Guide
Two showers at once on a tankless heater
The flow is fixed; the incoming water temperature decides the size.
Short answer
Yes, if it is sized for it. Two 2.1 GPM showers need 4.2 GPM of hot water. With 50°F water coming in and a 120°F setting, that takes a 160,000 BTU condensing gas unit; in the cold North, where winter water arrives near 40°F, 199,000 BTU. A heater sized for one shower will run the second one cold.
What two showers need, by climate
| Climate | Winter inlet | Rise | Gas unit | Electric |
|---|---|---|---|---|
| Warm South (Florida, Gulf coast) | 70°F | 50°F | 120,000 BTU | 36 kW, 150 A |
| Mild (Carolinas, Oklahoma) | 58°F | 62°F | 150,000 BTU | beyond one unit |
| Middle of the US (Ohio, Kansas) | 50°F | 70°F | 160,000 BTU | beyond one unit |
| Cold North (Minnesota, Maine) | 40°F | 80°F | 199,000 BTU | beyond one unit |
The rise is the difference between the water coming in and the temperature you set. It is what sizes a tankless heater, and it is largest in winter, which is why the same two showers need a bigger unit in Minnesota than in Florida. Incoming water temperature by state has the figure for yours.
Why the box’s GPM rating does not settle it
Manufacturers quote maximum flow at a small rise, usually 35°F. At a winter rise of 70 or 80°F the same unit delivers roughly half that. A heater sold as "9.8 GPM" is not a two-shower heater in a cold climate; it is about a 4.5 GPM heater there. Why the GPM rating misleads works through it.
Electric units and two showers
Two showers at a large rise are the limit of whole-house electric tankless. The biggest common units are about 36 kW and need 150 A of dedicated circuits, often more than a house’s whole service can spare. In warm climates electric works; in cold ones, gas or a heat pump tank is usually the practical choice. Low-flow showerheads help: at 3.5 GPM for the pair, a cold-climate house needs 160,000 BTU instead.