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 raise the incoming water to the setpoint, and the calculator uses the recommended 120°F. It assumes 65°F incoming water where the average January low is 40°F or more, 52°F where it is from 25°F up to 40°F, and 42°F below 25°F. Portland’s January low of 36°F puts it in the 52°F band, for a rise of 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.
It depends on what runs together. One shower at a time, 2.5 gallons per minute at a 68°F rise, is about 85,000 BTU per hour and fits the small class. Two showers at once double the flow to 5 gallons per minute, about 170,000 BTU per hour, which lands in the large class.
Yes, a large gas unit can. The calculator puts two showers at a 68°F rise at about 170,000 BTU per hour, inside its large class. Run a faucet or a dishwasher on top of both showers and the result becomes two linked units.
It is the number of degrees the unit adds to the incoming water. Water arriving at 52°F and leaving at 120°F means a 68°F rise. Tankless flow ratings are always given at a stated rise, and a bigger rise means fewer gallons per minute from the same unit.
No, not when two showers run together. The largest common electric units, about 36 kW, deliver about 3.6 gallons per minute at a 68°F rise, short of the 5 gallons two showers draw, 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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