Tankless sizing is two numbers multiplied together: how much hot water you want at once, and how far the heater has to raise its temperature. Both matter, and the second one is the one people skip.
The number on the box is measured at a 35 degree rise. A 199,000 BTU unit is advertised at about 10.6 GPM. At a 70 degree rise, which is an ordinary northern winter, the same unit delivers 5.3 GPM. Nothing is wrong with it: that is the physics. Why this happens.
The same house, two climates
One shower and a kitchen sink running together, 3.6 GPM in total, heated to 120 F.
Identical demand, different incoming water temperature| Location | Incoming water | Rise needed | BTU required | Unit |
|---|
| Florida | 72 F | 48 F | 93,038 | 120,000 |
| Minnesota | 40 F | 80 F | 155,063 | 160,000 |
The demand is identical. The energy required is 1.7 times higher in Minnesota, because the rise is 1.7 times higher. This is the whole of tankless sizing in one table.
Where it stops working
Two showers and a kitchen sink in a cold climate is 5.7 GPM at a 80 F rise, which needs about 245,516 BTU of input. The largest residential gas tankless unit is 199,000 BTU.
So that household cannot be served by one unit in winter. The honest answers are two units in parallel, a smaller simultaneous-use expectation, or staying with a tank. A calculator that returns "199,000 BTU" for this case is lying by rounding down, and most of them do.
Fixture flow rates
Typical and water-efficient flow rates by fixture| Fixture | Typical | Efficient | Note |
|---|
| Shower | 2.1 GPM | 1.8 GPM | Federal maximum is 2.5 GPM. |
| Bathtub | 4.0 GPM | 4.0 GPM | Tub fillers are unrestricted and are the largest single draw. |
| Kitchen sink | 1.5 GPM | 1.2 GPM | |
| Bathroom sink | 1.0 GPM | 0.8 GPM | |
| Dishwasher | 1.5 GPM | 1.0 GPM | Many modern machines heat their own water. |
| Washing machine | 2.0 GPM | 1.7 GPM | Only draws hot on a warm or hot cycle. |
Frequently asked questions
What size tankless water heater do I need for a family of 4?
Household size is the wrong variable, and this is the most common mistake in tankless sizing. A tankless heater has no tank, so it does not care how much hot water a family uses in a day: it cares what is running AT ONCE, which is set by the number of bathrooms, not the number of people. A family of four in a two-bathroom house has a realistic peak of two showers and a kitchen tap, about 5.7 GPM. In a warm climate with a 70 F inlet that is a 50 F rise and the 160,000 BTU class covers it. At a 50 F inlet the rise is 70 F and no single residential gas unit covers it, including the 199,000 BTU maximum, which delivers 5.3 GPM at that rise. That is a real answer, not a gap in the calculator: a cold-climate two-bathroom house either accepts one shower at a time or installs two units in parallel. The same four people in a one-bathroom house need 3.6 GPM and one ordinary unit covers it.
How many GPM do I need for a 2 bathroom house?
Plan for about 5.7 GPM: two showers at 2.1 GPM each plus a kitchen tap. Two bathrooms is the point where simultaneous showers stop being hypothetical, and it roughly doubles the requirement over a single-bathroom house. A three bathroom house is usually planned at 6.7 GPM, because the third shower running at the same time as the other two is rare enough not to size for.
Is a 7 GPM tankless water heater enough?
It depends entirely on your incoming water temperature, and that is the whole trap. "7 GPM" is a rating at a small temperature rise. A 199,000 BTU condensing unit, the largest residential gas size, delivers 10.6 GPM at the 35 F rise manufacturers quote, and only 5.3 GPM at the 70 F rise a northern winter produces. So a unit sold as 7 GPM is a 7 GPM unit in Florida in August and a 3.5 GPM unit in Minnesota in February. Ask what it delivers at YOUR rise, which is 120 minus your coldest incoming water temperature, and compare that against what you actually run at once.
What size tankless water heater replaces a 50 gallon tank?
There is no conversion, because the two are rated on different things: a tank is rated on stored volume and a tankless on flow. A 50 gallon tank usually serves a two to three bathroom house, and at a mild 50 F rise that is the 160,000 BTU class; in a cold climate the same house needs the 199,000 BTU maximum and may still run short with two showers going. What changes most is the failure mode: the tank gave you a fixed number of gallons and then went cold, and the tankless never runs out but drops in temperature the moment you exceed its flow.
Why does my tankless water heater run cold in winter?
Because incoming water is colder, so the heater has to raise it further, and a tankless unit trades flow for temperature rise. A unit that comfortably ran two showers in summer may only manage one in January. The unit is not faulty; it was sized against a summer rise.
Can one tankless water heater run two showers?
At a mild 35 degree rise, yes: a large unit delivers around 10.6 GPM, which covers two showers and more. At a 70 degree rise the same unit delivers 5.3 GPM, which is one shower and little else. The answer genuinely depends on the month.
How many GPM do I need for a whole house?
Add the fixtures you expect to run simultaneously, not every fixture you own. A shower is about 2.1 GPM, a bathtub filler 4 GPM, and a kitchen sink 1.5 GPM. Most households size for one or two showers plus a sink.