes — the margin that makes 120 °F the default recommendation in households with children or older adults.
Frequently asked questions
What is the average groundwater temperature for water heater sizing?
It follows the local mean annual air temperature: roughly 40–50 °F across the Midwest and New England, 50–60 °F in the Mid-Atlantic and Pacific Northwest, 60–70 °F in Southern California and Texas, and 70–75 °F on the Gulf Coast and in Florida. Size for your winter figure, which typically arrives a month or two after the coldest air.
How do I calculate temperature rise?
Subtract your incoming water temperature from the temperature you want delivered. For a 120 °F setpoint with 50 °F groundwater, the rise is 70 °F. Measure the inlet by running a cold tap for two minutes and holding a thermometer in the stream — in winter, since that is the condition that decides whether the heater is big enough.
What temperature should I set my water heater to?
120 °F is the standard recommendation: comfortable for showering, low standby loss, and a five-minute margin before a scald injury. If you want Legionella protection or more usable capacity from the same tank, set 140 °F and fit a thermostatic mixing valve to blend the outlet back down to 120 °F. Raising the setpoint without a mixing valve is how scald injuries happen — 140 °F water burns adult skin in about five seconds.
Why does my tankless water heater struggle in winter?
Because the temperature rise it has to produce grows by 20–30 °F, and flow capacity falls in direct proportion. A unit that handles two showers in September may only manage one and a half in February. This is normal physics, not a fault — the fix is either fewer simultaneous fixtures, a low-flow showerhead, or a larger unit sized on your winter rise.
Does temperature rise matter for tank water heaters?
Yes, through recovery rather than flow. Recovery rate is input × efficiency ÷ (8.33 × rise), so colder groundwater slows the refill of a drained tank by the same proportion it cuts tankless flow — a 45 GPH gas tank in a warm climate does about 30 GPH in a cold one. First-hour rating drops accordingly against its certified figure.