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 up to the setpoint, and we use 120°F, the commonly recommended setting. Dearborn has an average January low of 21°F, under 25°F, so we figure incoming water at 42°F. That is a rise of 78°F, the biggest lift the calculator uses. The same house asks more of a tankless unit here than it would in a warm climate.
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.
Plan on the flow of two showers at once, about 5 gallons per minute. At a 78°F rise that is more than one large gas unit gives, so it reads as two linked units. One shower at a time reads as a standard whole-house unit. Set your own fixtures in the calculator.
Not quite. The calculator sets Dearborn’s incoming water at 42°F, and at that rise one large gas unit delivers about 4.9 gallons per minute against the 5 that two showers use. Two linked units cover it, or the showers can run one after the other.
It is the gap between the incoming water and the 120°F setpoint. With water coming in at 42°F, the rise is 78°F. Tankless ratings are stated at a rise, so a unit rated for a smaller one delivers fewer gallons per minute in a Dearborn January.
Not for two showers at once at this rise. The largest common electric units, near 36 kW, give about 3 gallons per minute at 78°F, which is a little more than one shower. 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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