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.
We heat to 120°F, the setting the US Department of Energy recommends, and we set the incoming water temperature from the town’s average January low in three bands. A low of 40°F or more means 65°F water coming in, a low from 25°F up to 40°F means 52°F, and a low under 25°F means 42°F. Yonkers, at about 25°F, sits in the middle band, which calls 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.
Yes, if two showers run at the same time. That is 5 gallons per minute, and at the 68°F rise we use for Yonkers it comes to about 170,000 BTU per hour of heat, which falls in the large high-capacity class. Add a faucet or a dishwasher to those two showers and the result moves to two linked units.
Yes. Two showers draw 5 gallons per minute, and one large gas unit delivers about 5.6 at a 68°F rise. A faucet or an appliance running at the same time pushes the load past what a single unit can give.
Yes. Every flow rating is stated at a temperature rise, the number of degrees the heater adds. Water entering at 52°F and leaving at 120°F needs a 68°F rise, and the colder the water coming in, the fewer gallons per minute the same unit can heat.
Not quite. The largest electric units, about 36 kW, give about 3.6 gallons per minute at a 68°F rise. That covers one shower but not two at once, and the unit also 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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