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 unit has to bring incoming water up to the setpoint, and we use the recommended 120°F. Incoming water is taken as 65°F where the average January low is 40°F or more, 52°F where it sits between 25°F and 40°F, and 42°F below 25°F. Indianapolis has a January low of 22°F, the coldest of those bands, so the unit has to add 78°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.
Not quite. One 2.5 gallon per minute shower at a 78°F rise calls for about 97,500 BTU per hour, just past the calculator’s small unit class, so even a single shower lands on a standard whole-house unit.
No, not at a 78°F rise. Two showers make 5 gallons per minute, about 195,000 BTU per hour, which is past the large unit class, so the calculator suggests two linked units. One shower with a faucet, about 167,700 BTU per hour, still fits a large unit.
Yes. The rise is the gap between incoming water and the setpoint, and with water arriving at 42°F and leaving at 120°F that gap is 78°F. A bigger rise means fewer gallons per minute from the same unit, which is why every tankless rating names its rise.
No, not for more than one shower at a time. At a 78°F rise the largest electric units, about 36 kW, give about 3.1 gallons per minute, 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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