What size tankless water heater for a family of 6?
Size to 7.5 GPM: three showers running at once. At that draw, no single residential gas unit covers that moment at any US winter inlet temperature, so this household plans around two units or a trimmed peak. The table below runs the number against every US region's winter water.
The size by region, at 7.5 GPM
Delivered heat is 500 × 7.5 GPM × the rise from your winter inlet water to 120°F. Gas classes assume 82% efficiency; "two units" means the draw is past a single 199,000 BTU residential unit.
| Region | Winter rise | Heat needed | Gas unit (input) |
|---|---|---|---|
| Far north (MN, ND, MT, ME, VT) | 78°F | 292,500 BTU/hr | two units |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 262,500 BTU/hr | two units |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 232,500 BTU/hr | two units |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 202,500 BTU/hr | two units |
| Gulf coast and Florida | 48°F | 180,000 BTU/hr | two units |
Here is the honest tension in this household size. The scenario draw of 7.5 GPM needs 262,500 BTU/hr of delivered heat at the DOE-default 70°F rise, and the biggest residential unit delivers about 163,180 BTU/hr. The fix is rarely a second unit, though that works. It is usually behavioral: the third shower moves fifteen minutes, and the draw drops inside one unit's range. Decide whether your household will actually do that before you buy the architecture that assumes it.
Our scenario is an assumption, stated plainly: three showers running at once is the worst moment we consider realistic for 6 people. If your household's collisions are bigger (a garden tub plus two showers, say), run the calculator with your own moment; a tub filler alone is 4 GPM.
Three showers at once is a two-unit house
Three simultaneous showers needs three bathrooms, and by the time a house has three bathrooms the tankless conversation has usually become a two-unit conversation on its own. 7.5 GPM is past one 199,000 BTU unit in every region in the table, so a family of six sizing to its real worst moment is manifolding.
Two units is not a consolation prize. A manifold splits the draw, so each unit runs in the middle of its modulation range instead of pinned at maximum, which is where a non-condensing unit is least efficient and hardest on its heat exchanger. It also means a failure is half a house rather than no house, which at six people is the difference between an inconvenience and a hotel.
This is also the size where a conventional tank stops looking old-fashioned. A 75-gallon gas tank rates about 133 gallons in its first hour, which is six standard showers, and it costs a fraction of two tankless units plus the gas and vent work they ask for. The usual case against a tank is that it holds water hot for nothing. At six people almost nothing is held hot for nothing.
Questions people ask
How many GPM does a family of 6 need from a tankless?
About 7.5 GPM: three showers running at once, at 2.5 GPM per shower head. If your peaks are calmer than that, size to your real worst moment instead; the calculator takes any combination.
Does the brochure GPM rating apply to my house?
Only if your winter inlet water is 85°F, because ratings are quoted at a 35°F rise. Real winter inlets run 42 to 72°F depending on region, so real capacity is roughly half to three-quarters of the brochure number.