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.
Hot water leaves a tankless unit at a 120°F setpoint, the recommended setting, so the work it does depends on how cold the water arrives. The calculator reads that from the average January low and assumes 42°F below 25°F, 52°F from 25°F to 39°F and 65°F at 40°F or above. Cedar Hill’s 35°F January low lands 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.
Size depends on simultaneous showers. Two of them draw 5 gallons per minute, which at Cedar Hill’s 68°F rise requires about 170,000 BTU per hour, a figure the calculator files under large units. A dishwasher or another faucet added to that flow moves the result to two linked units.
Yes, if the unit is a large gas model. A unit like that delivers about 5.6 gallons per minute at Cedar Hill’s 68°F rise, which covers the 5 gallons two showers use. Electric units top out near 3.6 and fall short.
Yes, because colder incoming water means a bigger temperature rise, the degrees a unit adds on the way to the tap. For Cedar Hill the calculator assumes 52°F in and 120°F out, a 68°F rise, and any given unit delivers fewer gallons per minute when that rise is bigger.
No, an electric unit cannot keep two showers going. Around 36 kW is the largest size, and at a 68°F rise it gives roughly 3.6 gallons per minute, enough for one. Such a unit also loads the electrical service heavily, 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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