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 incoming water to a 120°F setpoint, the commonly recommended setting. The calculator sorts places into three bands by average January low: 40°F or higher means water coming in at 65°F, 25°F up to 40°F means 52°F, and below 25°F means 42°F. With a January low of 23°F, Cleveland falls in the coldest band, so the rise is 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 always. If two showers run together, that is about 5 gallons per minute, and at a 78°F rise the load is about 195,000 BTU per hour, just past the calculator’s large class, so it suggests two linked units. With one shower at a time the load is about 97,500 BTU per hour, which a standard whole-house unit covers.
Not with 2.5 gallon per minute showerheads at a 78°F rise, where two showers need about 195,000 BTU per hour. Low-flow 2.0 gallon per minute heads bring the pair down to about 156,000 BTU per hour, which sits inside the large unit class.
Yes. Water arriving at 42°F has to climb 78°F to reach 120°F. Tankless ratings are quoted at a stated rise, and the larger the climb, the fewer gallons per minute a given unit can heat.
Not for more than one shower at a time. The largest common electric units, about 36 kW, heat roughly 3.1 gallons per minute at a 78°F rise, enough for one 2.5 gallon shower but short of a shower and a faucet together. They also 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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