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.
The unit has to lift incoming water to a 120°F setpoint. We set the incoming temperature from the town’s average January low. Below 25°F we use 42°F water, from 25°F up to 40°F we use 52°F, and at 40°F or warmer we use 65°F. Meridian’s January low of 25°F puts it in the 52°F band, which means 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.
Yes, if it is a large unit. Two showers at once come to 5 gallons per minute, and at Meridian’s 68°F rise that is about 170,000 BTU per hour, inside the calculator’s large-unit band. Add a faucet or a dishwasher at the same time and the result moves to two linked units.
Yes. Two showers draw about 5 gallons per minute, and one large gas unit delivers roughly 5.6 gallons per minute at a 68°F rise, so it covers them with a little to spare. A third shower or an appliance on top of the two calls for two linked units.
No. The rise is the gap between the incoming water and the 120°F setpoint. With water coming in at 52°F, as the calculator assumes for Meridian, the rise is 68°F, while a place in the warm band at 65°F needs a rise of only 55°F.
No, not for two showers at once. The biggest standard electric units, at about 36 kW, deliver about 3.6 gallons per minute at a 68°F rise. That covers one shower, and the unit needs 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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