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.
Heaters in the calculator raise incoming water to a 120°F setpoint, and the town’s average January low tells it how cold that water starts. Below a 25°F January low the calculator assumes 42°F incoming water, from 25°F up to 40°F it assumes 52°F, and at 40°F or warmer it assumes 65°F. Caldwell’s 25°F January low is in the 52°F band, which makes the rise 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, when the unit is large. Caldwell’s 68°F rise turns two showers’ 5 gallons per minute into about 170,000 BTU per hour, which falls in the calculator’s large-unit band. An extra faucet or a dishwasher running with them moves the result to two linked units.
Yes. At a 68°F rise a large gas unit delivers about 5.6 gallons per minute, and two showers draw roughly 5, so there is some to spare. A third shower or an appliance as well would call for two linked units.
No, the rise is set by the town’s winter instead. It is the gap between the incoming water and the 120°F setpoint, so Caldwell’s assumed 52°F water means a 68°F rise, where a town in the 65°F warm band would need only 55°F.
Not for two showers together. A standard electric unit tops out around 36 kW, which at a 68°F rise gives about 3.6 gallons per minute, enough for one shower. It also takes a very large electrical service, and an electrician has to confirm that.
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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