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.
A tankless heater is set to deliver 120°F water, the temperature the US Department of Energy recommends. Because incoming water is colder in a cold winter, the calculator sorts the January low into three bands, 65°F water at 40°F or more, 52°F water from 25°F up to 40°F and 42°F water below that. East Orange’s 26°F low 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.
Start from two showers at once, about 5 gallons per minute. Heating that flow through East Orange’s 68°F rise takes roughly 170,000 BTU per hour, and one large gas unit covers it. If you also add a faucet or an appliance, the calculator calls for two units linked together.
It can, with a large gas unit. Two showers use about 5 gallons per minute and one large gas heater supplies about 5.6 at a 68°F rise. Run a dishwasher or another faucet at the same moment and a single unit can no longer keep up.
It is the number of degrees the heater adds. Water entering at 52°F and leaving at 120°F has gone up 68°F. Flow ratings are quoted at a stated rise, and a unit delivers fewer gallons per minute when the rise is bigger.
Not for a whole house. At East Orange’s 68°F rise, the top of the range electric model (36 kW) supplies only about 3.6 gallons per minute, which is one shower and a little more. Its demand on the electrical service is very large, so an electrician has to confirm the home can carry it.
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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