How tankless sizing works
A tankless water heater has no stored hot water. It heats water as it flows, so it must be sized for two numbers at once:
- Flow rate in gallons per minute (GPM): the total of every fixture running at the same time.
- Temperature rise: the difference between the cold water entering the house and the hot water temperature you want.
Heating one gallon of water by one degree takes 8.33 BTU, so the heat needed per hour is GPM × 500 × temperature rise. A condensing gas unit turns about 95% of its input into hot water; an electric unit about 99%.
Fixture flow rates
| Fixture | Typical flow | Note |
|---|---|---|
| Shower (WaterSense) | 1.8 GPM | Most homes built or remodeled after 2010 |
| Shower (standard) | 2.5 GPM | Federal maximum for showerheads |
| Bathroom faucet | 1.0 GPM | WaterSense faucets use 1.5 or less |
| Kitchen faucet | 1.5 GPM | |
| Bathtub | 4.0 GPM | Tub spouts run at higher flow |
| Dishwasher | 1.5 GPM | Only while filling |
| Washing machine (hot) | 2.0 GPM | Only while filling |
Groundwater temperature by region
Incoming water is roughly the temperature of the ground it flows through. Northern homes start with much colder water, so the same unit delivers far less flow in Minnesota than in Florida.
| Climate zone | Groundwater | Rise to 120 °F | Examples |
|---|---|---|---|
| Hot · IECC 1–2 | 72 °F | 48 °F | Florida, south Texas, Arizona |
| Warm · IECC 3 | 62 °F | 58 °F | Georgia, Dallas, southern California |
| Mixed · IECC 4 | 55 °F | 65 °F | Tennessee, Virginia, Missouri |
| Cold · IECC 5 | 47 °F | 73 °F | Illinois, Ohio, Colorado |
| Very cold · IECC 6–7 | 40 °F | 80 °F | Minnesota, Wisconsin, Maine |
Winter water is often 5–10 °F colder than these averages. If you can, measure it: run the cold tap for a minute and check it with a kitchen thermometer.
Tankless size chart
Gas input BTU needed at 120 °F with a 95% efficient condensing unit.
| Demand | Hot · 72 °F | Mixed · 55 °F | Very cold · 40 °F |
|---|---|---|---|
| 1 shower (2.0 GPM) | 120,000 BTU | 120,000 BTU | 120,000 BTU |
| Shower + kitchen faucet (3.5 GPM) | 120,000 BTU | 120,000 BTU | 150,000 BTU |
| 2 showers (4.0 GPM) | 120,000 BTU | 140,000 BTU | 180,000 BTU |
| 2 showers + faucet (5.5 GPM) | 140,000 BTU | 199,000 BTU | 2 × 120,000 BTU |
| 3 showers (6.0 GPM) | 150,000 BTU | 2 × 120,000 BTU | 2 × 140,000 BTU |
Residential gas tankless heaters top out at 199,900 BTU. Homes that need more install two units side by side.
Gas or electric?
- Gas units deliver much more hot water and suit whole-house use in any climate. They need venting (PVC for condensing models) and often a larger gas line.
- Electric units are cheaper to buy and need no venting, but they draw a lot of current. A 27 kW unit needs about 113 amps at 240 volts, usually across three dedicated breakers, and many older 100–150 amp panels cannot support it.
- In cold climates, whole-house electric tankless heaters often cannot keep up with two showers. They work best in warm regions or as point-of-use heaters for a single sink.
Frequently asked questions
What size tankless water heater do I need for a family of four?
Most families of four plan for two showers at once, about 4 GPM. That needs about 122,000 BTU of gas input in a warm climate and about 154,000 BTU in a cold one. A 199,000 BTU unit covers almost every home of that size.
How many GPM do I need from a tankless water heater?
Add up the fixtures you use at the same time. Two showers and a faucet is about 5 to 6 GPM. Check the manufacturer's flow chart at your temperature rise, not the maximum flow on the box, which assumes warm incoming water.
Can an electric tankless water heater run a whole house?
In warm climates, yes: a 27 to 36 kW unit can run two showers. In cold climates the temperature rise is too large for most homes' electrical service.
What size gas line does a tankless water heater need?
Most whole-house units need a 3/4 inch gas line, and long runs may need 1 inch. A standard 1/2 inch line for an old tank heater is usually too small.
Why does my tankless water heater go cold with two showers?
The unit is undersized for your flow and temperature rise, often because winter groundwater is colder than it was sized for. Reducing flow with 1.8 GPM showerheads or lowering the setpoint can help.
These results are planning estimates. Check the manufacturer's flow-versus-temperature-rise chart and local code for gas line, venting and electrical requirements.