What size tankless water heater for a family of 5?
Size to 7 GPM: two showers running while the clothes washer fills. 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 GPM
Delivered heat is 500 × 7 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 | 273,000 BTU/hr | two units |
| North (NY, MA, MI, PA, WA, CO) | 70°F | 245,000 BTU/hr | two units |
| Middle band (VA, KY, MO, KS, NV) | 62°F | 217,000 BTU/hr | two units |
| South (GA, AL, TX, AZ, most of CA) | 54°F | 189,000 BTU/hr | two units |
| Gulf coast and Florida | 48°F | 168,000 BTU/hr | two units |
Here is the honest tension in this household size. The scenario draw of 7 GPM needs 245,000 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 non-shower fixture in the scenario 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: two showers running while the clothes washer fills is the worst moment we consider realistic for 5 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.
No region fits this on one unit, so check the scenario first
7 GPM is past a single residential gas unit at every winter inlet temperature in the country, Florida included. There is no version of the table above where one unit covers it, so the question is not which unit to buy. The question is whether the moment is real.
In this scenario it usually is not, because the fixture creating it is the clothes washer, and a washer is the most schedulable draw in any house. It fills for about two minutes, the collision it makes is brief, and it has a delay-start button on the front of it. Take the washer out of shower hour and the same family of five is at 5 GPM, which one unit carries through the middle of the country.
If the washer genuinely cannot move, and in houses with small children it often cannot, the honest options are two units manifolded or a conventional tank. A tank answers simultaneity by having the gallons hot already rather than making them as they go past, and at five people that is not the sentimental choice it sounds like.
Questions people ask
How many GPM does a family of 5 need from a tankless?
About 7 GPM: two showers running while the clothes washer fills, 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.