Tankless water heater sizing chart
What each unit delivers, at every temperature rise. Read down to your rise, across to the unit, and compare with the flow you actually need.
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.
Gas units, condensing (0.93 efficiency)
| Rise | 120k BTU | 140k BTU | 150k BTU | 160k BTU | 180k BTU | 199k BTU |
|---|---|---|---|---|---|---|
| 30 F | 7.4 | 8.7 | 9.3 | 9.9 | 11.1 | 12.3 |
| 40 F | 5.6 | 6.5 | 7.0 | 7.4 | 8.4 | 9.2 |
| 50 F | 4.5 | 5.2 | 5.6 | 5.9 | 6.7 | 7.4 |
| 60 F | 3.7 | 4.3 | 4.6 | 5.0 | 5.6 | 6.2 |
| 70 F | 3.2 | 3.7 | 4.0 | 4.2 | 4.8 | 5.3 |
| 80 F | 2.8 | 3.3 | 3.5 | 3.7 | 4.2 | 4.6 |
Gas units, non-condensing (0.82 efficiency)
| Rise | 120k BTU | 140k BTU | 150k BTU | 160k BTU | 180k BTU | 199k BTU |
|---|---|---|---|---|---|---|
| 30 F | 6.6 | 7.6 | 8.2 | 8.7 | 9.8 | 10.9 |
| 40 F | 4.9 | 5.7 | 6.1 | 6.6 | 7.4 | 8.1 |
| 50 F | 3.9 | 4.6 | 4.9 | 5.2 | 5.9 | 6.5 |
| 60 F | 3.3 | 3.8 | 4.1 | 4.4 | 4.9 | 5.4 |
| 70 F | 2.8 | 3.3 | 3.5 | 3.7 | 4.2 | 4.7 |
| 80 F | 2.5 | 2.9 | 3.1 | 3.3 | 3.7 | 4.1 |
Electric units
| Rise | 11 kW | 14 kW | 18 kW | 24 kW | 27 kW | 36 kW |
|---|---|---|---|---|---|---|
| 30 F | 2.5 | 3.2 | 4.1 | 5.5 | 6.1 | 8.2 |
| 40 F | 1.9 | 2.4 | 3.1 | 4.1 | 4.6 | 6.1 |
| 50 F | 1.5 | 1.9 | 2.5 | 3.3 | 3.7 | 4.9 |
| 60 F | 1.2 | 1.6 | 2.0 | 2.7 | 3.1 | 4.1 |
| 70 F | 1.1 | 1.4 | 1.8 | 2.3 | 2.6 | 3.5 |
| 80 F | 0.9 | 1.2 | 1.5 | 2.0 | 2.3 | 3.1 |
How much flow is enough
Shower2.1 GPM
Bathtub4.0 GPM
Kitchen sink1.5 GPM
Bathroom sink1.0 GPM
Dishwasher1.5 GPM
Washing machine2.0 GPM
Add only what runs at once. A house with three bathrooms rarely has three showers running simultaneously, and sizing for the theoretical maximum buys equipment that spends its life idling.
Frequently asked questions
What size tankless water heater for 2 bathrooms?
Size on simultaneous use rather than bathroom count. Two bathrooms typically means one shower plus a sink, about 3.6 GPM. In a warm climate that is comfortably within a mid-range unit; at a 70 degree winter rise it needs close to the largest residential unit available.
How many GPM is a 199,000 BTU tankless water heater?
About 10.6 GPM at the 35 degree rise it is advertised at, 7.4 GPM at a 50 degree rise, and 5.3 GPM at a 70 degree rise.
Is a condensing tankless water heater worth it?
It delivers about 13 percent more flow from the same gas input, and correspondingly lower running costs. Against that it costs more and needs different venting. In a cold climate, where you are short of capacity anyway, the extra output is often the deciding factor rather than the efficiency.