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 incoming water up to the setpoint, which we take as the recommended 120°F. Where the average January low is 40°F or higher we figure incoming water at 65°F, between 25°F and 40°F at 52°F, and below 25°F at 42°F. Kannapolis, with a January low of 30°F, sits in the middle band, so the rise there is 68°F.
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.
Yes, if one shower runs at a time. A single 2.5 gallon per minute shower at a 68°F rise needs about 85,000 BTU per hour, inside the calculator’s small unit class. Add a faucet running at the same time and the load moves up to the standard class.
Yes, a large gas unit can. Two showers come to 5 gallons per minute, and at a 68°F rise that is about 170,000 BTU per hour, which the calculator places in its large unit class. Add a faucet or a washer to the two showers and the result moves to two linked units.
Yes. The rise is the gap between the water coming in and the setpoint, and with 52°F in and 120°F out it is 68°F. Every tankless flow rating is stated at a rise, so the same unit delivers fewer gallons per minute when the rise is bigger.
No, not for two showers at once. At a 68°F rise the largest electric units, about 36 kW, deliver about 3.6 gallons per minute, less than two 2.5 gallon showers, and they need 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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