Tankless Water Heater Size Calculator

Add the flow of every fixture you run at once, then find a heater that delivers that flow at your temperature rise. Two 2.0 GPM showers plus a 1.0 GPM faucet is 5 GPM; with 50 F groundwater heated to 120 F, that needs 175,000 BTU/hr of heat, which a 199,000 BTU gas unit covers.

Most families need 5 to 8 GPM. A shower uses about 2.0 GPM (2.5 is the federal maximum), a bathroom faucet about 1.0, a kitchen faucet about 1.5 and a dishwasher about 1.0 while filling. Count only what overlaps at your busiest time.

The heater has to raise the water from its incoming temperature to about 120 F. With 70 F water that is a 50 F rise; with 40 F water it is 80 F, which takes 60% more heat for the same flow. A heater's headline GPM is usually rated at a 35 to 45 F rise, the best case.

BTU per hour = GPM x 500 x temperature rise in F. The 500 is 8.33 pounds per gallon times 60 minutes. Divide by the heater's efficiency (about 0.93 for condensing gas) to get the gas input it needs, or divide by 3,412 to get kW for electric.

In warm climates, often yes. In the north, usually not. Two showers with 70 F water need about 29 kW, which a 27 to 36 kW unit handles on about 150 A of breakers. The same two showers with 40 F water need about 47 kW, beyond most homes' electric service.

Whole-house units need two to four 40 to 60 A double-pole breakers, 80 to 180 A in total. A 27 kW unit typically uses three 40 A breakers (120 A). Most need 200 A service; an electrician's load calculation decides whether your panel can take it.

It pays off best when you replace a gas tank with gas tankless and keep it 20 years. DOE estimates tankless units are 24 to 34% more efficient for homes using 41 gallons or less a day, because there is no tank losing heat all day. The install costs more than a tank, mostly for venting and gas line upgrades.

DOE puts it at more than 20 years, against 10 to 15 for a tank heater. Descale it yearly in hard water areas. Hard water scales the heat exchanger, so a water softener or descaling kit helps it reach that life.

Yes. Most need their own vent kit from the maker, matched to the model. Condensing units vent through PVC; non-condensing units need stainless steel. They also draw up to 199,000 BTU, so the gas line often needs to be upsized from what fed the old tank.

Only with the maker's conversion kit, or by buying the propane version. Propane and natural gas run at different pressures and orifice sizes, so a heater set for one cannot safely burn the other as shipped.