What size tankless water heater do I need?
Flow rate and temperature rise, and what a unit actually delivers at that rise.
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.
| 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
| 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. |