A tankless heater makes hot water as you use it, so its size comes down to flow and temperature rise. Choose the showers and fixtures that run at the same time to see what a unit has to deliver.
Pick your place and we set the incoming water temperature from its winter weather.
A tank stores hot water. A tankless unit makes it as you go, so it is sized in gallons per minute, not gallons. Add up what realistically runs together. We use 2.5 gallons per minute for a showerhead, which is the federal limit, about 1.8 for a faucet and about 2 for a dishwasher or washer. Two showers at once already means 5.
The unit has to lift cold incoming water to a 120°F setpoint, the temperature the US Department of Energy recommends. Winter changes the starting point, so the calculator sorts towns into three bands by January low. At 40°F or more the water arrives at 65°F, from 25°F up to 40°F it arrives at 52°F, and anything colder gives 42°F. Bayonne’s 27°F puts it in the middle band, a 68°F rise.
The heat a unit has to deliver each hour is the flow in gallons per minute, times the rise in degrees, times 500. That figure comes from the weight of a gallon of water. We round it and place it in one of our own planning bands, based on that delivered heat. Up to 90,000 BTU per hour is a small unit. Up to 150,000 is a standard whole-house unit. Up to 190,000 is a large unit. Above that, two units are linked together.
A tankless unit trades flow for rise. The same unit gives more gallons per minute at a small rise than at a big one, which is why every tankless rating is stated at a rise.
Gas units go up to about 199,000 BTU per hour of input, and two can be linked. Electric units top out near 36 kW, about 123,000 BTU per hour, and they need a very large electrical service. The result shows how many gallons per minute each one delivers at your rise. A plumber or an electrician checks the gas line, the electrical service and the venting before any unit is chosen.
Plan for two showers running together, about 5 gallons per minute. At Bayonne’s 68°F rise that takes roughly 170,000 BTU per hour, which is a large single gas unit. Adding a faucet or an appliance in the calculator moves the answer to two linked units.
Yes. Two showers draw about 5 gallons per minute, and one large gas unit delivers about 5.6 at a 68°F rise. Add a running faucet or a dishwasher and the demand goes past what one unit supplies.
Temperature rise is the difference between the water entering the heater and the water leaving it. Water arriving at 52°F and leaving at 120°F is a 68°F rise. Ratings are quoted at a stated rise, and a bigger rise lowers the flow a unit can deliver.
No, not on its own. A 36 kW electric unit, near the top of the range, delivers about 3.6 gallons per minute at a 68°F rise. That serves one shower with a little to spare, and it needs a very large electrical service that an electrician has to confirm.
Often. A 199,000 BTU per hour unit draws several times what a tank heater does, and the existing gas line and venting may not be sized for it. A plumber checks both first.
Data: fixture flow rates from the federal showerhead limit and typical faucet and appliance flows, the 120°F setpoint recommended by the US Department of Energy, and incoming water bands set from Daymet winter data 2015 to 2024 for each place.
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